Mercury

The laws of mot ion of Mercu ry are extre mely

complicat ed; they do not take place

exactly in the plane of the ecli ptic

(Pierre-Simon Laplace)

Mercury is a unique planet even by the standards of the Solar System. Like Mars, it is a survivor of many similar bodies, possibly a dozen or more, that were formerly present in the inner Solar System before the hierarchical assembly of the Earth and Venus. Thus the investigation of this planet may yield important insights into the early stages of the accretion of planets between the assembly of kilometer-size objects and of the Earth-sized bodies. Because so little is known about this smallest planet, one might question the wisdom of including here a separate chapter on Mercury. Although it was tempting to include this discussion in a section under minor bodies, we decided on separate treatment. This conclusion was driven by the similarities between the mercurian crust and that of the lunar highlands, so that this chapter follows on naturally from those dealing with the Moon. It also provides some interesting problems about primary or secondary crusts. Further, the Messenger mission is already en route to this innermost planet, so that it is useful at this stage to summarize more thoroughly our current understanding.

The geology of Mercury, a one-plate planet like Mars and Venus, shows some similarities with that of the Moon. Accordingly, the current interpretation of the geology and stratigraphy of Mercury depends strongly on the well-known lunar analogs. This has important implications for extrapolating from the well-dated sequence of events on the Moon to other bodies; the surface of Mercury is therefore highly significant in the interpretation of the early history of the Solar System. However, there are many differences as well, contrary to frequently expressed opinions that Mercury is “just like the Moon.

However, we understand Mercury less well than any other planet in the Solar System, including the ice giants, Uranus and Neptune. The orbit and rotation of Mercury are exotic. Mercury slowly rotates with a period of 59 days while orbiting the Sun every 88 days so that the planet rotates three times during each two orbits of the Sun. The current inclination of the orbit is, at 7°, the highest among the planets while the eccentricity is 0.2056, a value only a little lower than that of Pluto; the value has exceeded 0.325 during the past 4 Gyr.

The surface temperature on the sunlit side of this airless planet reaches 725 K, close to that of Venus (743 K). The night-time temperature falls to 90 K. The temperature in some deep craters at the poles may be as low as 60 K so that if water ice were trapped in such locations, it might be stable. Indeed Earth-based radar reflections may indicate the presence of some trapped ice