Introduction To Earth And Space Science

Introduction to Earth and Space Science

Teaching Material Prepared for Physics Diploma Students by

Mesfin Tadesse

Kotebe College of Teacher Education Adds Ababa, Ethiopia

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Chapter 1. 1.1.

The Shape and Motions of The Earth

The Shape of the Earth

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Early Greeks (C. 1600 B.C.) believed that the Earth was disklike, floating on water, with the dome of heavens above it. They also hypothesized that there was an underworld comparable in scope and complexity to the heavens.

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Pythagoras (C. 500 B.C.) is thought to have been the first to assert that the Earth is round in shape and that all heavenly bodies move in circles. 

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Plato (428 - 347 B.C.) and his followers believed that all motions in the universe are perfectly circular and that all astronomical bodies are spherical. 

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Plato's fundamental percept was that what we see of the material world is only an imperfect representation of ideal creation. The implication was that we can learn more about the universe by reason than by observation.

Aristotle (c. 384 - 322 B.C., a student of Plato) was the first to adopt physical laws and used them to demonstrate that both the universe and the Earth are spherical. 



The Pythagoreans believed that the sphere is the perfect shape and that the gods utilized the perfect form to create the Earth.

Aristotle taught that circular motions are the only natural motions and that the center of the Earth is the center of the universe -- geocentric point of view.

Aristotle had three ways of proving that the Earth was spherical: i.

Only at the surface of a sphere do all objects seek the center by falling straight down. According to Aristotle, falling objects follow their natural inclination to reach the center of the universe.

ii.

The view of the constellations changes as one moves to the north or south poles.

iii. During lunar eclipses, it can be seen that the shadow of the Earth is curved. 

Aristotle is a bit different from Plato in that he mixed reason (theory) with observation.

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Today there are various direct evidences that show the Earth is almost spherical. Pictures of the Earth taken from stratospheric balloons, ionospheric rockets and satellites show a spherical image of the Earth.



A very good theoretical evidence concerning the shape of the Earth can be obtained from Newton's law of universal gravitation:

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FG

m1m 2 r2

The force of gravity on a body on the surface of the Earth is called the weight of the body. Measurements show that the weight of a body has the same value everywhere on Earth except for minor variations. If W = Weight of body on Earth, M = Mass of Earth, m = mass of body, then we can obtain the radius R of Earth from the law of universal gravitation:

R

GMm W

Since the right hand side is a combination of constants, we conclude that R is a constant as well, i.e., the Earth must be a sphere. Reading Assignment 1. Eratosthenes (c. 300 B.C.) was able to determine the size of the Earth. Write a short account of his method and the result obtained. 2. The Earth is oblate spheroid rather than a perfect sphere. The deviation from a perfect sphere is known as oblateness. Find the Earth's oblateness. 3. Discuss how to determine the mass of the Earth using the law of universal gravitation.

1.2. 

Motion of the Earth and the Seasons From geocentric to heliocentric view 

Ancient Greek philosophers believed that the Earth is at the center of the universe. They argued that the Sun, the Moon, and the planets (Mercury, Venus, Mars and Jupiter) all move in circular orbits around the Earth. This idea is Aristotle's geocentric (Earth-centered) view of the universe.



Eudoxus (408 - 356 B.C.) constructed a series of concentric spheres on which the Moon, the Sun and the planets moved in perfect circular motions. However, his model did not account for the observed retrograde motion of Mars.



Apollonius (265 - 190 B.C.) was able to explain the retrograde motion of Mars by including a small circle called an epicycle. An epicycle is a small circle that revolves on a larger circle around the Earth.

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Planet Deferent

E Epicycle

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

Aristarchus (c. 310 - 230 B.C.) is believed to have been the first scientist to adopt the idea that the Sun, not the Earth, is at the center of the universe. This heliocentric (Sun-centered) hypothesis of Aristarchus failed for lack of concrete evidence of the motion of the Earth.



Some 2000 years after Aristarchus, during the period of renaissance, the heliocentric view was reconsidered and adopted by Copernicus (1473 1543).



Johannes Kepler (1571 - 1630) studied Tycho Brahe's (1546 - 1601) massive collection of planetary data and developed the laws of planetary motion.

The Motion of the Earth 

Revolution The Earth moves around the Sun at an average speed of 30km/s in an elliptical orbit. This motion is called the revolution of the Earth. Since the Earth orbit is elliptical, its distance from the Sun varies throughout the year. The position of closest distance is called perihelion (about 145 million km). The Earth passes through this point around January 3rd. The farthest point from the Sun is called aphelion (about 151 million km). The Earth is at this point around July 4th. The average distance between the Sun and the Earth is 149.6 million km. This distance is defined as 1AU (Astronomical Unit): 1 AU = 149.6  106 km The orbital period of the Earth is 365.25636 mean solar days (see Rotation).

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Earth

Perihelion January 3

145mil km

151mil km

Aphelion July 4

Sun

Earth's orbit

Consequences of the Earth's revolution 1. Different views of the sky. As the Earth changes its position in space we see different star groups at different times of the year. 2. Occurrence of different seasons. (Discussed below) 

Rotation We see the Sun, Moon and the planets set and rise each day because of the Earth's rotation about its axis. The period of time required for one rotation of the earth on its axis is called a day. Two types of days are defined based on whether the sun or a star is used as a reference object. These are the solar day and the sidereal day. The solar day is the length of time between two consecutive passages of the sun across the meridian. The solar day varies in length because of the variation of the Earth's orbital speed. The length of the solar day averaged over a year is known as the mean solar day and is defined to be exactly 24h00m00s. The sidereal day is the time it takes the Earth to rotate once relative to a distant star not the Sun. It is defined as 23h56m4.098s, which is about 4 min shorter than the mean solar day. The length of the sidereal day is known to vary because of a number of reasons. 1. Regular slowing down of the Earth's rotation due to energy losses in tidal actions. The following table shows that the length of the day is keeping increasing.

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Period Cambrian, 500 million years ago Devonian, 300 million years ago Upper Carboniferous, 290 million years ago Mesozoic, 70 million years ago Today

Length of day 21 hours 22 hours 22.6 hours 23.67 hours 23.93 hours

2. Redistribution of mass in the Earth's ice sheet and glacier coverage 3. Atmospheric and oceanic circulations due to seasonal variations. Assignment 1. 2. 3. 4. 5.



Why does the Earth have day and night? Why do day and night alternate? Why are day and night unequal in length most of the year? Why do day and night change in length from day to day? Will the length of the day keep increasing as shown in the table above or will it stop increasing at some point in time? Precession Over a period of many years the direction of the Earth's axis shifts slowly. As a consequence, the positions of the stars change. For example, at present, the North Pole points toward Polaris and at about 15,000 A.D., Draco will be the Pole star. Polaris will be over the North Pole once again after one precessional period, which is about 26,000 years. The precession of the Earth's axis is caused by the gravity of the Sun and (especially) the Moon acting on Earth's equatorial bulge. The planets also have a small influence on precession.



Nutation The direction of the Earth's axis undergoes an additional small wavy motion called nutation, with a period of 18.6 years. The cause of nutation is the varying pull of the Moon on the equatorial bulge as the inclination of the Moon's orbital plane to the plane of the equator varies in a period of 18.6 years, due to the regression of the nodes of the Moon's orbit.

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

Chandler Wobble The body of the Earth wobbles slightly relative to its axis causing the latitudes of all places to change by small amounts. Evidences suggest that the positions of the poles have very slowly wondered for long distances over the Earth's surface during hundreds of millions of years of geologic time.

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The Seasons 

Seasons are caused by the constant tilt of the rotation axis of the Earth as it orbits the Sun. As a result, each hemisphere receives different amounts of heat during the year.

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On June 21, the Northern Hemisphere is tipped toward the Sun. Viewed from the Earth, the Sun will be over the Tropic of Cancer that is 23½º north of the equator. June, July and August are the hottest months and the days are longer in the Northern Hemisphere. These months make summer time in the north and because of this June 21 is called summer solstice. In the Southern Hemisphere conditions are quite opposite. Week Sun and short days prevail and they make June, July and August the winter months in the Southern Hemisphere.



Six months later, on December 21, the Earth is at the opposite side of its orbit and the North Pole is tipped away from the Sun. This date is called winter solstice. During winter solstice, the Sun will be located at the tropic of Capricorn, 23½º south of the equator. It is wintertime in the Northern Hemisphere and weak Sun and short days prevail through December, January and February. The same months make summertime in the Southern Hemisphere.



From June 21 to December 21, the Sun appears to move from Cancer to Capricorn passing the equator on September 23. Then, from December 21 to June 21, the Sun appears to move from Capricorn to Cancer passing the equator again on March 21. On March 21 and September 23, when the Sun is over the equator, daytime and nighttime are approximately equal throughout the world and the strength of the sunshine is the same in both hemispheres. On March 21, spring begins in the Northern Hemisphere and fall begins in the Southern Hemisphere. This date is called spring (vernal) equinox. On September 23, the Northern Hemisphere begins its fall and the Southern Hemisphere begins its spring and this date is called fall (autumnal) equinox.

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N

Sun Rays

N

S

S

December 21 Winter Solstice

1.3. 

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June 21 Summer Solstice

Phases and Eclipses The Moon 

The Moon is a natural satellite of planet Earth. It revolves the Earth in an elliptical orbit in a period of one month. The length of the month depends on the reference frame in which it is measured. When measured with respect to the stars, it takes the Moon 27d7h43m11.5s to complete one cycle around the Earth. This period of time is called the sidereal month. The synodic (lunar) month, on the other hand, is the time it takes the Moon to complete one cycle of phases, say, from one full Moon to the next full Moon. The synodic month is 29d12h44m2.8s long.



The nearest position of the Moon is 356,410 km from the Earth and is referred to as the perigee. The farthest position is known as the apogee and it is 406,697 km from the Earth.

Phases of the Moon 

While the Moon is orbiting the Earth, we see varying amounts of its illuminated face. This causes the Moon's apparent shape through the month. The sequence of shapes we see from Earth are known as the phases of the Moon. The full cycle of phases is completed in one synodic month.



We always see the same face of the Moon because the rotational period is equal to the orbital period.

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The different positions of the Moon with respect to the Earth-Sun direction are called configurations. The diagram below shows the configurations and phases of the Moon.

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First Quarter Moon Waxing Gibbous Moon

Waxing Crescent Moon

S U N

New Moon Invisible

Full Moon

Waning Gibbous Moon

R A Y S

Waning Crescent Moon Last Quarter

Moon

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Eclipses 

Eclipse is the obscuring of one celestial body by another, particularly, that of the Sun or a planetary satellite. An observer on Earth could see two types of eclipses: lunar and solar eclipses.



A lunar eclipse occurs when the Earth is between the Moon and the Sun and its shadow falls on the Moon. Lunar eclipses could be partial or total. Lunar eclipses are observed at full Moon.

