Mercury

Mercury Terms Albedo - The albedo measures the reflectivity of a planet or a satellite. Reflectivity is the percentage of the incoming light that the body reflects back in all visible wavelengths. A darker body has a smaller albedo than a brighter body. Cratered - Usually refers to a body whose surface is covered with craters, or holes. Isaac Newton - British physicist (1643-1727) who invented modern mechanics and wrote the law of gravity. To learn more, read the SparkNote on Isaac Newton. Elliptical orbits - An orbit in the shape of an ellipse. An ellipse can be constructed by finding the point such that the summation of its distance from two fixed points, called the foci, is constant. Major axis - The longest axis of an ellipse. Perihelion - The point in an orbit closest to the Sun. General Relativity - Einstein's theory of gravity, in which he explains that space and time curve under because of the presence of masses. Albert Einstein - German physicist (1878-1955) who invented the Theory of Relativity and helped to define the foundations of quantum mechanics. Read the SparkNote on Einstein. Prolate - Descriptive term used to describe long tubular bodies shaped like cigars. Internal heat - The heat contained within a planet, generated by nuclear decay processes. Meteoric - Relating to meteors. Surface gravity - The acceleration of gravity caused by a planet at its surface.

Differentiation - The process in which heavier rocks sink toward the center of a planet while lighter rocks end up forming its crust. Mantle - The portion of a planet beneath the crust and above the core. The mantle is made mostly of silicates, though these minerals are denser on average than those in the crust. The mantle is mostly in a semi-solid viscous phase, allowing very slow convective currents. The mantle's convection motion is similar to that of water boiling in a pot, though much slower. Core - The core of a planet is its central, spherical portion. It can be divided into two regions. The inner region is made of a mixture of nickel (Ni) and iron (Fe), while the external portion is made of iron (Fe) and sulfur (S). Crust - The external, solid portion of the planet, literally its 'skin'. It is made of silicate minerals, but lighter than the mantle underneath. Radar - Radio waves emitted and received by radar are used to find the distance of objects by measuring the delay due to the time of travel of the waves. The longer the delay, the greater the distance. Comets - Cosmic remnants of the formation of our solar system. When they get close to the Sun, comets vaporize. Comets leave a trail of gas and dust visible to the eye. Planetesimal - The current theory of the formation of planets involves the collision of smaller bodies, called planetesimals, which formed the planets we observe today. Such collisions would have been common in the very first 500 million years of our solar system's history. Silicates - Minerals that contain silicon oxide and metals, such as Fe2SiO4. Solar wind - A wind of particles emitted by the Sun that permeate the solar system. X-ray spectrometer - An instrument that determines the composition of rocks by emitting beams of X-rays.

Fact Sheet Average distance from the Sun: 0.39 AU Orbital Period: 88 days Mass: 0.06 Earth masses Radius: 2440 km Density: 5.5 g/cm3 Rotation Period: 59 days Surface Temperature: 150 K to 750 K

Introduction The closest planet to the Sun, Mercury is also one of the smallest planets in the solar system, with a mass about 6% that of Earth's. Since Mercury is closer to the Sun than Earth, it is visible in the evening sky just after sunset or in the morning sky before sunrise and close to the horizon, like Venus. The turbulence of Earth's atmosphere close to the horizon makes it difficult to distinguish any feature on Mercury, even with a telescope. So far, the largest amount of data we have on Mercury comes from the American probe Mariner 10, which approached Mercury in 1974 and 1975. In appearance, Mercury is deceptively similar to the Moon. The rocks at its surface reflect about the same amount of light, with an albedo of only 10%, and they are probably made of a mixture of minerals similar to the Moon's. Mercury, like the Moon, is heavily cratered by the impact of asteroids. It has no atmosphere and displays no geological activity on its surface. Unlike the Moon, Mercury is on average as dense as the Earth, indicating that its overall composition is strikingly different from the Moon's. Mariner 10 also discovered that Mercury possesses a relatively strong magnetic field, like Earth. The Moon, on the contrary, has no magnetic field.

