Mercury is a small planet that is quite hot (approximately 800°F [427°C] during a Mercurian day) when the Sun shines on its surface. It has a very thin atmosphere of oxygen, potassium, and sodium vapors. The surface pressure of atmosphere is too low to have wind. Without wind, running water, and flowing ice, the range of surface processes is limited to physical weathering effects of heating and cooling and meteoritic impact.
Mariner 10 is the only spacecraft to have photographed the surface of Mercury. Completing a total of three close encounters with the planet, in March and September 1974, and March 1975, the space probe was able to record details over about 45% of Mercury's crater strewn surface. The remaining half of Mercury's surface has never been photographed.
Mercury's surface is very similar to that of the Moon. There are, however, some important differences in features. Mercury has, for example, relatively fewer craters larger than 15.5-31 mi (25-50 km) in diameter. There are no extensive highland regions on Mercury, and the surface is subdivided simply into "cratered terrain" and "intercrater plains" based upon differences in crater density and size. Resurfaced regions on Mercury are rare (<15% of the surface), and are referred to as "smooth plains." Unlike the Moon, Mercury exhibits many scalloped cliffs, or lobate scarps that can run for several hundred kilometers and be as much as 0.6 mi (1 km) high. Only one lunar-like mare, the Caloris Basin, has been discovered on Mercury, however, many large multi-ring crater basins (124-373 mi [200-600 km] in diameter) are filled with flood basalts like the lunar mare. Sporting a relatively flat but wrinkled floor, and being surrounded by a ring of 1.24 mi-(2 km) high mountains, the Caloris Basin, with a diameter of about 807.5 mi (1,300 km), is the largest Mercurian feature.
The prominent scarp features recorded on Mercury by Mariner 10 are unique to the planet. The scarps are interesting from a geological standpoint, because they run cross other surface markings, such as craters. This suggests that the scarps were formed through the shrinkage of the planet's outer mantle or some other stresses perhaps due to change in planet shape upon attainment of its current 2:3 spin orbit couple with the Sun. The rises of the observed scarp features suggests that since Mercury formed it has shrunk by about 2 mi (3 km) in radius.
The geological history of Mercury starts with accretion and differentiation about 4.6 billion years ago. Now long after the planet's mass came together there was a global melting episode for the crust and mantle and planetary differentiation so that an iron core formed and a mantle and crust developed. Mercury, as the other planets, went through the "heavy bombardment" period in which much of the impact structure of the old cratered terrains of Mercury were formed. The Caloris Basin impact was a punctuation mark in Mercury's history. The impact dispersed a huge amount of ejecta over large areas of the planet and the passage of shock waves directly through Mercury caused an "antipodal" (opposite side) effect of crustal shattering and disruption. Since Caloris, flood basalts filled the large impact basins, including Caloris, and the global system of scarp features formed due to planetary contraction. Light impact cratering followed the scarp formation event and continued for the last three billion years of Mercury's history.
The International Astronomical Union has established a guide to the naming of planetary features, and for Mercury the convention is that craters are named after artists, musicians, painters, and authors; plains are given names corresponding to Mercury in various languages; scarps are named after famous ships of scientific discovery, and valleys are named after radio telescopes.