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A solar eclipse occurs when the Moon is between the Earth and the Sun and its shadow falls on the Earth. Solar eclipses could be partial, total or annular. Solar eclipses are observed at new Moon.



A planet or a satellite lit by the Sun casts a conical shadow in space. At any point within that cone the light of the Sun is wholly obscured. This dark shadow cone is called the umbra. Surrounding the umbra is a region of partial shadow called penumbra.

Penumbra

SUN

Umbra Planet or Satellite



A total lunar eclipse occurs when the Moon completely passes into the umbra. The period of totality depends on how the Moon passes through

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the umbra. If it moves right through the center, it is obscured for about 2 hrs.

1.4. 



A partial lunar eclipse occurs when part of the passing Moon enters the umbra and part in the penumbra.



Total solar eclipses occur when the Moon's umbra reaches the Earth. The area in which a total solar eclipse is visible on the surface of the Earth is never more than 270 km across. A total solar eclipse is usually visible for about 3 min.



In areas where the penumbra of the Moon's shadow but, the Sun is only partly obscured, and a partial solar eclipse occurs.

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At certain times when the Moon passes between the Earth and the Sun, its shadow does not reach the Earth. At such times an annular eclipse of the Sun occurs, that is, only a ring of the Sun's disk is visible around the black disk of the Moon.



We don't see eclipses every new and full Moon because the Moon's orbit is inclined by about 5 relative to the Earth's. Often, in its new or full phase, the Moon lies below or above the Earth' s orbit and its shadow cannot fall on the Earth, nor can the Earth's shadow fall on it. However, there are times, at least twice each year, when the Moon, the Earth and the Sun line up and eclipses can occur.

Rockets and Satellites Rockets 

Rockets are devices that burn solid or liquid chemical propellants in their motors. The burnt hot gases are ejected in a jet through a nozzle at the rear of the rocket at extremely high speed. The rockets forward motion (thrust) is produced by reaction to the rearward expulsion of the hot gases. Assignment 1. What physical principle applies to rocket propulsion? Discuss in detail and give other examples. 2. The types of rockets discussed in this section are spacecraft launch vehicles. Give examples of other type of rockets.

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Satellites 

Satellites are natural or artificial objects orbiting a larger astronomical object, usually a planet.

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

Artificial satellites are used for scientific research and other purposes, such as communication, weather forecasting, earth resources management, and military intelligence. Assignment 1. What is a geostationary satellite? 2. How can a satellite be used to determine the mass of the Earth?

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Chapter 2. 2.1. 

The Earth's Atmosphere

Origin and Composition of the Atmosphere Present Atmosphere 

The atmosphere is a mixture of gases that surround the Earth. Almost 99% of the atmosphere lies within 30 km of the Earth's surface.



The Earth's atmosphere is colorless, odorless and tasteless. It is mobile, elastic and compressible.



The atmosphere is suitable for living things: It contains O2 and CO2 needed by animals and plants. It has a temperature range that helps living organisms survive.

 

The atmosphere protects the Earth from dangerous radiations and bombardments (UV radiations, meteors).

Origin 

By applying the methods of radiometric dating, it was found that the Earth's original atmosphere developed about 4.6 billion years ago when the planet formed.



The original atmosphere of the Earth was dominated by Hydrogen (H2), Ammonia (NH3) and Methane (CH4). It is believed that these lightweight gases have escaped during the early ages of the Earth. There are several hypothesis: i. UV radiation from the Sun breaks the hydrogen atoms from these gases and subsequently hydrogen escapes for the Earth's gravity is too week to hold it. (Huge planets like Jupiter and Saturn can hold such light gases.) ii. The original atmosphere was wiped by explosive waves from the Sun.



If the original atmosphere has gone, where did today's air come from? [The atmosphere we have today is known as a secondary atmosphere.] There are two theories: i.

Comet impacts -

Comets are made of frozen water and gases. Numerous comet impacts could have delivered enough water and gas to form the atmosphere and gases.

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ii.

Volcanic eruptions -

-



Around the first billion years of the Earth's history, gases trapped in the Earth's hot interior were expelled by a large number of volcanoes. Volcanic eruptions contain water vapor (~ 85%), CO2 (~10%), a relatively small amount of nitrogen, some other gases and dust particles. Through hundreds of millions of years, the volcanic water vapor condenses into a thick cloud. When the Earth cooled sufficiently, rain started to fall for 40,000 years and filled the earliest oceans. The rain dissolved much of the carbon dioxide leaving out nitrogen as the principal constituent of the atmosphere. CO2 would have reacted with the rocks of the Earth's crust to form the carbonate minerals.

The present atmosphere of the Earth is primarily a mixture of two gases: Nitrogen (78%) and oxygen (21%). Neither of the theories discussed above (comet impacts and volcanic eruptions), can account for the large amount of oxygen in today's atmosphere. Where, then, did the oxygen originate? There are two possible answers: Photosynthesis and Photodissociation. i.

Photosynthesis Geological evidences show that primitive life capable of photosynthesis evolved in the oceans. They used Sun light, water and carbon dioxide to produce oxygen. [Photosynthesis combination of H2O and CO2  O2] (c. 570 million years ago, the oxygen content of the atmosphere and oceans became high enough to permit marine life capable of respiration. c. 400 million years ago, when the atmosphere contains enough oxygen, air breathing animals emerge from the waters.)

ii.

Photodissociation -

-

Plants have created most of our oxygen by photosynthesis but not all. Water molecules are split into hydrogen and oxygen molecules by UV radiation from the Sun, a process known as photodissociation. The hydrogen, being light, escaped the Earth's gravity and oxygen remained behind. This process, however, is too slow to account for the present percentage of oxygen.

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

Composition 

The principal components of the atmosphere are Nitrogen (78%) and Oxygen (21%).



The remaining 1% includes Argon (0.9%), CO2 (0.03%), water vapor, hydrogen, ozone, methane, carbon monoxide, helium, neon, krypton, and xenon.



The atmosphere also contains small particles called aerosols.



Oxygen -- is vital for all life forms -- combines with all other elements -- is a reason for combustion



Nitrogen -- is chemically inactive -- is a constituent of all living matter (organic compounds) -- regulates oxygen by diluting oxygen -- scatters the blue portion of Sun light and makes the sky appears blue



Carbon dioxide -- is a product of combustion -- is constantly added to the atmosphere by the burning of fossil fuels (coal, oil, natural gas) and from fermentation and animal respiration -- used by green plants in photosynthetic processes -- absorbs IR radiation from the ground and, therefore, reduces heat loss. This phenomenon is known as the greenhouse effect.



Water vapor -- varies from 0% in cold dry air to 4% in humid hot air -- is significant for climate changes (it is the source of all clouds and precipitations) -- absorbs IR radiation, causing a greenhouse effect -- 99% is found within 6 km of the ground

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Visible radiation

IR radiation absorbed by CO2 and H2O

Greenhouse effect

Ground absorbs visible radiation; it warms up and reradiates IR



Ozone -- is found in a very small quantity (0.00005%) at a height of 25 km. -- it is the most efficient absorber of UV radiation -- is formed by the following reactions: i. During the day, atomic oxygen is formed by photodissociation of O2. O2 + h  2O ii. During the night ozone forms by the reaction O2 + O + M  O3 + M where M is any other molecule needed to conserve momentum. -- Ozone is destroyed by the following reactions: i. Photodissociation during the day by UV radiation O3 + h  O2 + O O is immediately converted to O3 by reaction (ii) above. ii. Reaction with NO O3 + NO  O2 + NO2 iii. Reaction with chlorofluorocarbons Cl + O3  ClO + O2 ClO + O  Cl + O2

Cl is reformed and the reaction can be repeated any number of times.

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

Aerosols -- are small particles from deserts, volcanic eruptions, smoke from forest fires, pollutants of different types, salt crystals from evaporating sea sprays and debris of numerous meteors. -- Scatter short wavelengths and make the sky appear blue and the Sun yellowish and reddish in the evening and morning sky.

2.2. 

Structure of the Atmosphere According to its composition the atmosphere is divided into two parts: homosphere and hetrosphere. 

Homosphere The gases are well mixed Extends up to 80 km



Hetrosphere Gases stratify according to density Part of the atmosphere above 80 km

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Temperature and altitude 

The variation of temperature with vertical distance is called lapse rate. Two types of variations are distinguished: A normal lapse rate occurs when the temperature decreases with altitude. A negative lapse rate (inversion) is an increase of temperature with height.

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Thermal layers The atmosphere has four thermal layers: 

Troposphere 16 km thick at the equator and 10 km thick at the equator Supports weather processes and life Temperature decreases with altitude (ground heats up first) At the top of the troposphere (tropopause) the temperature is – 60ºC



Stratosphere Layer of atmosphere next to the troposphere Extends from 12 to 50 km

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Temperature varies from – 60ºC at the base to 0ºC at the top (Stratopause) due to absorption of UV by O3. 

Mesosphere Extends from 50 to 80 km Temperature falls to a minimum of – 100ºC at the mesopause



Thermosphere Outermost layer extending from 80 km to the outer space Temperature rises to 500ºC at an altitude of 150 km. Above this height the temperature remains constant. It has two parts: ionosphere and exosphere Ionosphere Layer of ionized gases Ranges from 100 to 400 km of altitude Reflects radio signals back to Earth Solar electrons pulled by the Earth's magnetic field toward the poles interact with the ionosphere and produce a colorful display in the sky known as aurora. Exosphere Portion of the atmosphere above the ionosphere The magnetic field traps charged particles from the Sun in two regions at altitudes of 3000 km and 25000 km. These regions of trapped particles are known as Van Allen radiation belts

2.3.

Major Air Circulation The two main causes of air circulation are unequal heating by the Sun and Coriolis effect due to the rotation of the Earth.

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Unequal heating (global effect) 

The heat absorbed per unit area near the equator is much greater than that absorbed near the poles.



As a result, pressure (density) differences appear in the atmosphere. These pressure differences cause the air to move from one region to the other.



Generally, air in hot areas rises, cools at high altitudes and descends somewhere else. 17

Cool air Convection in large-scale air circulation

Warm air rises in the tropics

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Unequal heating (local effect)



Locally, unequal heating produces coastal breezes, which are small-scale air circulations Land breeze

See breeze

Water is cooler

Land is warmer

Daytime circulation of air results in sea breezes

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Water is warmer Land is cooler

Nighttime circulation of air results in land breezes

Coriolis effect 

The Coriolis effect is an apparent force due to the Earth's eastward rotation. It is the reason for the deflection of winds blowing toward the equator or toward the poles.

Coriolis effect in the N.H.

Coriolis effect in the S.H.

At the equator speed = 1670km/hr = 464 m/s Winds veer to the right

Winds veer to the left

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

Because of the Coriolis effect, air moving out of a high pressure center deflects to the right forming anticyclones in the northern hemisphere. Similarly, air pulled in toward a low pressure center deflects to the right in the northern hemisphere forming cyclones.



In the southern hemisphere, the wind circulates counterclockwise aroud a high and clockwise around a low.

