Magnetic Field Of Mercury

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. ???, XXXX, DOI:10.1029/,

Another Look at Mercury’s Surface and Magnetic Field Trevor Dolinajec Mercury is the only terrestrial planet besides Earth to have a magnetic field. This magnetic field produces a magnetosphere which use to be thought of as a small version of Earth’s magnetosphere but is in fact very different. Mercury’s magnetosphere, unlike that of Earth, is full of holes called magnetic flux transfer events that reach the planet’s surface and possibly the interior. Mercury’s magnetic field is also coupled to the sun’s magnetic field (the interplanetary magnetic field, IMF) in ways very much unlike that of Earth. Mercury’s magnetic field does appear to have some striking similarities to Earth’s magnetic field despite these differences in the two planets’ magnetospheres. Both magnetic fields appear to be predominately dipole fields and, according to the conclusions drawn in this paper, both magnetic fields appear to created by active geodynamos. Other explanations for Mercury’s magnetic field such as a frozen-in field and a non-active geodynamo exist, but given recent evidence from the probe MESSENGER’s fly-bys regarding a more active planet than previously supposed, the active geodynamo explanation prevails as the most plausible. Abstract.

chemical composition of Mercury’s surface, the geological history, the nature of the magnetic field, the size and state of the core, the volatile inventory of the core and the nature of the planet’s exosphere and magnetosphere over a nominal orbital mission of one Earth year (1). This last goal of the mission entails the space probe, after making three fly-bys, will enter orbit around Mercury in 2011. To date, the three fly-bys have been completed successfully, the last having occurred on September 29, 2009. All three of these fly-bys were at 200km minimum-altitude owing to the increased sophistication of propulsion and navigation technologies in the twenty-first century. MESENGER has revealed an entire side of Mercury that was in the dark during Mariner 10’s fly-bys. Now Mercury’s surface can be examined as a whole and numerous inferences can begin to be drawn. That, however, is just the tip of the iceberg. MESSENGER has not only viewed Mercury within the visible light spectrum but through a large range of the electromagnetic spectrum as well as gravitationally, suppling an overwhelming amount of data. Consequently many of Mercury’s mysteries are on the threshold of being revealed or at least better understood. Not least of all the nature of the planet’s magnetic field and the ostensible geodynamo that causes it as well as the nature of the planet’s surface and its connection to volcanic activity and its implications.

1. Introduction Mercury is a intriguing planet. It has an interesting dayyear ratio with one day approximately equal to two years. More significantly it is the only terrestrial planet besides Earth in the solar system to possess a magnetic field and a resulting magnetosphere. This magnetic field has implications about Mercury’s interior such as the possible existence of an active geodynamo. In turn, the existence of an active geodynamo would imply heat within Mercury’s interior as well a convecting liquid metal outer core. This would indeed make Mercury a sort of kindred planet to Earth and a desirable field of study. The intrigue, however, does not stop there, Mercury’s position to the Sun’s own magnetic field offers a entirely different hypothetical origin to Mercury’s field. Furthermore, the intricate layering of craters, ridges and planes on Mercury offers great insight into the geological nature of the planet and it’s history as they do on any terrestrial planet. The story of this small atmosphere-less, solar-wind-bombarded planet may well add to the repertoire of how terrestrial planets react to their surroundings. The first mission to Mercury was the Mariner 10 robotic space probe launched in 1973. This mission expanded greatly what was known about the closest planet to the sun. Nothing had been known about Mercury’s surface up till that point. Mariner 10 revealed that Mercury’s surface was moon-like with craters, ridges and planes. (1) The other major discovery of the Mariner 10 mission was the existence of a magnetic field. This makes Mercury the only other planet besides Earth to have a magnetic field. This field has a strength of 400nT as compared to Earth’s magnetic field of strength 45µT (2). Thus Mercury can be said to have a relatively weak magnetic field. Of Mariner 10’s three fly-bys it was only the first and third that were close enough measure Mercury’s magnetic field (they were at altitudes of 703 and 327km respectively). The second fly-by at an altitude of 48,069km was too distant to measure the magnetic field (1). Mariner 10 would remain the only mission to mercury for 31 years until August of 2004 when the MESSENGER (MErcury Surface ENvironment GEochemstry and Ranging) mission was launched. This mission sought to characterize the

