Mercury, the planet closest to the Sun, is a cratered world. It is the smallest of the inner planets, perhaps because the heat of the nearby Sun as Mercury formed, about 4.6 billion years ago, prevented most of the gases present in the vicinity from becoming part of the protoplanet. Mercury's surface is very hot, sometimes reaching extremes of more than 470 degrees C (more than 1,380 degrees F)--especially at two "hot spots" opposite one another on the equator. This heat and the planet's low gravity make it impossible for Mercury to retain any significant atmosphere. Trace amounts of hydrogen, helium, and oxygen above the surface probably derive from the solar wind, while similar traces of sodium and potassium atoms may represent gases diffusing up through the planet's crust.

Mercury orbits the Sun once every 88 days at distances varying from 70 million to 46 million km (43 million to 29 million mi.). Because of the great difficulty in observing this small and distant planet, which never appears more than 28 degrees from the Sun in the sky, it was thought as late as the early 1960s that Mercury also rotated with an 88-day period, so that one hemisphere always faced the Sun. Radar observations have since shown, though, that the true rotation period is 58.6 days. Mercury rotates three times for every two trips around the Sun, so that during every alternate perihelion (closest approach to the Sun) the same face points directly at the Sun. The perihelion of Mercury's orbit advances 43 seconds of arc per century. This effect is only fully explained by Einstein's general theory of relativity.

Mercury's diameter is about 40 percent that of the Earth, and its mass is about 6 percent of the Earth's. Its high density implies that there is a large iron or nickel-iron core inside the planet. Mercury is thought to contain a higher percentage of iron than the Earth does. Current computer models set Mercury's core radius at 1,800 km (1,120 mi.)--75 percent of the radius of the planet. (The Earth's core has a radius that is only about 55 percent of the planetary radius.) This large iron core, part of which is probably molten, is undoubtedly responsible for Mercury's intrinsic magnetic field. Discovered in 1974 by the Mariner 10 spacecraft, the field is only about 0.7 percent as strong as the Earth's at the surface, but this is enough to disturb the solar wind as it streams past Mercury.

Mariner 10 photographed about 40 percent of Mercury's surface in detail. Extensive cratered highlands cover much of the surface, making Mercury look like the Moon. Such highlands are probably quite ancient. Dark, smooth plains that look like those on the Moon are also seen. They are probably younger than the highlands. The largest plain, 1,300 km (800 mi) wide, is called the Caloris Basin because it is located at one of Mercury's "hot spots"--either of the two points where the Sun can be directly overhead when Mercury is nearest the Sun. Radar observations in 1991 suggested that despite Mercury's nearness to the Sun, some water ice may exist at the planet's polar areas on shadowed crater floors--a possibility strengthened three years later by further radar analyses.

Mercury is quite different from the Moon, however. It is 40 percent larger, 4.5 times more massive, and much richer in iron. Even Mercury's surface is different from that of the Moon in two important ways. First, although heavily cratered, Mercury's highlands are not saturated with craters. Extensive, gently rolling plains predominate. These plains may represent the original crust of Mercury showing through the effects of cratering, because Mercury's higher surface gravity prevented meteoric impacts from spreading their ejecta as far as they did on the Moon. Second, there are large, long, and winding scarps, or one-sided ridges, that cross the surface of the cratered highlands for hundreds of kilometers. These scarps are thought to have formed during contraction of the crust as Mercury's large core cooled and partially solidified.

Microwave and infrared studies indicate that Mercury's surface is largely devoid of basalts. This suggests that Mercury has kept its molten iron interior--and related magnetic field-- because there has been minimal heat loss from the interior through volcanic action, which would have led to a more basaltic surface on the planet.