H

Anticyclones in the N.H. around a high

2.4.

L

Cyclones in the N.H. around a low

Global Wind Systems

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Warm, humid air rises at the equator, creating a low-pressure belt known as the doldrums (windless zone around the equator).



The warm air rises to the top of the troposphere and moves toward the poles. The Coriolis effect deflects it eastward until it reaches 30 lat North or South. At this latitude the air descends and a major portion returns to the equator. The 30 latitudes are known as the horse latitudes where we have the major deserts of the world. [Saharan, Arabian and Iranian deserts in the northern hemisphere and Atacama, Kalahari and Australian deserts in the southern hemisphere.]



The ground flow coming from the horse latitudes to the equator forms the northeasterly trades and the southeasterly trades. The cells between the equator (0) and the horse latitudes (30) are known as the Hadley cells.



Part of the air descending at 30 lat advances as a ground flow toward the 50 lat forming the prevailing westerlies. The low-pressure regions near 50 lat north and south of the equator are known as the polar fronts. The convection cells between the horse latitudes (30 lat) and the polar fronts (60 lat) are called Ferrel cells.



Cold air sinks at the poles and streams outward to the polar fronts forming the polar easterlies in both hemispheres.

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2.5.

Factors of Weather



Weather is the state of the atmosphere at a particular place during a short period of time.

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The factors that determine the weather conditions of an area are:



i.

Temperature

v.

Precipitation

ii.

Pressure

vi.

Visibility

iii.

Humidity

vii. Wind

iv.

Clouds

viii. Sunshine

All weather factors are interdependent. 

Sunshine amount varies with seasons, latitudes and cloud cover determines the level of temperature 47% is absorbed by land and water bodies Assignment Write a short note on the solar radiation budget our planet receives



Precipitation amount of rain, drizzle, snow or hail that falls from clouds



Visibility greatest horizontal distance at which large objects on the ground (buildings, mountains) cab be recognized by the naked eye reduced by smoke, fog and other particles in the atmosphere



Humidity the amount of water vapor in the atmosphere warm air holds more air vapor than cold air. For example, air at 30C can hold up to 30 g/m3 of water vapor while air at 0C can hold only 4.8 g/m3 of water vapor. Absolute humidity is the amount of water vapor per cubic meter of air. Relative humidity is the percentage of the maximum amount of water vapor that the air can hold at a given temperature. Re lative Humidity 

Absolute Humidity

 100%

Maximum amount of water vapor

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2.6. 

Water Cycles and Clouds The different sources of water on planet Earth include: i.

oceans

ii.

glaciers and ice sheets

iii.

lakes

iv.

rivers

v.

ground water

vi.

clouds

vii. plants and animals viii. atmospheric water vapor 

These sources of water constitute the hydrosphere of the Earth.



The hydrologic (water cycle)





The processes in the hydrosphere that starts as evaporation, passes through condensation and ends as precipitation.



Despite the continuous change of state and location, the total water budget remains in balance.

Condensation nuclei 

Aerosols (dust particles) that facilitate the condensation process



Relative humidity is nearly 100% when water vapor condenses



Size

 

-

Small (Aitken) nuclei are less than 0.2m in diameter

-

large nuclei range from 0.2m to 1.0m in diameter.

-

giant nuclei have sizes larger than 1m.

Could be hydroscopic (water-seeking) or hydrophobic (water-repelling)

Clouds 

Formed when the atmosphere cools below its dew point, the temperature at which relative humidity is 100%.



Consist of water droplets and ice crystals (2 - 100m) sustained by upward moving air currents



There are 10 main cloud families, divided into groups based on their altitudes.

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i.

High clouds (13 - 5 km) Cirrus: fine, feathery clouds consisting of ice crystals Cirrocumulus: aligned in rows with a gap of clear sky Cirrostratus: thin sheet of ice crystals that may produce halos

ii.

Middle clouds (7 - 2 km) Altocumulus: white and gray with wavy or parallel appearance Altostratus: gray; cover the sky partly or wholly; show Sun/Moon vaguely Nimbostratus: dark; produce steady and continuous rain or snow

iii.

Low clouds (2 - 0 km)

Stratocumulus: gray lumps or rolls; often covering the whole sky Stratus: uniform base Cumulus: thick, wool-like, flat bases Cumulonimbus: lighting and thunder accompanied by heavy rain, snow or hail 

2.7.

A shallow layer of cloud at or near ground level is called fog.

Change of Weather



The changes in the weather elements are associated with the formation and transportation of huge bodies of air known as air masses.



Air masses



Large bodies of air having nearly uniform temperature and humidity at any given altitude



Form whenever the atmosphere remains in contact with a large, relatively uniform land or sea surface



The Earth's major air masses originate in polar and subtropical latitudes known as source regions.



Examples of source regions:

i. Polar:

Snow-covered plains of Canada, Siberia

ii. Tropical and subtropical: Oceans at 30 lat, hot and arid Sahara 

Major air masses In terms of their temperature air masses are classified as Polar (P) and Tropical (T). 22

In terms of their humidity air masses could be continental (c) or maritime (m). The following table describes the four major air masses.



Air mass

Description

cP

Cold, dry, stable

cT

Hot, dry, stable

mP

Cold, moist, unstable

mT

Warm, moist, unstable

Fronts 

A front is the interface or transition zone between two air masses of different characteristics.



Frontal zones are boundaries of highly active weather changes. These weather changes are: Sharp temperature changes Pressure changes Changes in relative humidity Shifts in wind directions Considerable cloudiness and precipitation



There are four types of fronts: cold fronts, warm fronts, stationary fronts and occluded fronts.



The difference between a cold front and a warm front is the direction in which the front moves. A cold front moves to the warm area. A warm front moves to the cold area.



Consequences of a cold front: Sudden drop in temperature Heavy rain and thunderstorms from cumulus and cumulonimbus clouds



Consequences of a warm front: Formation of cirrus, stratus and nimbostratus clouds Gentle rain from nimbostratus clouds

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

Stationary fronts are weather fronts that remain over an area for several days.

cb

C O L D FR ON

Warm Air Mass

T

Cold Air Mass

Warm Air Rising

c

Ns

St WA

FR RM

ON

T

Cold Air Receding



An occluded front forms when warm air is squeezed aloft between two merging cold air masses.

Warm Colder

Cold Occluded Cold Front



Cold

Colder Occluded Warm Front

Weather Forecasting



It is possible to tell in advance the weather conditions of an area using the principles of physics and meteorology. The prediction of the weather based on such principles is known as weather forecasting.



Scientific weather forecasting relies on empirical and statistical techniques, such as 2.7...i.1. measurements of temperature, humidity, atmospheric pressure, wind speed and direction, and precipitation, 2.7...i.2. and computer-controlled mathematical models.

Write a report on 1. the different techniques of weather forecasting. 2. the causes of climate change

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Chapter 3. Geology of the Earth 3.1.

Minerals 

A mineral is any naturally occurring homogeneous solid that has a definite chemical composition and crystal structure.



Four conditions must be satisfied for a substance to be a mineral: 

It must be a crystalline solid.



It must occur naturally.



It must be inorganic.



It must have a definite chemical composition.

Crystalline solid  The atoms of a mineral are arranged in a 3D array forming a regularly repeating, orderly pattern known as a crystal structure. For example, the mineral halite (NaCl) has cubic (isometric) crystal structure. [See Mekbib, the basic crystal systems, Page 138, Appendix F] Natural substance  Human-made crystalline compounds are not regarded as minerals.  There are synthetic equivalents of various minerals, such as emeralds and diamonds, manufactured for commercial purposes. Inorganic substance  Minerals are formed by inorganic processes.  Organic substances (compounds of carbon oxygen and hydrogen) made by leaving organisms are not regarded as minerals. Definite chemical composition  The composition of a mineral is expressed as chemical formula.  The formula always shows the same ratio of elements.  Although most minerals are chemical compounds, a small number are elements.  Examples of minerals Quartz has the chemical formula SiO2 Halite, known as sodium chloride, has the formula NaCl The formula for olivine is (Fe, Ma)2SiO4

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

There are six basic crystal systems known to crystallographers: Triclinic, Monoclinic, Orthorhombic, Hexagonal, Tetragonal and Cubic (Isometric)



Nearly 3000 distinctive mineral types have been discovered. The most important ones are divided into five families based on their chemical composition. These are Silicates, Carbonates, Oxides Sulfates and Sulfides.



Silicates are composed of the most abundant elements in the Earth's crust, oxygen (~ 45%) and silicon (~ 28%).



Second in abundance are the oxides which are combinations of oxygen with other elements.



Combination of oxygen with carbon and sulfur produces the mineral groups carbonates and sulfates, respectively.



The halides are formed by combinations of sodium, chlorine, iodine, bromine and fluorine. [See Mekbib, page 57]



The physical properties of minerals  Physical properties are used to identify an unknown mineral. These are color, streak, luster, hardness, external crystal form, cleavage, fracture, specific gravity and other properties. Color: For some minerals color is a useful property. For many other minerals such as quartz color is extremely variable and, therefore, it is a poor procedure to totally depend on color to identify such minerals. Streak A mineral in a powder form (pulverized mineral) has a different color than the color of the specimen itself. This color of a pulverized mineral is known as a streak. For instance, hematite (Fe2O3) always leaves a reddish brown streak though the sample may be brown or red or silver. Luster The quality and intensity of light that is reflected from the surface of a mineral is known as luster. Luster is either metallic or nonmetallic. Hardness The relative ease or difficulty with which a smooth surface of a mineral can be scratched is known as Hardness. Hardness is measured by Mohs' scale shown below. On Mohs' hardness scale, talc, the softest substance, is designated as 1. Diamond, the hardest substance on Earth, has a hardness of 10 on this scale.

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Mohs' hardness scale 1. Talc

6. Feldspar

2. Gypsum

(Finger nail)

7. Quartz

3. Calcite

(Copper Coin)

8. Topaz

4. Fluorite 5. Apatite

(File)

9. Corundum (Knife blade, glass)

10. Diamond

External crystal form The crystal form of a mineral is the arrangement of its faces in a definite geometric relationship. For example, halite maybe regarded as a series of cubes stacked in three dimensions and thus its crystal form is usually a cube with faces at 90 to each other. Cleavage cleavage is the ability of a mineral to break, when struck along preferred directions. A mineral splits apart along certain planes because at these planes the atomic bonding is weakest. Quartz has no cleavage because its bonds are equally strong in all directions. Fracture Minerals that do not have cleavage break in irregular surfaces called fractures. Specific gravity Specific gravity is the ratio of the mass of a substance to the mass of an equal volume of water. Other properties properties such as taste, smell, magnetic and optical properties can be used to identify a mineral. For example, Halite tastes salty, clay minerals smell earthy when moistened and magnetite (an iron oxide) attracts pieces of metal.

3.2.

Rocks 

A rock is a naturally formed, consolidated material composed of grains of one or minerals. For instance, granite is made of quartz and feldspar.