2. Mercury’s Magnetic Field It was a surprise in 1974 when Mariner 10 reveled that Mercury has a coherent, intrinsic magnetic field (3). Although Mercury is generally stated to have a magnetic field strength of 400nT this value is far from constant or even necessarily accurate. It is true that Mariner 10’s third and final fly-by measured a magnetic field of 410nT but MESSENGER’s first fly-by measured only 159nT. Furthermore, this measurement was made at a lower altitude of 201km versus Mariner 10’s altitude of 327km on the third fly-by. This magnetic field strength decrease of 2.5 times is quite significant, especially when one considers that the lower measurement was made at an altitude a third again lower. This is not consistent with a centered dipole in a vacuum because the maximum variation in field magnitude from a dipole is a factor of 2 at constant altitude (3). This leads to questions regarding how significant are contribution of a quadrupole moment or higher multipole moments. Comparison of Mariner 10’s first fly-by and MESSENGER’s first flyby shows no statistically significant evidence that Mercury’s

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magnetic field has changed between 1974 and 2008 (3). Thus assuming the field is unchanged, data from Mariner 10’s flybys can be combined with those of MESSENGER, creating a greater data pool. In the end, although initial data does not fit a dipole model, when pressure gradients due to solar wind and magnetospheric plasma is taken into account along with correction using the Tsyganenko and Sitnov model, the need for an intrinsic quadrupole nearly disappears (3). In

Figure 1. The magnetic field and magnetosphere of Mercury as observed during MESSENGER’s first fly-by (3).

Figure 2. The magnetic field and magnetosphere of Mercury as observed during MESSENGER’s second flyby (7). Notice the increase in flux transfer events and the flux rope.

Figure 3. A volcano on Mercury (8).

fact, Mercury’s magnetosphere appears structurally to be a miniature of Earth (4), a magnetosphere known to be intimately related to a magnetic dipole. Despite similarities between Mercury’s magnetosphere and that of Earth, the solar wind conditions in which the two magnetosphere’s exist are markedly different. MESSENGER has revealed Mercury’s magnetosphere to be immersed in a cloud of commit-like planetary ions and these ions may exert influence from kinetic to magnetohydrodynamic scale lengths (5). This magnetosphere is not the relatively stable type that Earth possesses but rather perforated form (6).This magnetosphere is home to multiple large scale reconnection flux tubes (see Figure 2). This tubes at around 900km in diameter behave as magnetic twisters and interact with Mercury’s surface (6).This twisters are very interesting in their own right, indicating a magnetospheric process that determines the weather on Mercury and exogenic geological processes that formed the surface of Mercury over past eons (6). In fact, the magnetosphere is to Mercury what the troposphere is to Earth (6). The implication of these flux transfer events does not stop there; these events imply that Mercury has a strong coupling to its local interplanetary environment (6). This is not the case with Earth but that stands to reason if we consider that the solar wind around Earth is not nearly as intense and does not behave like a cloud of commit-like planetary ions. The extension of this coupling concept are observations made during MESSENGERS second fly-by that indicate that Mercury’s magnetosphere is much more responsive to interplanetary magnetic field directions and dominated by effects of reconnection than that of Earth (7). We see that Mercury may not have such an earth-like magnetosphere after all. Perhaps Mercury’s magnetosphere looks like that of Earth from a distance and at low resolution but upon further inspection significant differences arise. We have talked a good deal about the magnetosphere of Mercury, which is undeniable relevant to a discussion of the magnetic field, but what of the magnetic field itself? We have to wonder if this strong evidence of magnetosphere influenced by the intense solar wind can reveal to us the nature

Figure 4. Scarps on Mercury. A sign of Mercury’s geological history of contraction and the contradictions that presents with regards to recent evidence of young volcanism (12).

TREVOR DOLINAJEC: MERCURY of the magnetic field and its generating process. It is possible that since the magnetosphere of Mercury is so heavily dependent on the interplanetary magnetic field (IMF) that a good place to look for better understanding of the magnetic field of Mercury is possible magnetization by the IMF itself. Furthermore, if the magnetic field of Mercury is the result of a geodynamo, we must answer the question whether this dynamo is currently operating or operated only in the past and imparted a long-wavelength remnant of a frozen field to Mercury’s outer crust (4). As mentioned above, there is no statistically significant evidence that Mercury’s magnetic field has changed from 1974 to 2008. Whereas the magnetic field of Earth has certainly changed and moved during that time period. This certainly indicates that there is something different about Mercury’s magnetic field as compare to Earth and consequently there should either be something different about the nature of Mercury’s geodynamo or the origin of Mercury’s magnetic field. One model of Mercury’s geodynamo suggests that the presence of a stagnant layer at the top of the molten outer core may suppress higher-order structure and yield variations over time scales of centuries not decades (3). To say that observed behavior of Mercury’s magnetic field matches a model is not enough; one successful model does not represent a unique solution. What we require is some collaborating evidence. Let us look for such evidence in a discussion of the level of activity in Mercury’s interior.

