Mercury is the closest planet to the sun. As such, it circles the sun faster than all the other planets, which is why Romans named it after the swift-footed messenger god Mercury.
Mercury was known since at least Sumerian times roughly 5,000 years ago, where it was often associated with Nabu, the god of writing. Mercury was also given separate names for its appearance as both a morning star and as an evening star. Greek astronomers knew, however, that the two names referred to the same body. Heraclitus believed that both Mercury and Venus orbited the sun, not Earth. [Latest Photos: Mercury Seen by NASA's Messenger Probe]
Mercury's physical characteristics
Because the planet is so close to the sun, Mercury's surface temperature can reach a scorching 840 degrees F (450 C). However, since this world doesn't have a real atmosphere to entrap any heat, at night temperatures can plummet to minus 275 F (minus 170 C), a temperature swing of more than 1,100 degrees F (600 degree C), the greatest in the solar system.
Mercury is the smallest planet — it is only slightly larger than Earth's moon. Since it has no significant atmosphere to stop impacts, the planet is pockmarked with craters. About 4 billion years ago, an asteroid roughly 60 miles wide (100 kilometers) struck Mercury with an impact equal to 1 trillion 1-megaton bombs, creating a vast impact crater roughly 960 miles (1,550 km) wide. Known as the Caloris Basin, this crater could hold the entire state of Texas.
As close to the sun as Mercury is, ice may exist in its craters. In 1991, astronomers using radar observations discovered that water ice may lurk at Mercury's north and south poles inside deep craters that are perpetually shadowed and cold. Comets or meteorites might have delivered ice there, or water vapor might have outgassed from the planet's interior and frozen out at the poles.
Mercury apparently shrank about 0.6 to 1.2 miles (1 to 2 km) as it cooled in the billions of years after its birth. This caused its surface to crumple, creating lobe-shaped scarps or cliffs, some hundreds of miles long and soaring up to a mile high.
Mercury is the second densest planet after Earth, with a huge metallic core roughly 2,200 to 2,400 miles (3,600 to 3,800 km) wide, or about 75 percent of the planet's diameter. In comparison, Mercury's outer shell is only 300 to 400 miles (500 to 600 km) thick.
A completely unexpected discovery Mariner 10 made was that Mercury possessed a magnetic field. Planets theoretically generate magnetic fields only if they spin quickly and possess a molten core. But Mercury takes 59 days to rotate and is so small — just roughly one-third Earth's size — that its core should have cooled off long ago. The recent discovery from 2007 Earth-based radar observations that Mercury's core may still be molten could help explain its magnetism.
Although Mercury's magnetic field is just 1 percent the strength of Earth's, it is very active. The magnetic field in the solar wind — the charged particles streaming off the sun — periodically touches upon Mercury's field, creating powerful magnetic tornadoes that channel the fast, hot plasma of the solar wind down to the planet's surface.
Instead of a substantial atmosphere, Mercury possesses an ultra-thin "exosphere" made up of atoms blasted off its surface by solar radiation, the solar wind and micrometeoroid impacts. These quickly escape into space, forming a tail of particles.
Mercury's orbital characteristics
Mercury speeds around the sun every 88 Earth days, traveling through space at nearly 112,000 mph (180,000 kph), faster than any other planet. Its oval-shaped orbit is highly elliptical, taking Mercury as close as 29 million miles (47 million km) and as far as 43 million miles (70 million km) from the sun. If one could stand on Mercury when it is nearest to the sun, it would appear more than three times as large as it does when viewed from Earth.
Oddly, due to Mercury's highly elliptical orbit and the 59 Earth days or so it takes to rotate on its axis, when on the scorching surface of the planet, the sun appears to rise briefly, set, and rise again before it travels westward across the sky. At sunset, the sun appears to set, rise again briefly, and then set again.
Composition & structure
Atmospheric composition (by volume)
No atmosphere: Mercury possesses an exosphere containing 42 percent oxygen, 29 percent sodium, 22 percent hydrogen, 6 percent helium, 0.5 percent potassium, with possible trace amounts of argon, carbon dioxide, water, nitrogen, xenon, krypton and neon.
Roughly 1 percent the strength of Earth's.
Orbit & rotation
Average distance from the sun: 35,983,095 miles (57,909,175 km)
By Comparison: 0.38 Earth's distance from the Sun.
Perihelion (closest approach to sun): 28,580,000 miles (46,000,000 km)
By Comparison: 0.313 times that of Earth
Aphelion (farthest distance from sun): 43,380,000 miles (69,820,000 km)
By Comparison: 0.459 times that of Earth
Length of Day: 58.646 Earth days
Research & exploration
The first spacecraft to visit Mercury was Mariner 10, which imaged about 45 percent of the surface and detected its magnetic field. NASA's MESSENGER orbiter is the second spacecraft to visit Mercury. When it arrived in March 2011, MESSENGER became the first spacecraft to orbit Mercury, where it continues to study the planet. [First Photos of Mercury from Orbit]