There are three major classes of rocks grouped based on the processes that formed them. These are igneous, sedimentary and metamorphic.

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

Igneous rocks  Are formed when hot, molten material called magma cools and crystallizes  May be intrusive or extrusive  Intrusive rocks are igneous rocks formed when magma solidifies below the Earth’s surface.  Extrusive rocks are igneous rocks formed either by effusive eruption or volcanic eruption of magma that crystallizes on the surface of the Earth.



Sedimentary rocks  Sedimentary rocks are formed at the Earth’s surface where sediments of preexisting rocks are deposited long enough to become compacted and cemented into hard beds or strata.  What is sediment? Sediment is the collective name for loose, solid particles that originate from Weathering and erosion of preexisting rocks Chemical precipitation from solution, including secretion by organisms in water 

Formation process Weathering refers to destructive processes that change the physical and chemical character of a rock at or near the surface of the Earth. Physical (or mechanical) weathering includes processes that break a rock into smaller pieces. Water freezing and expanding in the rock cracks causes physical disintegration of the rock. Chemical weathering is the decomposition of rock by agents like water and atmospheric gases into new chemical compounds.

 Transportation is the movement of eroded particles by agents such as rivers, glaciers, waves or wind. When the transported material settles deposition occurs. Sediment is deposited when running water, waves glaciers, or wind loses energy and can no longer transport its load.  Lithification is the process by which loose sediment is converted into sedimentary rock. Most sedimentary rocks are lithified by a combination of compaction, cementation and crystallization. Compaction is a process of packing loose sediment grains tightly together Cementation: Precipitation of cement around sediment grains binds then into a firm, coherent rock.

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Crystallization is the development and growth of crystals by precipitation from solution at or near the Earth’s surface. Such rocks lack cement and have no pore space. 

Sedimentary rocks may be clastic, nonclastic or evaporites Clastic: a sedimentary rock composed of fragments of preexisting rocks. Example: sandstones (contain ferric ions) Nonclastic: a sedimentary rock formed by chemical and biological agencies from material in solution (ions dissolved in sea water). Example: calcite which later become lime stones, dolomite Evaporites: sedimentary rocks resulting from the evaporation of seawater. Example: rock salt, gypsum



Metamorphic rocks  Metamorphism is the transformation of preexisting rock into texturally or mineralogically distinct new rock as a result of high temperature, high pressure or both, but without the rock melting in the process.  Metamorphic rock is a rock formed by metamorphism  A metamorphic rock has characteristic texture and particular mineral content due to several factors. The most important ones are: Composition of the parent rock before metamorphism Temperature and pressure during metamorphism Effects of tectonic forces Effects of fluids such as water 

Types of metamorphism Contact (thermal) metamorphism  High temperature is the dominant factor. The heat energy of the magma rising from deep within the Earth is very high. It increases sharply the temperature of the rocks with which it becomes in contact.  The confining pressure is relatively low. This is because contact metamorphism mostly takes place near the Earth’s surface (< 10km depth). Regional (dynamothermal) metamorphism  Takes place at considerable depth underground

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 Formed under the combined effect of heat, confining pressure and differential stress  Depending on the pressure and temperature conditions during metamorphism, a wide verity of metamorphic rocks could be formed. The rock cycle Weathering and Erosion Sediment

Igneous rock

Lithification

Solidification

Sedimentary rock

Magma Melting

3.3.

Metamorphic rock

Metamorphism

The Earth’s interior 

We do not have direct access to the interior of the Earth. The deepest hole drilled so far is less than 15km, which is only about 0.2% of the Earth’s radius.



Then how do we know about the deep interior of the Earth? Mostly by use of indirect techniques. Data about the Earth’s interior can be gathered by  Information from heat flow measurements  Studies of seismic waves  Other sources such as volcanic rocks that come from great depths



Density, pressure and temperature  The average density of the Earth is 5.5 g/cm3. Most rocks on the surface, however, have densities from 2 g/cm3 to 3.5 g/cm3. This indicates that the Earth’s interior must be denser than 5.5 g/cm3.  Pressure increases with depth to about 3.5 megabars at the center. The average pressure in the Earth’s interior is 1Mb. The enormous pressure

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crushes the material inside the Earth causing its density to increase. It also changes the mineral structure of rocks and resolidifies molten material.  The heat in the Earth’s interior has its origin from radioactive decays and heat remaining from the time of the Earth’s formation. Measurements in mines show that as one moves inward the temperature increases by about 30K per km. Direct evidence for the increase in temperature with depth comes from volcanic magmas that reach the surface at temperatures greater than 1000K. 

Magnetic field  It is long known that the Earth has a magnetic field. This magnetic field could not be produced by a powerful permanent magnet at the center (core) of the Earth because the high temperature of the core destroys the permanent magnetism of any magnetic material.  What, then, is the source of the Earth’s magnetism? One widely accepted hypothesis (dynamo model) for the Earth’s magnetic field suggests that somewhere with in the Earth there must be a large-scale motion of conducting (probably metallic) material.  The magnetic poles are displaced about 11½˚ from the geographic poles and they appear to be moving slowly around the geographic poles.  Evidences show that the Earth’s magnetic field has periodically reversed its polarity in the past.



Seismic waves  Earthquakes produce seismic waves that travel at great speeds through the Earth. Earthquakes occur when sections of the Earth’s crust move suddenly relative to each other.  Study of seismic waves gives information about the density at all levels in the Earth and the depths of different layers. Regions in which density changes abruptly with depth indicate changes in chemical composition, mineral structure or physical state.



Internal structure of the Earth  The study of seismic reflection and seismic refraction shows three main zones of the Earth’s interior: the crust, the mantle and the core.  The Earth’s crust The crust is the uppermost layer of the Earth. The oceanic crust varies from 5 to 8 km Continental crust averages 30 to 50 km in thickness. 31

Seismic waves travel faster in oceanic crust than in continental crust indicating that the two crusts are made of different types of rocks.  The mantle The layer below the crust The speed of seismic waves increases sharply at the boundary between the crust and the mantle The mantle has three parts  The upper mantle, like the crust, is solid and rigid. It extends down to about 70 km. Together with the crust it forms the lithosphere.  Between depths of 70 and 250 km the mantle is partially melted because of high temperature. This partially melted zone is called asthenosphere. The rocks here are likely to flow with relative ease and zone is where magma is likely to be generated.  Below the asthenosphere is the solid part of the mantle where pressure compresses the rock to form denser minerals. Here the speeds of seismic waves increase rapidly. The mantle material has an average density of 3.5 g/cm3.  The core The Earth’s metallic core begins at a depth of about 2900 km. Seismic wave refractions within the Earth’s core suggest that the core has two parts: a liquid outer core and a solid inner core. The inner core is subjected to a pressure so great that the metallic material is solid. Because of its high density (13 g/cm3), this metal is most likely iron and nickel mixed with small amounts of the lighter elements such as sulfur, silicon, or oxygen.

3.4.

Continental drift 

The Theory of Plate Tectonics  Tectonics is the study of the origin and arrangement of the structural features of the Earth’s surface (folds, faults, mountain belts, continents, sea floors, earthquake belts).  Plate tectonics is a theory that the crust of the Earth is divided into large regions (plates) that move very slowly relative to each other and change in size. Interactions between the plates at the boundaries create mountains, earthquakes, volcanoes and other geologic activities.

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 Some 30 years before, most geologists thought that the continents and oceans were stable, permanent features of the planet. This point of view changed by the theory of plate tectonics that brings together and unifies many observed facts about the Earth’s outer layers. It describes the outer layers as dynamic and changing.  The theory of plate tectonics is a combination of two previous ideas: continental drift and sea-floor spreading. 

Continental drift 

Observed facts In 1620, Francis Bacon noticed that the eastern and western shores of the Atlantic Ocean were parallel and could be fitted together rather snugly. In the1920s, Alfred Wegener noted the strong similarity of rocks (mineral types) and fossils on opposite shores of the Atlantic. Geologists observed similarities between magnetic alignments in the rocks on opposite sides of the Atlantic.

 These observed facts suggest that the continents were once joined together and have split and moved apart from one another.  Wegener proposed that the present continents had been parts of a supercontinent called Pangaea. (About 200 million years ago)  Pangaea began to break apart about 135 million years ago into two parts called Laurasia and Gondwanaland. Laurasia was the northern part of Pangaea that formed North America, Europe and Asia (excluding India) Gondwanaland was the southern part of Pangaea that formed South America, Africa, India, Australia and Antarctica. India has drifted north later. 

Recent evidence of continental drift A much more precise fit between continental shelves, rather than the present shoreline. Glacier movement. The ice that moved onto South America has been traced to a source that is now in Africa. Since the continental glaciers cannot move across the oceans, how cloud we have similar glaciers on the costal lines of Africa and South America? The answer seems simple: the two continents had been joined together.

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Broad belts of rock that match in type and age from one continent to another 

Sea-floor spreading  Sea-floor spreading is the splitting of the oceanic crust and subsequent creation of new ocean floor by magma forces.  By 1960, geologists learned that there are ridges on the mid ocean floor that extend for 60,000 km  Harry Hess proposed that the sea floor moves away from the mid oceanic ridge as a result of mantle convection.  Why does the sea floor move? According to Hess’s hypothesis the sea-floor spreading is driven by deep mantle convection. A slow convection of molten material is set up by temperature differences deep in the earth’s interior and when it comes near the crust it spreads out and drags the surface layers with it breaking the crust apart in a phenomenon called rifting.



Plates and plate motion  A plate is a large mobile slab of rock that is part of the lithosphere. Plates are made of oceanic crust (sea floor) and/or continental crust.  The plates, which are composed of blocks of lithosphere, move relative to one another. They ride on the partially molten asthenosphere.  There are three types of plate boundaries: A divergent plate boundary is a boundary between plates that are moving apart. Here plates grow and separate and such boundaries coincide with the crests of submarine mountain ranges called midoceanic ridges. A convergent plate boundary lies between plates that are moving toward each other. When the plates collide, one of them overrides the other or they form mountains. When overriding occurs, the edge of the overridden plate is driven down into the mantle and melted. This process is called subduction. Subduction zones appear as deep ocean trenches. A transform plate boundary is one at which plates move horizontally past each other.

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3.5.

Earthquakes 

Causes of earthquake  An earthquake is a trembling and shaking of the ground caused by the sudden release of energy in the rocks beneath the Earth’s surface  The waves of energy produced by an earthquake are called seismic waves.