3. Mercury’s Volcanic Activity Mercury has been often compared to the Moon. This comparison seemed appropriate up until about a year ago Mariner 10’s observations largely supported it and such conclusion were even drawn from MESSENGER’s first fly-by. On the simplest level Mercury was comparable to the Moon as a crater covered planet. The comparison went further, however, both solar bodies were thought of as cold and dead (8). Thus the prevailing theory explaining large planes on the moon held true for Mercury as well. This theory is based on methods of laser altimetry and stereo image analysis and basically states that large basins or planes on the Moon are caused by thick ejecta deposits that are emplaced during impact events (9). MESSENGER’s first fly-by began to suggest that Mercury’s plains were not the result of impact ejecta ponding but rather an effect of volcanic flooding. The presence of flow-like scarps and ghost craters, the lack of basin ejecta sculpture, and the distance from known basins all combined to favor a volcanic origin for the emplacement of crater interior planes (10). This suggests a substantial role played by volcanism in the geological history of Mercury but still does not separate this volcanic style from that of the moon, which also has volcanic filled craters despite some ejecta created basins. In fact, it was stated after MESSENGER’s first fly-by that the style of volcanism on Mercury was similar to that of the Moon and contrasted huge volcanic edifices and more a extended duration of volcanism on Mars and the plate-boundry and hot spot volcanism on Earth (10). Laser altimetry during MESSENGER’s first fly-by did show that sampled craters on Mercury were shallower than their counterparts on the Moon, which may be explained by Mercury’s greater gravity or might be indicative of the fact that Mercury is in fact a different type of body than the Moon (11). A major reason that Mercury was so often associated with the Moon is a prevailing theory that volatiles could not have been delivered and retained on a planet so close to the Sun during the formation of the planets (12). The scientific community has consequently been very hesitant to

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suggest more active volcanism on Mercury because of the necessity of explaining how volatiles could have been retained on Mercury. Nevertheless, MESSENGER’s second and third fly-bys have gone a long way to show that indeed Mercury does have a much more active volcanism than the Moon and something more like Mars. The difficulty in explaining existent volatiles on Mercury was not the only deterrent to theorizing about active volcanism on Mercury; the other was evidence of contraction indicating cooling. Scarps indicating a cooling planet were fist confirmed by Mariner 10 during its three fly-bys and active volcanism was though to have ended three billion years ago (12). In fact, during MESSENGER’s first fly-by, summed lengths of such scarps showed that contraction was an average of one-third greater than previously estimated (4). The evidence against active volcanism on Mercury seemed to be overwhelming but that has begun to rapidly change with new models and new data from MESSENGER’s two most recent fly-bys. Indeed, evidence has been rolling in of a much more active Mercury than previously thought. There is suggestions of a very youthful age for the Raditladi basin of only one billion years or less (13). This is two billion years younger than previously observed evidence of volcanism on Mercury’s surface. One model that seeks to explain such a young volcanic phenomenon is volcanic heat-piping (14). This theory is at odds with contraction theory, however, because contraction squeezes things shut, closing off vents and pipes. So, perhaps volcanism early on in Mercury’s history was not enough to preclude volcanism, which would require a new model for contraction and maybe new explanations for the crisscrossed system of cracks and faults (8). Apparently Mercury’s geological history has become quite a quagmire with simultaneous evidence of increased contraction and more youthful volcanism; the two of which are at odds with each other. In general Mercury has begun to look like not a dead world after all but rather a planet that has been very active in spite of its size (8). Mercury is no longer looking like there was once a primordial magma ocean as there is solid evidence of having been on the Moon. This is back up by Mercury being much duller in reflectance than previously thought and thus not composed of a feldspar surface as the Moon (8). If a floatation crust of feldspar ever existed then recent observations from MESSENGER suggest it has long since been completely covered by volcanic resurfacing events (8). During MESSENGER’s third fly-by a new crater was investigated that is being called Twin because of its similarity to the Raditladi crater which shows the strongest evidence of recent volcanism yet (12). This crater due to its lack of covering craters seems to date to less than a billion years ago and the greater number of small covering craters in Twin’s outer ring than in the inner ring suggests successive stages of volcanic activity (12). This is yet another compelling piece of evidence that Mercury has been much more volcanically active than assumed. Features that seem to be volcanoes are being discovered (see Figure 3) and an area of overlapping cracks in the Rembrandt Basin may indicate four stages of crustal movement and a much more tectonically interesting place than suspected from Mariner 10 or the first two MESSENGER fly-bys (12). It is not such a stretch that this evidence of a active rather than cold dead planet may correlate to activity in the outer core of the planet and the existence of a active geodynamo.