History of Mercury The Greeks associated Mercury with the messenger God Hermes most likely because Mercury has a very short period of revolution around the Sun--only about 88 days. Mercury moves swiftly in the sky, changing its position so fast that daily differences can even be detected by the naked eye. The spotlight fell on Mercury toward the end of the 19th century, when some irregularities were discovered in its motion around the Sun that could not be explained by using the

classical Newtonian laws of physics. In the 19th century, such laws were believed to yield an exact description of the motion of any body subject to gravity. The partly unsolved problem of Mercury's motion did not constitute a sufficient discrepancy to worry the physicists of the time. Here is the problem they faced: As was well known at the time, the planets follow elliptical orbits around the Sun, with the Sun at one of the foci. If Newtonian laws were exactly true, and if the Sun were the each planet's only attracting body, planetary motion would be strictly elliptical. The Sun is the predominant mass in the solar system (about 1000 greater than Jupiter, which the most massive planet). The attraction of the Sun onto the planets is far greater than that of the planets onto each other, but the planet-to-planet attraction is not completely negligible. Planets' orbits are slightly perturbed by other planets. In the 19th century, astronomers had painstakingly calculated the perturbations affecting Mercury's orbit, using the Newtonian laws of physics. They could explain only part of the observed 'drift' of Mercury's major axis, called also the advance of Mercury's perihelion. The effect is very small, just 43 arc seconds per century, but only half of it could be explained by perturbations due to known planets. Could it be that an undiscovered planet had escaped detection because it was too close to the Sun to avoid its glare? Astronomers named this hypothetical planet Volcan, and estimated that they could explain Mercury's motion by calculating the gravitational perturbations caused by this hypothetical planet. They never found this planet, however. This puzzle was solved in 1917, when the Albert Einstein's theory of General Relativity explained the irregular motion of Mercury by changing the laws of gravity themselves. Einstein showed that the well-known law of gravity and the laws of motion that Newton had laid out are only a very good approximation. They can be applied quite successfully to predict any motion in our solar system, but they reveal tiny inaccuracies when applied to objects very close to the Sun, such as Mercury. Einstein realized the importance of Mercury in proving his theory quite early, and he used the solution of this puzzle as evidence in favor of his theory of General Relativity. See the SparkNote on Albert Einstein.

Mercury's Rotation Mercury's rotation is quite slow, about 59 days, and its axis is roughly perpendicular to the plane of its orbit. A given region of the planet is exposed to sunlight and then remains in the shade for a very long time.

Mercury practically does not have an atmosphere--given its small mass and gravitational attraction, it was never able to sustain one. This phenomenon is similar to what happened to the Moon and to most of satellites in the solar system. Because of the lack of the moderating influence of an atmosphere and its slow period of rotation around its axis, Mercury's surface temperatures are the most extreme in the solar system. In areas illuminated by the sunlight, the temperature can soar to 750 K, while on the dark side of the planet it plummets to 150 K.

Figure 3.1: A global view of Mercury, from images of Mariner 10

The reason for the slow rotation of Mercury is the action of 'tidal forces' between the planet and the Sun. This is an important effect when two celestial bodies are relatively close to each other. Tides are important, for instance, between the Earth and the Moon, deforming both from a perfectly spherical shape. It is quite possible that the period of rotation of Mercury was much shorter long ago, at the time of the planet's formation. The Sun attracts the closest side of Mercury slightly more than the farthest side. Such difference causes the planet to assume a slightly prolate shape in the direction of the Sun. For the sake of the argument let us assume that Mercury did have a fast period of rotation, with an axis roughly perpendicular to that of the planet's orbit around the Sun. Concentrate

your attention on one particular point of the planet surface, say a point at the equator. As the rotation of the planet proceeds, the distance between that point of the surface and the center of the planet periodically changes. Layers of rock in any given region of the planet's interior get stretched and contracted on a massive scale. Since the forces of friction resist such a motion, the energy of the planet's rotation gradually gets transformed into internal heat, while the rotation slows down. If Mercury's orbit had been circular, the process we have just described would have slowed down the rotation to a period equal to the period of revolution of the planet around the Sun. Our Moon has the two periods in a 1 : 1 ratio, and that it why we always see the same half of our satellite's surface. But Mercury has an eccentric orbit, and the period of rotation of Mercury stabilized when it reached a value 2 : 3 of the period of revolution, instead of being in the simple ratio 1 : 1.