Seismic waves  The point within the Earth where seismic waves originate is called the focus (or hypocenter) of the Earthquakes. The point on the Earth’s surface directly above the focus is the epicenter.  Two types of seismic waves are generated during earthquakes: Body waves that travel through the earth’s interior Surface waves that travel on the Earth’s surface  There are two kinds of body waves: A P wave and an S wave A P wave  Compressional wave in which the rock vibrates back and forth parallel to the direction of wave propagation  The first (or primary) wave to arrive at a recording station  Traveling at speeds of 4 to 7 km/s  Passes through both solid and fluid materials An S wave  Transverse wave in which rock vibrates perpendicular to the direction of wave propagation  Slower compared to a P wave, moving at speeds ranging from 2 to 5 km/s  Cannot travel through a fluid  Surface waves are the slowest waves set off by earthquakes and cause more property damage.  The two most important kinds of surface waves are Love waves and Rayleigh waves.  Love waves are most like S waves in that the particle motion is perpendicular to the direction of the wave travel along the surface of the Earth. Like S waves, Love waves do not travel through liquids

35

 Rayleigh waves are like ocean waves causing rolling on the Earth’s surface. They are very destructive. 

Earthquakes And Plate Tectonics  The theory of plate tectonics explains most earthquakes as being caused by interactions between two plates at their boundaries.  Divergent plate boundaries are marked by a narrow zone of shallow earthquakes along normal faults.  Convergent boundaries are marked by a very broad zone of shallow quakes  Transform boundaries are marked by shallow quakes cause by strike-slip motion along one or more faults.

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Chapter 4.

The Solar System

The Solar System consists of the Sun and other bodies orbiting in its gravitational field. These other bodies are the nine planets, their moons, and swarms of asteroids and comets. The two most fundamental features of the Solar System are its flattened structure and the orderly orbital and spin properties of its planets.

4.1 The Sun  The Sun is a star composed of hot incandescent gas and formed at about 4.5 billion years ago.  It is the largest body in the solar system and its gravitational force holds all the other bodies in the system in their orbits.  The light and heat energy are generated by nuclear reactions in its core.  Size, composition and other physical properties o Diameter = 1.39  106 km, about 110 Earth-sized planets put in a row! Mass = 1.99  1030 kg, about 300,000 times as massive as the Earth. Average density = 1.41g/cm3. (Why smaller than the density of the Earth?) Density at the center = 150,000kg/m3. Luminosity (amount of energy emitted per second) = 3.6  1026 watt. o The Sun is so huge that it makes about 99.9% of the mass of the solar system. o The Sun is a sphere of hot gas held together by gravity. While gravity pulls the gas inwards, the hot gas exerts pressure, pushing outward and balancing gravity. This balance between pressure and gravity keeps the Sun in hydrostatic equilibrium. o The spectrum of the Sun shows that it is mostly composed of hydrogen and helium (more than 99%). It also contains very small proportions of all the other chemical elements.  Hydrogen -------- about 74% of the total mass of the Sun.  Helium ----------- about 25% of the total mass of the Sun.  All the rest ------- about 1% (oxygen, carbon, iron, nitrogen, etc)  Due to the extremely large temperature, all elements are in gaseous state, completely ionized in the inner parts and partially ionized in the outer parts of the Sun.

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 Internal structure The Sun has different layers. o The Core  The Sun’s energy is produced within the core, the central part that is 25% of the Sun’s radius.  The temperature and pressure of the core are extremely large. The temperature at the center of the core is 15.6 million K and falls to 8 million K at the outer part of the core. Virtually no energy is produced beyond this region  The energy in the core is produced by the fusion of hydrogen into helium. The fusion occurs through a series of nuclear reactions known as proton-proton chain.  When four hydrogen nuclei (protons) combine to make one helium nucleus, a tremendous amount of energy is released:  MH converted per second = 600 billion kg  MHe produced per second = 596 billion kg  Difference = 4 billion kg  Energy equivalent = mc2 = 4  109  9  1016 = 3.6  1026J/s. Assignment: if the central 10% of the Sun’s mass is capable of nuclear reactions, for how long will the Sun continue to radiate? o The Radiative Zone  The part of the Sun next to the core is known as the Radiative Zone, whose outer edge is at about 70% of the Sun’s radius.  In this region energy is transported by radiation in which  photons are emitted at one spot and absorbed at another within an average distance of 1m. The numerous absorptions and random emissions slow down the flow of energy towards the surface of the sun.  At about 70% of the Sun’s radius temperature drops to 1.5 million K. o The Convection Zone  Next to the radiative zone is the Convection Zone. Here the solar energy is transported by convection, that is, the overturning motions of the gas carry nearly all of the energy outward.

38

 The Sun’s convection zone extends to the surface of the Sun where the Sun’s energy escapes into space.  It takes about 170,000 years for the energy produced at the Sun’s center to reach the surface.  The Outer Layers of the Sun o The Photosphere  The Photosphere is the layer next to the convection zone that we see in the visible range of wavelengths.  It is a very thin layer of gas where most of the radiant energy is converted into visible radiation and escapes.  In the photosphere temperature drops from 6500K to 4000K.  Since the photosphere is only about 0.07% of the radius of the Sun, some take it as the surface of the Sun. o The Solar Atmosphere The atmosphere of the Sun consists of two parts: the chromosphere and the corona both of which are clearly visible during total solar eclipse.  The Chromosphere The chromosphere appears as a reddish ring during total solar eclipse; it is rich in hydrogen with a temperature range of 4500K to 50,000K.  The Corona It is a very hot rarified gas that makes the outer part of the Sun’s atmosphere with no outer boundary; it emits x-rays and its temperature shoots up to 1 million Kelvin. Like the chromosphere, we can see the corona during total solar eclipse.  Surface Features of the Sun o Prominences  These are flames of gas that shoot outward from the chromosphere and fall back due to the Sun’s strong gravity  They result from the disturbances in the strong magnetic field of the Sun and may last for months. o Sunspots  Sunspots are regions of intense magnetic fields where the magnetic field lines break through the surface.

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 Sunspots appear relatively darker because they are cooler than the surrounding gas. They move across the Sun’s disk. After formation, they may last for a few days or months and then disappear.  It was observed that the number spots rises and falls in a cycle of 11 years. o Solar flares  These are brief, bright eruptions of hot gas in the chromosphere due to sudden releases of energy stored in the Sun’s magnetic field.  The sudden eruption may result in a gas of particles escaping the Sun’s gravity and rushing across the solar system. o The solar wind is ionized gas, mainly hydrogen and helium, moving outward from the Sun into interplanetary space. It arises because the corona’s high temperature gives its atoms enough energy to escape the Sun’s gravity. The solar wind creates comet tails and auroras.

4.2 Planets  The planets orbit about the Sun in elliptical orbits and are much smaller than it. They do not emit light of their own but shine by reflecting sunlight. The nine planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto in order of their distance from the Sun.  All the planets move around the Sun in nearly the same plane and they all travel in the same direction: counterclockwise as seen from above the Earth’s North Pole. As a planet orbits the Sun, it also spins on its rotation axis generally in the same sense as its orbital motion, with the exception of Venus.  Spectral analysis of light reflected from the planets is used to understand the composition of their atmospheres and surface rocks.  The internal planetary composition is studied by indirect methods such as density measurements.  The planets fall into two families called inner and outer planets based on their size, composition and location in the Solar System.  The inner planets o The inner planets are Mercury, Venus, Earth, and Mars. o They are small rocky bodies with relatively thin or no atmospheres. o The rocky planets are mostly composed of Silicon and Oxygen (SiO2) and other heavy elements such as aluminum (Al), magnesium (Mg), sulfur (S) and iron (Fe). 40

o Astronomers sometimes use the word terrestrial to refer to the inner planets. The terrestrial planets are so named because of their resemblance to the Earth, that is, they have solid surfaces like the Earth. o Mercury (Named for the Roman deity who was the messenger of the gods) Mercury is the smallest terrestrial planet, closest to the Sun. It resembles our moon in both size and appearance and is always viewed in the morning and evening twilight. Mercury was visited by Mariner 10 in 1974 and 75.  Size  R = 2439 km = 38% of Earth’s radius  M = 3.33 × 10 23 kg = 5.5% of Earth’s mass  Mean density = 5.40 g/cm3  Atmosphere Has essentially no atmosphere because of its small gravitational attraction (small size) and high temperature (proximity to Sun).  Temperature At the equator, noon temperatures reach about 700 K. Nighttime temperatures drop to 100 K.  Surface features  Like our Moon, Mercury’s surface is dominated by impact craters, large plains, and scarps.  Large-scale movements or surface distortions are not observed. (Dead planet?)  Interior  Mercury’s high density cannot be attributed to gravitational compression because of mercury’s small mass. It rather indicates an iron-rich core and a thin silicate mantle  The small size of mercury may have allowed heat to escape readily resulting in no tectonic activities.  Mercury’s magnetic field is an indication of a liquid metallic core, though there is no seismic information,  Rotation Mercury’s orbit is more elliptical and its spin is very slow. Rot. Period = 58.646 Earth days, Orbital period = 87.969 Earth days Rotation period = ⅔ of Orbital period Long solar day of 176 Earth days Think: what would happen if Mercury’s rotational and orbital

41

period were the same? o Venus (Named for the Roman goddess of love) Venus is the brightest object in the sky, visible above the western horizon in the evening and above the eastern horizon in the morning. Venus is most like the Earth in radius, mass and density. Despite this similarity, however, the two planets are fundamentally different in many ways.  Size R = 6051 km = 95% of Earth’s radius M = 4.87 × 10 24 kg = 82% of Earth’s mass Mean density = 5.24 g/cm3  Atmosphere  The Venusian atmosphere is composed of CO2 (92%), N2 (3.5%) and small amounts of water vapor and other elements.  The atmosphere is so dense that it exerts a pressure roughly 100 times the atmospheric pressure here on Earth.  Clouds are composed of concentrated sulfuric acid droplets  Temperature  The surface of Venus is so hot ( 750 K) because its carbon dioxide atmosphere creates a very strong greenhouse effect.  The upper atmosphere is very cool ( 300 K, room temperature here on Earth)  Surface feature (examined by Soviet landers, the Venera series, and the orbiter Magellan in 1990)  Venus’s surface shows evidence of volcanic activity and crustal distortion. But there is no evidence of plate tectonics.  There are many big impact craters on Venus but no oceans.  It is less mountainous with only two major highlands (Ishtar and Aphrodite). Most of the surface is congealed lava plains.  Interior  Venus’s size similarity to Earth implies similar internal structure, that is, Venus probably has a liquid iron core and rock mantle.  No seismic information, based on deductions from gravity and density  Very weak magnetic field due to extremely slow rotation  Rotation  Rotation period = 243 Earth days, longest in the solar system.