4. Conclusion We have suggested three possible explanations of how Mercury’s magnetic field is generated and we have laid out some of the data and hypotheses that are relevant to these three possible explanations. The first proposed generating phenomenon for Mercury’s

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magnetic field is magnetization of the planet by the interplanetary magnetic field (IMF), i.e. the magnetic field of the sun. This proposition was ventured after observing how interconnected Mercury’s magnetic field was with the IMF due to the bombardment of solar wind and the simple intensity of the IMF owing to proximity. The IMF may be a complex magnetohydrodynamic dynamo with the solar wind composed of a plasma but there is no evidence that it could have magnetized a planet. It seems that for Mercury’s magnetic field to have been created by the IMF there would have had to been a significantly different and ostensibly stronger magnetic field at some time in the solar system’s history of which there is no evidence. There is also a popular theory that Mercury’s magnetic field is a result of a geodynamo, but one that is no longer active. This would entail a frozen-in magnetic field that is residual in the outer crust of the planet. There is no simple or convenient way in which to disregard this theory. Nevertheless, the discussion of it is very relevant with regards to recent discoveries of non-primordial volcanism on Mercury. A frozen-in magnetic field is only a necessary construct if one needs to explain magnetic field activity for a planet without an active geodynamo. Mercury used to look very much like a planet without an active geodynamo; a cold planet that had shrunk significantly due to cooling and probably had lost much of its heat during that process. The kilometer high scarps that scar the planet are still best explained by contraction and cooling but their currently exists a pressing need for new hypotheses. Given the MESSENGER observations of craters and rises best explained by relatively recent volcanism (in the last billion years) the cool Moon-like view of Mercury is being reinvented as a surprisingly active little planet. The new propositions of Mercury housing significantly more valtiles than previously supposed matches up nicely with a liquid outer core and thus an active geodynamo. Indeed, the same adjustments to theory (explaining volatiles in the solar system) that help to explain volcanism past the one billion years of age mark for the planet serve to justify an active geodynamo. Thus given recent observations the active geodynamo explanation for Mercury’s magnetic field seems the most likely. This does not change the fact that the resulting magnetosphere is perforated by magnetic twisters and coupled to the

IMF; it only suggests that the small solar-wind-pelted field was created and, most importantly, is maintained by an active geodynamo. There remains numerous unanswered questions about the magnetic field of Mercury. If it is truly maintained by an active geodynamo and Mercury has not cooled as much as previously supposed than the nature of the scarps require a good deal of explanation. The process for delivering volatiles to a planet so close to the sun also does not fit into current theory and requires further modeling and investigation. The fascinating existence of a planetary magnetic field so close to the magnetic field of the sun is also sure to raise new questions about plasma under such circumstances. Hopefully, when MESSENGER inters orbit in 2011 the prolonged observing time will begin to answer many of these questions and more. We can also expect may new questions, contradiction and controversies to arise with an increased understanding of Mercury and its magnetic field, but this is exactly what stimulates human understanding of the universe around us.

References (1) http://solarsystem.nasa.gov/missions (2) http://www-ssc.igpp.ucla.edu/personnel/russell/papers (3) B.J. Anderson et al., Science 321, 82 (2008) (4) S.C. Solomon et al., Science 321, 59 (2008) (5) J.A. Slavin et al., Science 321, 85 (2008) (6) K.H. Glassmeier, Science 324, 597 (2009) (7) J.A. Slavin et al., Science 324, 606 (2009) (8) E. Hand, Nature, Mercury not so like the Moon (2009) (9) M. Wieczorek et al., Reviews in Mineralogy and Geochemistry 60, 221-364 (2006) (10) J.W. Head et al., Science 321, 69 (2008) (11) M.T. Zuber et al., Science 321, 77 (2008) (12) R. Lovett, Nature, Probe uncovers Mercury’s youthful secret (2009) (13) R.G. Storm et al., Science 321, 79 (2008) (14) K. Multhaup, Geophysical Research Letters, DRAFT (2009)