Mercury's Geology Because of Mercury's vicinity to the Sun's glare, observations of its surface have always been difficult. Scientists curious about the planet's geology had to wait until the probe Mariner 10 flew-by Mercury in 1974-1975 and took images of about 40% of the planet's surface. From these photos it became immediately clear that Mercury's surface had not changed for most of its history. Mercury's surface is covered with meteoric (not volcanic) craters. From the number density of these craters, it seems that for about 4 billion years there was no volcanism on Mercury. By comparing Mercury's pattern of meteoric craters with the Moon's we see plenty of superficial similarities between the two bodies. Many of the differences can be attributed simply to their different surface gravity. Gravity on the Moon is a lot weaker than on Mercury. When an asteroid impact occurs, the debris flies out and wide before raining back on to the surface of the Moon in characteristic rays, which can extend hundreds of miles from the main crater. On Mercury, which has a larger surface gravity, craters are more self-contained and the debris remains closer to the main craters. The low albedo of Mercury's surface--similar on average to that of the Moon--suggests that the bulk of its rocks are of similar composition. However, unlike the Moon, Mercury does not have maria. The maria on our satellite are due to large meteoric impacts that gave rise to a partial melting of the Moon mantle. Maria are literally seas of solidified lava. This suggest that the Mercury underwent a relatively short time of differentiation in the initial phase of its life, but its surface quickly and permanently solidified.

When asteroids hit the planet, the effects of their impact remained superficial, rather than causing the eruption of mantle material to the surface that happened on the Moon. That was even true for the very large impact that gave rise to the largest crater we see on Mercury, called Mare Caloris. We think that, soon after Mercury's formation there probably was a period of widespread magmatic activity, which gave rise to the volcanic plain completely covering the planet. From that time onward the meteoric bombardment carved the multitude of craters we see.

Figure 4.1: Mare Caloris

Several cliffs, which make Mercury's surface look a bit like an old apple, can be explained by the cooling of Mercury's interior after the crust of the planet had already solidified. Since many craters are crossed and cut by the cliffs, the contraction that produced these features

must have taken place some time after many of the craters had formed. The cooling would have been associated with a contraction of the core of Mercury, which would have cooled down and almost certainly solidified. The problem with this hypothesis is that we have reasons to think the iron core of Mercury is still in a liquid, molten, state (see Mercury's Interior).

Figure 4.2: Impact craters and cliffs on Mercury.

Just as on the Moon, some craters close to the poles of Mercury are never exposed to sunlight; radar measurements made from Earth indicate that there probably is some ice within the rims of these craters, though in small quantities. The origin of the ice is uncertain: one might speculate that the ice may have been deposited on Mercury by comets that impacted the planet some time after its formation. Comets are mostly made of ice, and part of that ice could have remained intact within some of the craters at the poles.

Mercury's Interior and Formation Little is known about Mercury's interior, but indirect evidence gives us some clues about its composition. These clues give us a picture of the planet that differs widely from the one we have about the Moon. Mercury is very dense, about 5.5g/cm3 (this value is almost the same as the Earth's). The Earth has much larger mass, of course, and gravity makes any material in Earth's interior much more compressed than in Mercury. Mercury has a larger percentage of the heavy elements iron and nickel that make up the core of all inner planets. Scientists are tempted to speculate that the core may be divided into a