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 Orbital period = 224.7 Earth days, solar day = 117 Earth days  Axial tilt = 177.4  Doppler shifts of reflected radar waves show that Venus has a slow, retrograde rotation. Some astronomers thought that Venus might have been struck by a huge planetesimal. Think: What can you say about Venusian sunset and sunrise? o Mars (Named for the Roman god of war) The idea that Mars is a habitable planet was gradually abandoned as new observations with better telescopes continue to progress.  Size R = 3397 Km  ½ the radius of the Earth M  10% of the mass of the Earth Mean density = 3.9 g/cm3  Atmosphere  Spectra reveal that the atmosphere of Mars is mostly CO2 (95%) with small amounts of Nitrogen (3%) and traces of oxygen and water.  Martian atmosphere is clear enough to see its surface from Earth.  Drifting clouds of dry ice (frozen CO2) and water-ice crystals (H2O)  No rain despite clouds (little water, cold atmosphere)  Dust storms keep dust suspended in Mars's atmosphere at all times  Temperature  The density of the atmosphere is so low ( 1% of the Earth’s) that the carbon dioxide creates only a very weak greenhouse effect.  Little greenhouse effect and Mars’s great distance from the Sun make the planet very cold. Equatorial noon temperature  273 K (0C)  The average of daytime and nighttime temperatures is about 218 K.  Surface features  Mars has many volcanoes and craters. Ejected materials flowed away from many craters, an indication that there may be subsurface water.  Martian faults and rifts indicate substantial crustal motion in the past.  Mars has permanent polar caps. The northern cap is mainly water ice while the southern cap contains largely dry ice (frozen CO2). 43

 Orbiter images revealed numerous channels and dry riverbeds apparently formed by running water. This indicted that considerable amounts of water once existed on Mars.  Immense deserts with dunes blown by Martian winds.  Interior  The Martian interior is differentiated like the Earth’s into crust, mantle and iron core.  The crust of Mars is rich in iron, giving Mars its reddish color.  Its interior is probably cooler due to its small size (mass and radius)  Therefore, low level of tectonic activity and weak magnetic field  Rotation  Solar day = 24.66 hrs --- Compare to Earth’s solar day  Orbital period = 687 Earth days  Axial tilt = 24 --- Compare to Earth’s axial tilt Think: Are there any seasons on Mars?  Martian Moons  Mars has two tiny moons (about 10 km across) which might be captured asteroids  Is there life on Mars?  Biological experiments on the Viking landers produced no conclusive evidence for biological activities so far. o Conclusion Although the terrestrial planets are alike in being rocky, they differ from one another because of differences in their masses, radii and distance from the Sun. The terrestrial planets are heated by impacts and radioactive decays until much of the iron sank to form a core, and light rocky material floated to the surface to become the crust. Size determines which planet has cooled at a faster rate and become volcanically inactive. The smaller planets quickly became inactive while the larger remain tectonically and volcanically active. o The outer planets  The outer planets are Jupiter, Saturn, Uranus, Neptune, and Pluto.  They are gaseous, liquid or icy. Except for Pluto, they are much larger than the inner planets and have deep, hydrogen-rich atmospheres.

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 The icy planets are frozen liquids and gases such as ordinary water ice (H2O), frozen carbon dioxide (CO2), frozen ammonia (NH3), frozen methane (CH4) and so on.  The outer planets, except Pluto, are sometimes called Jovian planets because of their resemblance to Jupiter. They do not have solid surfaces.  Pluto, by far the smallest planet in the solar system, is icy and rocky. o Jupiter (King of gods as it was called by the ancient Romans) Jupiter is the largest planet in the solar system and more massive than all the planets put together. Jupiter and the other giant planets were examined by the spacecrafts Pioneers 10 and 11, Voyagers 1 and 2, and Galileo starting in 1973.  Size  R = 71492 Km  11  radius of Earth.  M = 1.90  1027 kg  318  mass of Earth.  Mean density = 1.33 g/cm3 (despite its huge mass and tremendous internal pressure!)  Low density suggests large amounts of lighter gases  Atmosphere  Spectral analysis shows that hydrogen and helium make up almost 99.9% the Jovian atmosphere.  There are also hydrogen-rich compounds such as methane (CH4), ammonia (NH3), and water (H2O).  The mass ratio of hydrogen to oxygen is 4:1, the same ratio as in the Sun. this indicates that Jupiter retains almost all of the gas in its vicinity during formation.  Swiftly moving, dense, parallel clouds cover the planet. Jupiter’s clouds consist mainly of water, ammonia, ammonium hydrosulfide, all of which are colorless. The clouds are colored by complex organic molecules yet unidentified.  Jupiter shows many cloud features. One of these, the Great Red Spot, is an atmospheric storm that has persisted for centuries.  The gas rising and falling due to convection in the upper layers is subject to Coriolis effect due to the rapid rotation of Jupiter, and as a result the gas deflects into powerful winds called jet streams seen from here as cloud belts.  Interior  Invisible through the layers of clouds; impossible to probe with

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seismic detectors. (Think: why?)  The low average density of Jupiter implies that its interior consist mainly the lighter elements: hydrogen and helium. Moreover, since in the deep interior pressure and temperature are very high, hydrogen becomes denser and liquefies. There we have metallic hydrogen.  Metallic hydrogen has abundant free electrons. Vigorous convection in the interior of Jupiter together with its rapid rotation generate the planets large magnetic fields.  The average density of Jupiter is too large to say the planet is entirely made of hydrogen and helium. Consequently, Jupiter’s core must be rocky and metallic.  Jupiter’s interior is extremely hot (30,000 K). The heat is probably generated by continued gravitational contraction. The heat rises to the surface and escapes into space as IR radiation. That is, Jupiter is self-luminous.  Jupiter looses more energy than it receives from the Sun. 60% of the energy it emits is due to internal energy sources.  Rotation  Orbital period = 11.86 yrs, Rotation period = 9.9 hours  Fast rotation results in bulged equator.  Rings  Jupiter has very thin rings made of numerous particles following individual orbits.  Moons  So far 16 Jovian moons are identified.  Four of the moons – Io, Europa, Ganymede, and Callisto – are very large. They are called Galilean satellites.  The large average densities of the Galilean satellites suggest that their interiors are composed of mainly rocky materials. o Saturn (Named for an ancient Roman harvest god, later identified by the ancient Greeks as the father of the gods) 

Size  The second largest planet, surrounded by magnificent rings.  R = 57316 Km  9  radius of Earth.  M = 5.69  1026 kg  95  mass of Earth.  Mean density = 0.69 g/cm3  Low density implies that, like Jupiter, Saturn’s chemical makeup is dominated by hydrogen and helium, the lighter gases.

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 Atmosphere  The atmosphere Saturn, like Jupiter, is made primarily of hydrogen and helium.  On Saturn, however, the ratio of hydrogen to helium is about 7 to 1, indicating that helium is less abundant in Saturn’s atmosphere than in Jupiter’s atmosphere.  Saturn also has clouds with distinctive circulation patterns. It has layers of ammonia, ammonium sulfide and water clouds are colored by unknown molecules.  Interior  In the deep interior of Saturn, like Jupiter, hydrogen gradually transformed from a gas to a liquid becoming an electrically conducting metallic liquid.  The metallic hydrogen, high convection in Saturn’s interior and its rapid spin together generate the planet’s strong magnetic field.  The core of Saturn contains large amounts of rock, metal and ice. Energy is generated deep in the atmosphere probably by the condensation of helium droplets  Radiates more energy than it gains from the Sun. 50% of the energy emitted by Saturn comes from internal energy sources.  Rotation Orbital period = 29.46 yrs, Rotational period = 10.7 hours  Rings  Saturn has very thin but very wide rings. In addition, it has faint inner and outer rings  The rings are a swarm of individual particles only a few centimeters to a few meters across.  The ring particles are primarily composed of water and ice.  Moons  Saturn has several large moons and a dozen of smaller moons. Titan is the largest Saturnian moon.  Astronomers believe that Titan’s surface is covered with oceans of liquid nitrogen or hydrocarbon ethane (C2H6), or both and that ethane rain may fall from its clouds. o Uranus (Known as the God of the Heavens in ancient Greek mythology) Uranus was discovered in March 1781 by William Herschel who was a professional musician and later became an astronomer.

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 Size  R = 4  radius of Earth, M  15  mass of Earth  Average density = 1.2 g/cm3, larger than Jupiter’s and Saturn’s  Atmosphere  Rich in hydrogen and methane; methane gives the planet its deep blue color. Crystals of frozen methane form Uranus’s atmosphere which absorb red light and scatter blue light from the Sun.  Uranus has almost no cloud features because of lack of atmospheric convention. Heat is not flowing outward from the interior of Uranus.  Interior  Astronomers believe that Uranus has a small rocky core covered by a thick layer of water and molten rock. The outer layer is mostly hydrogen and helium.  Uranus radiates essentially no internal heat.  Rotation  Rotation period (near the equator) = 17 hours, bulging the planet’s equator.  Orbital period = 84 years  axial tilt = 98 with respect to its orbital plane  Rings  Uranus has a set of narrow rings composed of a myriad of small particles. The rings are very dark implying that they may be rich in carbon particles  Moons  Uranus has 17 known satellites, 5 are large moons. The smaller ones are composed probably of ice and rock.  Many of them are heavily cratered like our moon except Miranda. o Neptune (Named for the Roman god of the sea) Neptune is the outermost of the Jovian planets. It was discovered Adams, a mathematics student at Cambridge, and Leverrier, a young French astronomer, by applying celestial mechanics.  Size  R  3.9  radius of Earth, M  17  mass of Earth  Average density = 1.67 g/cm3  Interior  Similar to Uranus’s structure.

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 Like Jupiter and Saturn, Neptune radiates more than it gains from the Sun. 60% of the energy it emits comes from internal sources.  Rotation  Rotational period = 16 hours  Orbital period  165 years  axial tilt = 30  Atmosphere  Neptune’s blue color is caused by methane in its atmosphere  It has cloud belts like Jupiter. The cloud belts are formed by gas rising to the surface and deflected into wind currents by the Coriolis effect  Rings  Neptune’s rigs are composed of debris from small satellites  Compared to the rings of either Saturn or Uranus, Neptune’s rings contain more dust. Moreover, the ring particles are not uniformly distributed but form an arc in some places  Moons  Neptune has 8 known satellites. Two of them, Triton and Nereid, are large. Triton is the second moon in the solar system known to have an atmosphere o Pluto (Named for the Greek and Roman god of the underworld) Pluto is the last of the nine planets. It was discovered by comparing images of objects the sky, which were taken a week apart, and by carefully examining which object has changed position.  Size  R  0.2  radius of Earth, M  0.002  mass of Earth  Average density = 1.8 g/cm3, a value that implies Pluto is a mix of water, ice and rock.  Rotation  Orbital period =248.5 years  Rotational period = 6.4 Earth days  Axial tilt = 122  Surface and atmosphere  Pluto is covered mainly by frozen nitrogen with small proportions of frozen carbon monoxide and methane.  Moons Pluto has one moon, Charon. 49

o Conclusion The outer planets, except Pluto, are formed far from the Sun in a region where the temperature is low enough for the planets to possess hydrogenrich material. Hydrogen-rich gases form a deep, colored atmosphere whose density increases with depth and eventually becomes a liquid. The giant planets are probably heated by continued gravitational contraction or settling of heavier matter toward their cores. This heat flows outward, generating convection of matter inside the planets. The spin of the planes creates Coriolis effect on the rising gas, drawing it into cloud belts. The spinning motion also creates equatorial bulge and when combined with convection in the planets’ metallic cores it generates powerful magnetic fields. All giant planets have satellites (made mostly of water, ice and rock) and rings composed of small orbiting particles.

