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.

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).

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.