liquid outer core of iron (Fe) and sulphur (S) and a solid inner core composed of a mixture of iron (Fe) and nickel (Ni), as on Earth. Mercury's iron core probably occupies about 40% of the planet's volume and about 60% of its total mass. Given the absence of geological activity at the surface, the mantle of the planet may be solid, but some portion of the core is almost certainly liquid, as it is on Earth. A large and partly liquid core could help explain why Mercury has a weak magnetic field (one hundredth that of Earth). No definitive explanation has been offered regarding Mercury's unusually large iron core. One hypothesis posits that just after its formation, Mercury was hit by a large planetesimal. Such collisions were commonplace in the primordial solar system. The collision would have stripped most of Mercury's mantle, but it would have left the iron core intact. Another hypothesis claims that the more volatile minerals, such as silicates, were blown away by a solar wind in the early history of the solar system. According to this idea, the dust grains that eventually formed the planet were richer in iron than the ones that formed the Earth, as reflected in Mercury's average composition. Still another theory posits that the dust grains containing silicates could not condensate and coalesce at the high temperatures present at that distance from the Sun around 4.5 billion years ago. This and other hypotheses about Mercury will be put to the test as soon as the new MESSENGER and BepiColombo space missions get under way. The American mission MESSENGER (the acronym stands for Mercury Surface, Space, Environment, Geochemistry and Ranging) is scheduled to start in 2004 and reach Mercury after 5 years. It will last about one year and will map the whole planet accurately while measuring the details of the magnetic and gravitational fields. The main aim of the MESSENGER mission is to use these data to determine if Mercury's core is partially molten and the primary cause of its the magnetic field. The mission could also confirm the existence of ice within the Mercury's craters. The European mission BepiColombo will send an orbiter and a lander. The lander will consist of a main station and of a little tethered rover. The rover will explore the surface and analyze Mercury's soil with an X-ray spectrometer, while the main station will pierce Mercury's surface to a depth of 2-3 m recording heat, quakes and surface magnetism. The orbiter's main task will be to map the planet's magnetic field.

Review Quiz

1. Mercury's surface is hardly visible from Earth because of: (A) the small size of the planet (B) its vicinity to the Sun (C) the clouds covering it 2. Mercury is similar the Moon because of: (A) its size and the number of craters on its surface (B) its mass (C) its size and mass (D) its distance from the Earth 3. Newton's laws of gravity: (A) explain the motion of Mercury as due to the attraction of the Sun (B) explain the motion of Mercury as due to the attraction of all bodies in the solar system (C) fail to explain the details of Mercury's orbit (D) explain the details of Mercury's orbit sufficiently 4. General Relativity is a physical theory of: (A) motion (B) motion and gravity (C) subatomic particles (D) electron transfer 5. The advance of the perihelion of Mercury is: (A) a periodic change in Mercury's axis of rotation (B) a periodic change in the plane of the orbit of Mercury (C) a slow change in the direction of the axis of Mercury's orbit, but not of its plane (D) a periodic change in the composition of the crust of Mercury 6. Mercury's period of rotation is influenced by: (A) volcanism (B) tidal forces between Mercury and the Sun (C) meteoric impacts, just like Venus (D) atmospheric pressure

7. The temperatures on Mercury's surface are extreme only because of: (A) its vicinity to the Sun (B) the slow rotation and the lack of an atmosphere (C) its vicinity to the Sun, slow rotation, and lack of an atmosphere 8. Mare Caloris is: (A) a large volcano (B) a large impact crater (C) a sea of molten lava (D) a long lost friend 9. The long cliffs on Mercury: (A) pre-date many of the meteoric craters (B) formed after many meteoric craters (C) are due to widespread volcanic activity (D) are due to glacial activity 10. Mercury probably has: (A) no water on its surface (B) some vapor in its atmosphere (C) ice within craters never exposed to sunlight (D) rivers beneath its crust 11.Mercury has: (A) a small core and a thick crust (B) a large core and thick crust (C) a large core and a thick mantle (D) a Napoleonic complex 12. Mercury has: (A) no magnetic field (B) a small, but non-negligible magnetic field (C) a magnetic field larger than the Earth's (D) A magnetic field that changes intensity periodically 13. The composition of Mercury's interior may result from: (A) the absence of water in its core

(B) volcanism (C) its vicinity to the Sun (D) its subterranean lakes 14. Scientists think that part of Mercury's core may be molten because: (A) of quakes on its surface (B) of volcanism, erupting lava from the core (C) Mercury's magnetic field could be caused by currents within the liquid interior (D) Mercury has no liquid on its surface 15. Future probes headed for Mercury will: (A) remain in orbit exclusively (B) take accurate readings of the planet's magnetic field (C) look for liquid water on Mercury's surface (D) return with samples from Mercury's buried glaciers

Further Reading P. Morrison et al. Exploration of the Universe. Saunders College Publishing. W. Hartmann. The Significance of the Planet Mercury. Sky & Telescope, May 1976. B. Murray. Mercury. Scientific American, May 1976. R. Cowen Forgotten planet. Science News, July 8, 2000.