4.3 Satellites, Meteors, Asteroids and Comets  Satellites o As the planets orbit the Sun, most are themselves orbited by satellites. o Jupiter, Saturn, and Uranus have 16, 22, and 15 moons, respectively. Neptune has 8, Mars has 2, and Earth and Pluto each only 1. Mercury and Venus are moonless planets.  Meteors, Meteoroids, Meteorites, and Meteor showers o The “shooting star” we see in the night sky is not really a star. Astronomers call it a meteor. It is a solid body heated to incandescence by its passage through the Earth’s atmosphere. o The solid body, while in space before entering the Earth’s atmosphere, is called a meteoroid. Meteoroids enter the Earth’s atmosphere at speeds that range from 11 km/s to 72 km/s. The friction between the atmosphere and the meteoroid increases the surface temperature of the meteoroid to thousands of Kelvin within seconds. The heat generated in this way vaporizes the meteoroid and it becomes a meteor. What we see as a white tail is light emitted by a trail of hot evaporated matter and atmospheric gas. o Astronomers estimate that hundreds of tons of meteors bombard the Earth each day. Small meteors completely vaporize in the atmosphere, but larger ones survive to reach the ground. Meteor fragments found on Earth are known as meteorites. o Meteorites are classified into three groups based on their composition: iron, stony and stony/iron.

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 90% of the meteorites that fall to the Earth are stony meteorites and most of these are chondrites, named for the chondrules (spheres of silicate rock) they contain.  One important type of stony meteorite is the carbonaceous chondrites, which contain water and high content of carbon in the form of organic compounds (about 20 are amino acids).  Iron meteorites make up about 5% of the meteorites that fall to the Earth. They are nearly pure alloys of iron and nickel.  Stony-iron meteorites make up the only 1% of the meteorites. They are a mixture of metal and silicate rock. o Analysis of radioactive elements in meteorites shows that most of them solidified about 4.6 billion years ago. o About ten times a year, many meteors could be observed in one hour. This increased rate at which meteors are observed is known as a meteor shower. Meteor showers happen when the Earth crosses a swarm of meteoroids.  Asteroids o Asteroids (also called minor planets) are much smaller objects that orbit the Sun within the planetary System. They are rocky or metallic bodies with diameters that range from a few meters up to about 1000 km. The largest known asteroid is Ceres, nearly 1000 km in diameter. o Most asteroids circle the Sun in the large gap between the orbits of Mars and Jupiter, a region called the asteroid belt. They are probably material failed to aggregate into a planet. o In addition of asteroids in the asteroid belt, there are many others that share Jupiter’s orbit, leading or trailing Jupiter by 60. These are known as the Trojan asteroids. o Three main compositions:  Carbonaceous bodies  Silicate bodies  Metallic iron nickel o Inner-belt asteroids are rich in silicates whereas outer belt asteroids are rich in carbon.

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 Comets o Comets are among the objects that orbit the Sun. They are small icy bodies about 10 km or less in diameter. When a comet passes near the Sun it becomes brighter and grows a huge tail stretching across the solar system. o Most comets orbit far beyond Pluto in a region of the Solar System called the Oort cloud, and only rarely do they move into the inner Solar System. Some comets come from a disk-like swarm of icy objects that may lie just beyond the orbit of Neptune, a region called the Kuiper belt. The Oort cloud and the Kuiper belt together contain probably more than 1 trillion (1012) comet nuclei. o Comets consist of three parts: nucleus, coma and tails.  The nucleus of a comet is an irregularly shaped chunk of mostly water ice and dust that has frozen in the extreme cold of interplanetary space. When a comet gets closer to the Sun, water and other molecules within the nucleus evaporate and flow outward, carrying the dust with them. The gas and dust that escape from the nucleus form the coma and tails.  The coma of a comet is a ball of outflowing gas and dust that surrounds the nucleus. The coma can be a million km across, much bigger than the nucleus that is only about 1 – 10 km in diameter. The coma appears bright because of a combination of emission from the gas and sunlight reflected by the dust.  Many comets have two tails. The white or yellow tail is made of dust swept from the nucleus, so it is called the dust tail. The blue tail is called the plasma tail because it is made of ions and electrons. The blue color is due to ionized carbon monoxide. The plasma tail is swept out by the solar wind away from the Sun to about 100 million km across the inner solar system. o Composition  Spectra of gas in the coma and tail show that comets are rich in water, CO2, CO, and small amounts of other gases. The water is broken up by the Sun’s UV radiation to create oxygen and hydrogen, and most comets are surrounded by a vast cloud of hydrogen created in this way. o Comets are divided into two groups based on their orbital periods. Shortperiod comets have periods less than 200 years and they are thought to come from the Kuiper belt. Long-period comets originate at the Oort cloud when passing stars disturb their orbits. They have periods greater than 200 years.

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4.4 Origin of the solar System  Clues from orbital properties: o The Solar System is flat, i.e., the planetary and satellite orbits lie almost in the Sun’s equatorial plane. o Almost prograde orbital and spin motions, i.e., all the planets orbit the Sun in the same direction and most planets rotate in the same direction that they revolve about the Sun. o Nearly circular orbits, i.e., planetary and satellite orbits are almost circular. o Axial tilts are nearly the same, i.e., spin axes of most planets and satellites are nearly perpendicular to the ecliptic.  Clues from the physical properties of the planets: o There are two types of planets: inner and outer. The inner (terrestrial) planets are near the Sun, small in size and rocky while the outer (Jovian) planets are further out from the Sun, much larger and gaseous. o Differences in composition: The giant planets consist primarily of the volatile elements, i.e., abundant lightweight gases like the Sun. The terrestrial planets, on the other hand, have low abundances of the volatile elements. o Satellite numbers and rings: Giant planets have more satellites than the terrestrial planets. Giant planets have ring systems but terrestrial planets do not. o Orbital speeds: giant planets rotate more rapidly than do terrestrial planets.  Other clues: o The Sun has more than 99% of the mass of the solar system, but less than 1% of the angular momentum. o All the bodies in the Solar System whose ages have so far been determined are less than about 4.5 billion years old. o The structure of asteroids o The number of craters on planetary and satellite surfaces o Chemical composition of surface rocks and atmospheres The best theory of the origin of the solar system must explain all observations of astronomers. The currently favored theory was independently proposed by Immanuel Kant and Pierre Simon Laplace and is now called the solar nebula hypothesis.

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 The solar nebula hypothesis o Kant and Laplace proposed that the Solar System originated from a rotating, flattened disk of gas and dust, with the outer part of the disk becoming the planets and the center becoming the Sun. o This theory offers a natural explanation for the flattened shape of he system and the common direction of motion of the planets around the Sun. o According to the modern form of this theory, the solar system was born 4.5 billion yeas ago from an interstellar cloud. Which is an enormous, rotating aggregate of gas and dust.  Interstellar clouds o Our galaxy has a large quantity of interstellar materials that contain both gas and small dust grains. These interstellar materials appearing here and there with different shapes and sizes are called interstellar clouds. o The inter stellar cloud that became our Sun and planets was probably a few light years in diameter and contained about twice the present mass of the Sun. it was initially very extended and rotating slowly. o It was made mostly of hydrogen (71%) and helium (27%) with tiny traces of elements such as carbon, oxygen and silicon. o Somehow the cloud began to contract on its way its way to becoming a star. This transformation into the Sun and planets was due to gravitational forces causing the cloud to collapse inward. o Regardless of the cause of the collapse, the shrinking cloud spun faster and flattened into a disk with a central bulge.  Formation of the Solar Nebula o The core of the cloud began to heat up from the energy of impact as the material fell in. While shrinking, it began to glow, first at infrared wavelengths and finally at visible wavelengths. Eventually, the temperature and pressure of the core became high enough to trigger nuclear reactions, and the Sun began its long lifetime as a star. o As the central part of the interstellar cloud collapsed into the Sun, the outer portions were forced into a rotating flat disk within a few million years. This disk, surrounding the Sun, is called the solar nebula. o As the solar nebula cooled, substances with the highest vaporization temperatures such as iron and silicate condensed first everywhere within the disk. But the temperature of the disk between the Sun and Jupiter never dropped low enough that water and the volatile elements could not condense within that part of the nebula. Thus the nebula became divided into two parts: an inner zone of silicate/iron particles and an outer zone of 54

similar particles on which ices also condensed. Water, hydrogen and other easily vaporized substances were present as gases in the inner solar nebula, but they could not form solid particles there.  Accretion and Planetesimals o In due course, the tiny particles that condensed from the nebula began to stick together into bigger pieces in a process called accretion. o The larger objects formed in this way are called planetesimals, that is, small planet-like bodies. Large planetesimals were sufficiently massive that their gravity attracted additional material making them larger yet. o Since the planetesimals near the Sun formed from silicate and iron particles and those farther out had in addition ice coatings, there were two types of planetesimals: rocky/iron ones and icy/rocky/iron ones.  The Formation of the Planets o As planetesimals move within the disk and collide with one another, planets formed. In most cases, the planet that was formed in this way rotated in the same direction as the over all rotation of the disk. o When a planet’s mass increased due to the merging of planetesimals so did its gravitational attraction. Violent impacts of planetesimals as they fell onto a growing planet generate heat by releasing gravitational energy. This heat together with radioactive heating in the planet’s interior melts the planet and allows it to differentiate. Thus all the inner planets (probably the outer planets too) ended up with iron cores and rock mantles. o Planet growth is especially rapid in the outer parts of the solar nebula. In this region ice is 10 times more abundant than silicate and iron compounds and thus the planetesimals in the outer solar nebula could become 10 times larger than those in the inner solar nebula. o Once a planet became large enough, it was able to attract and retain gas by its gravity. Larger and cooler planets were able to hold back the most abundant element in the solar nebula – hydrogen. The smaller and warmer planets could not capture hydrogen, and it was eventually swept away. Think: Which theory are you in favor of, Accumulation  Differentiation OR Differentiation  Accumulation?)  Formation of Moons and rings o The most recent theory of the formation of the giant planets suggests that their cores formed from the coalescence of planetesimals, just as the terrestrial planets formed.

