Mercury is the second-smallest planet after Pluto. It’s about a third the size of Earth, and about a third bigger than the Moon. Mercury orbits so close to the Sun that the Hubble Space Telescope might damage its optics if it attempted to photograph it. However, we did get some close-range photographs in the 1970s from the American Mariner missions, primarily Mariner 10. Only one hemisphere was photographed; the other still remains to be explored. Another robotic mission, the Mercury Messenger, will arrive in 2011 to complete the photographic exploration of the planet.
Orbital diagram of our solar system. Click here for original source URL
The photos we have of Mercury reveal a very Moon-like planet: it’s heavily cratered, and some smooth areas appear to be younger lava plains. Mercury has a large multi-ring basin called Caloris, similar to the large lunar impact basins. However, there are important differences between the geology of Mercury and the Moon, too. Unlike the contrast in color and composition between different terrains on the Moon, the smooth plains on Mercury are the same color as the cratered areas. Whether they are made of the same type of material is still unknown.
Scale model of the diameter of the planets. Click here for original source URL.
Mercury’s mean density is almost as high as those of larger planets like Venus and Earth. If the planets were all made of the same material, you would expect Mercury to have a lower density, since its interior isn’t as compressed by its lower gravity. So Mercury must have a much larger core relative to its size than the other terrestrial planets. In fact, its core is thought to take up 75% of its diameter! The fact that Mercury has a magnetic field (even though it’s about 100 times weaker than the Earth’s) also leads to the conclusion that there must be a large, metallic, iron-rich core beneath the surface. The leading theory as to how this happened involves a large impact early in Mercury’s history. If an asteroid hit Mercury with enough force after it differentiated, the impact may have knocked off a considerable amount of material from the outer layers, leaving behind a relatively large core.
The surface of Mercury shows a network of strange scarps. These unique features are hundreds of kilometers long and about a kilometer high. The scarps intersect impact craters, meaning they formed after the craters, and so are comparatively young. These scarps are most likely “wrinkles” that formed when Mercury’s large core cooled and shrank. This contracted the crust, and the pressure on the crust pushed up these scarps. Another unusual feature on Mercury is the “weird terrain,” an area characterized by jumbled hills and canyons. This terrain is found exactly opposite Caloris basin, so scientists believe it may be a result of the impact that created Caloris. Perhaps the shock wave from the large impact traveled all the way around the globe and converged on the other side to create the weird terrain.
As the closest planet to the Sun, it’s not surprising that Mercury gets very hot! Surface soil temperatures are well above 500 K (441°F) on the sunlit side. (Science fiction writers used to describe pools of molten metal on Mercury, but this is unlikely as the temperature isn't quite high enough to melt common metals, especially in the sub-surface soil.) There is essentially no atmosphere, and without a blanket of insulating air to hold in the heat, the surface temperature drops to an extreme low of about 100 K (-279 °F) at night.
Since the daytime side gets broiled by the nearby Sun, it was all the more amazing when radar signals returned strange signals from Mercury’s poles in 1991. Radar images revealed strange deposits located in the shadowed floors of craters. These deposits suggested the existence of ice caps at the poles, reminiscent of possible ice deposits in lunar craters. Like the Moon, Mercury has polar craters whose floors are in permanent shadow. These cold craters may trap water and other molecules that are vaporized when a comet hits the planet’s surface. The gas molecules might condense into solid frost on rocks in the cold, dark polar craters. It may be comforting to think that familiar terrestrial substances like water exist throughout the solar system — even on barren, lifeless worlds like Mercury and the Moon. But researchers studying Mercury’s surface debate whether these crater deposits are frozen water, or some other material. Some suspect that the deposits might be sulfur, or a mixture of the two materials. Both could have been brought to Mercury by comets, which sometimes hit the planet. If the deposits really contain H2O ice, they could be a welcome resource if human explorers ever attempt to land on the otherwise forbidding planet.