Mercury
DISTANCE FROM SUN: 36 million miles (57.9 million kilometers).
REVOLUTION AROUND SUN: 88 days.
ROTATION: 59 days.
DIAMETER: 3,030 miles (4,878 kilometers).
DENSITY: 5.4 x that of water.
MASS: 0.06 x that of earth
SURFACE TEMPERATURE: 806°F (430°C) on day side,-297°F (-183°C) on night side.

Zipping across the sky, the innermost planet was named after the fleet-footed messenger of the Roman gods. Lean, mean Mercury—smaller than Jupiter’s largest moon and enduring the most dramatic temperature swings of any planet in the solar system—does indeed move faster than any other planet.

Although Mercury is closer to Earth than most planets, it is among the most difficult to observe. It remains too close to the sun to be seen at the darkest time of night, and even when visible at dawn or dusk, it hugs the horizon so that a telescope must peer at it through the thickest part of Earth’s atmosphere. So close is Mercury to the sun that operators of the orbiting Hubble Space Telescope are not permitted to point it at the little planet for fear that the intensity of sunlight would damage the telescope.

When the Mariner 10 probe passed Mercury in the early 1960s, it found a planet that was very much like Earth’s moon. Craters from meteorite bombardment pock the surface. Dust from countless impacts covers everything.

Although it is the solar system’s speed demon, Mercury has the slowest period of rotation. The planet turns just one and a half times in a single orbit—and there are two Mercury years, equal to 176 Earth days, between sunrise and sunset. As a result, the sunny side of the planet is baked mercilessly, with daytime temperatures up to 806°F (427°C). At night, although the sun is literally just around the corner, the temperature plunges to -297°F (-183°C).

Amazingly, it’s possible that water exists on Mercury’s tortured surface. The bottoms of some craters at Mercury’s north pole never see sunlight, and their frigid temperatures could freeze water from comet impacts.

CALORIS BASIN Mercury's largest known landform, Caloris Basin, is near the point on the planet that comes closest to the sun. That makes it, in theory at least, the hottest spot on the planet, thus its name, which is Latin for heat.
Caloris Basin was formed when an asteroid-like object slammed into Mercury. The basin is 800 miles (1,300 kilometers) wide, with a smooth plain on its floor. The impact that created it also created visible fractures in the surface that get larger toward its center.
The impact shock focused on the opposite side of Mercury from the center of Caloris Basin. There the surface is broken into a series of complex blocks.

RIDGES AND FAULTS Mercury is crisscrossed with scalloped cliffs that run for hundreds of miles. These ridges and faults are most likely “wrinkles” in the planet, created as it shrank earlier in its history. At one point the planet had a liquid metallic core that included most of its interior mass. As the core cooled, it shrank to become a small, solid inner core. As the cooling progressed, the surface of Mercury also shrank slightly, like the skin of a dried orange, and heaved and buckled.

CRATERS AND MORE CRATERS A lunar explorer would feel at home on Mercury. Craters, and craters within craters, are everywhere. One crater in the Shakespeare region of Mercury measures 55 miles (90 kilometers) wide, and has a striking central peak created in the initial “splash” of a meteorite impact. Because it has no atmosphere, the craters of Mercury remain virtually intact until subsequent impacts disturb them. Younger craters display bright halos and rays, remnants of ejecta that was hurled outward in the crash.