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o However, in the outer solar system where the temperatures were very low, the solid objects that formed from the coalescence of planetesimals were able to gravitationally trap gas from large volumes in their vicinities, and these planets therefore built up extended gaseous atmospheres including hydrogen and helium. o These planets in accreting gas from their surroundings, formed disks much like the solar nebula itself. The moons of the outer planets probably were formed from planetesimals orbiting the growing planets much the same way as planets are formed around the Sun. All four giant planets have flattened satellite systems in which the satellites (with few exceptions) orbit in the same direction. o Ring systems formed wherever the disks extended closer to the parent planet. o The rapid spins of the giant planets are explained by the fact that angular momentum conservation caused them to rotate faster as the disks contract.  Final Stages o Although planet building consumed most of the planetesimals, some survived to form small moons, asteroids and comets. Impacts with the remaining planetesimals leave craters on the surfaces of planets and satellites. o Rocky planetesimals and their fragments remained between Mars and Jupiter and being stirred by Jupiter they were unable to assemble into a planet. We see them today as the asteroid belt. o Jupiter’s gravity also disturbed the orbits of the icy planetesimals, tossing some in toward the Sun and others outward. These tossed planetesimals form the swarm of comet nuclei we call the Oort cloud and Kuiper belt.  Formation of the Atmosphere o The atmospheres formed at the end of the planet-forming process. The outer planets probably captured most of their atmospheres from the solar nebula. The nebula was rich in hydrogen, so are the atmospheres of the giant planets. o The inner planets were not massive enough and were too hot to capture gas from the solar nebula. Venus, Earth and Mars probably created most of their original atmospheres by volcanic eruptions. Some of their gases may have come from evaporation of icy planetesimals on impact.  Cleaning Up the Solar system o In the final stage of the formation of the solar system, a solar wind swept the remnant gas and dust to the outer fringes of the solar system.

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Chapter 5.

Stars

 A star is any massive, celestial body of gas that shines by radiant energy generated inside it. Of the myriad of stars, only a very small fraction is visible to the unaided eye.  Most young stars are composed mainly of hydrogen. At the core of a star a small fraction of this hydrogen is converted to energy by nuclear fusion reactions. The energy generated inside the core makes its way out to the outer surface called the photosphere and from there it escapes into space.  In most stars, energy is transported from the inner to the outer surfaces by means of two mechanisms: radiative transport and convective transport.  Gases at the outer layers of a star are partially ionized. The interior, however, is at extremely high temperature and pressure, and fully ionized. The extreme pressure inside balances gravitational forces and the star will be maintained in hydrostatic equilibrium for a long time.  Single stars such as the sun are the minority; most stars occur in pairs (binary stars), multiple systems, or clusters.  Stars vary greatly in brightness, color, temperature, mass, size, chemical composition, and age. In nearly all, hydrogen is the most abundant element.  Properties of Stars o Measuring distance  To measure distances of nearby stars astronomers use a triangulation method known as parallax. Parallax is the apparent change in position of an object due to the observer’s motion.  Stellar parallax is defined as half of the angle by which a star’s position appears to shift. With definition, a star’s distance d becomes the reciprocal of its parallax p:

d pc 

1

parc seconds

,

where d is measured in parsec and p is measured in arc seconds. Think: Can you drive this relation? [1 parsec = 3.09  1013km]  For more distant stars, the standard candle method is used. This technique is based on the brightness of stars.

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o Temperature  The temperature of a star could be determined from the color of the light it emits. At low temperatures objects glow red and as the temperature increases they tend to glow blue.  If we look at the night sky with our naked eye, we learn that some stars have bluish color and some are reddish. This tells us that stars differ in color.  According to Wien’s law, the temperature of a body is inversely related to the wave length of the strongest radiation it emits, i.e.; T

3 10 6 , 

where T is in Kelvin and  in nanometer (nm).  The red star Betelgeuse, for instance, radiates most strongly at about 1000 nanometers. Its temperature, using the formula above, is therefore 3000K. o Luminosity and The Inverse Square Law  Luminosity (L) is the amount of energy radiated every second by a body.  The Sun has a luminosity of about 4  1026 watts, generated by the nuclear fusion of hydrogen into helium.  Knowledge of a star’s luminosity is important in determining the star’s radius, distance and lifetime.  The inverse-square law is used to determine a star’s luminosity (L) if its distance (d) and apparent brightness (B) are known: B

L 4 d 2

This law shows that brightness decrease inversely with the square of distance.  The brightness of a star is measured in magnitudes, a unit first used by Hipparchus in ancient times. He assigned magnitude 1 for the brightest stars and magnitude 6 for the dimmest ones, i.e., the brighter the star, the lower the number assigned as a magnitude.  In the present system, a difference of magnitude is used. Magnitude difference corresponds to brightness ratio. For example, the magnitude difference of a first-magnitude star and a sixth-magnitude star is 6 – 1 = 5, and the ratio of brightness of the former to the latter 58

is 100. Thus, a difference of five magnitudes corresponds to a brightness ratio of 100 to 1. Each magnitude difference corresponds to 5 100  2.512 . Think: The apparent magnitudes of Venus and Aldebaran are, respectively, -4.2 and 0.8.How brighter appears Venus to our eye than Aldebaran?  There are two types of magnitudes: apparent and absolute. The apparent magnitude of a star is its brightness as seen from Earth. The absolute magnitude is a star's brightness as seen at a standard distance of 10 parsecs. o Radius  If two stars have the same temperature, the larger one emits more energy and so has a larger luminosity than the smaller. A star’s luminosity is related to its radius R and temperature T by the StefanBoltzmann law: L  4R 2 σT 4

where  is constant value of 5.67  10 – 8 watts/m2K4 known as the Stefan-Boltzmann constant. This law is used to determine the star’s radius if we know its luminosity and temperature.  Astronomers call stars much larger than our Sun giants. Much smaller stars are called dwarfs. o Composition  The composition of a star is determined by comparing the absorption lines in its spectrum with the lines made by each atom. Spectral analysis show, after corrections of temperature effects, that virtually all stars including our Sun are composed mainly of hydrogen (about 71%) and helium (about 27%).  Stars are classified by their spectra, from blue-white to red, as O, B, A, F, G, K, or M; the sun is a G-type star. O stars are hot (more than 25,000K) and M stars are cold (less than 3500K). Hot class (O and B) stars are blue in color while cold class (K and M) stars are red. o Mass  Two orbiting around a common center of gravity are known as binary stars.

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 The orbital periods and semimajor axis of binary stars and additional observations of their orbital motions are used in the modified form of Kepler’s third law to determine the masses of the binary stars.  The H-R diagram o H-R diagram, worked out independently by Hertzsprung and Russell, is a graph in which the luminosities of stars are plotted against their temperature. On the diagram, stars are ranked from bottom to top in order of increasing brightness and from right to left by increasing temperature. o Stars tend to cluster in certain parts of the diagram. Most stars (about 90%) fall along a diagonal line called the main sequence, with hot luminous stars at the upper left and cool dim stars at the lower right. The main sequence is the locus of hydrogen-burning stars of different masses. The Sun lies almost at the middle of the main sequence. o Stars located at the upper right of the H-R diagram are cool and bright. These stars have the same temperature as those on the lower main sequence but they are thousands of times brighter. According to the StefanBoltzmann law such stars must be very huge. Moreover, since they are cool t he glow red. Astronomers call such huge cool stars red giants. o The stars lying below the main sequence are dimmer but so hot that they glow with a white heat. They are small, Earth-size stars and are known as white dwarfs.  Stellar Evolution o Gravitational collapse of interstellar cloud A star begins its life when a portion of a dense interstellar cloud of hydrogen and dust grains collapses inward from its own gravity. Initially, the cloud core is cool because heat generated by gravitational contraction escapes in the form of IR radiation. o Slow contraction of core A protostar develops at the center of a collapsing core. The protostar is originally transparent, but eventually becomes opaque to IR radiation as the collapsing cloud becomes more and more denser. The heat (IR radiation) trapped in this way causes the internal temperature to increase and thereby building up internal pressure. The pressure slows down the contraction. o Material infall The protostar stops collapsing and begins to grow in mass by accumulating infalling material. The infall creates heating and shock waves and hydrogen

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molecules breakup by absorbing the heat and causing a reduction in pressure and hence a second collapse. o Final contraction After the second collapse, the protostar begins a period of slow contraction due to a rise in the internal pressure. The protostar is still surrounded by gas and dust material. o Nuclear ignition That slow contraction ends when the star becomes hot enough for hydrogen fusion to occur. Enough energy is released during the nuclear reaction to provide pressure that balances gravity and the protostar stops collapsing o Shading the surrounding gas and dust A vigorous surface activity produces a wind that eventually blows away infalling gas and dust. The protostar then becomes a young visible star just entering the main sequence of the H-R diagram. The new star will be in main sequence for a long time and in hydrostatic equilibrium by “burning” hydrogen in a nuclear fusion process.  Stars Like the Sun o Stars as massive as the Sun remain in main sequence until the hydrogen fuel in the core is almost exhausted. After hydrogen is exhausted, the core shrinks and heats up while the star's outer layers expand significantly and cool. The cool, swollen star then becomes a red giant. o A red giant has a dense helium core surrounded by a burning shell of hydrogen. Rising temperature in the stellar core eventually initiates the fusion of helium. The fusion of helium is a triple-alpha reaction in which helium is converted into carbon. In stars like the Sun, the reaction happens as a rapid burst of nuclear reactions throughout the core and is called the helium flash. o

After the helium is used up, nuclear reactions stop and the core collapses while the expanded outer layers remain far behind. The remnant will be a compact, hot core called a white dwarf. A white dwarf is taken as the last stage in the life cycle of Sun-like stars.

 Massive Stars o Like the Sun, massive stars (6 – 10 solar masses), spend most of their lifetime on the main sequence while hydrogen fusion in the stellar core produces the energy necessary to balance gravity and supply the luminosity of the star.

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o The main differences with the evolution of a low-mass star are:  Rapid evolutionary steps  Several red-giant phases due to successive nuclear reaction stages that produce ever-heavier elements in the stellar core. o The sequence of nuclear reactions are: Hydrogen burning  Helium burning  Carbon burning  Neon burning  Oxygen burning  Silicon burning  Iron core o The sequence of nuclear reactions stops when finally iron is produced and the massive star runs out of fuel and collapses. The collapse merges protons and electrons into neutrons. The outer layers fall onto the hard neutron core and rebound producing a huge explosion called supernova. o The supernova is taken as the death of a massive star whose remnant becomes a neutron star. o Rotating neutron stars emit pulses of electromagnetic radiation and, therefore, they are called Pulsars. We often observe 1000 pulses/second. o If the core becomes too massive infalling matter does not rebound from the neutron star. Instead, the core becomes more and more denser with everincreasing gravitational field. The result is the transformation of the neutron core into a black hole.  Galaxies o Galaxies are classified using the Hubble system: 

Smooth, spheroidal galaxies are classified as ellipticals.

 Galaxies with flat disks and spiral arms are classified as spirals.  Spiral galaxies have much more interstellar gas and dust. than ellipticals  Spirals also have young stars, but ellipticals do not.  A small fraction of galaxies lack overall structure and are classified as irregular. o The different appearances of galaxies may be a result of the way in which they formed or their interactions with other galaxies. Collisions between galaxies can alter their appearances and also can merge galaxies into larger galaxies. o Our galaxy, the Milky Way, is a spiral galaxy.

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o Galaxies are usually grouped into clusters. A group of 30 galaxies is known as a local group. The Milky Way belongs to a local group. Rich clusters contain hundreds to thousands of member galaxies. o Galaxies are receding from us with speeds proportional to their distances.

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