Mercury

Sun-scorched Mercury is only slightly larger than Earth's Moon. Like the Moon, Mercury has very little atmosphere to stop impacts, and it is covered with craters. Mercury's dayside is super-heated by the sun, but at night temperatures drop hundreds of degrees below freezing. Ice may even exist in craters. Mercury's egg-shaped orbit takes it around the sun every 88 days.

Mercury is appropriately named for the swiftest of the ancient Roman gods. Mercury, the god of commerce, is the Roman counterpart to the ancient Greek god Hermes, the messenger of the gods.

Mercury speeds around the sun every 88 days, travelling through space at nearly 50 km (31 miles) per second - faster than any other planet. One Mercury solar day equals 175.97 Earth days.

Mercury's elliptical orbit takes the small planet as close as 47 million km (29 million miles) and as far as 70 million km (43 million miles) from the sun. If one could stand on the scorching surface of Mercury when it is at its closest point to the sun, our star would appear more than three times as large as it does when viewed from Earth. Because Mercury is so close to the sun, it is hard to directly observe from Earth except during twilight.

Mercury makes an appearance indirectly, however - 13 times each century, Earth observers can watch Mercury pass across the face of the sun, an event called a transit. The transits fall within several days of May 8 and November 10. The first two transits of Mercury in the 21st century occurred 7 May 2003 and 8 November 2006. The next will occur on 9 May 2016.

Temperatures on Mercury's surface can reach 800 degrees Fahrenheit (427 degrees Celsius). Because Mercury's atmosphere is so thin, the surface cannot retain that heat so nighttime temperatures can drop to -290 degrees Fahrenheit (-179 degrees Celsius).

Mercury's thin atmosphere, or exosphere, is made up of atoms blasted off the surface by the solar wind and micrometeoroid impacts. Because of solar radiation pressure, the atoms quickly escape into space and form a tail of neutral particles. Though Mercury's magnetic field has just 1 percent the strength of Earth's, the field is very active. The magnetic field in the solar wind episodically connects to Mercury's field, creating intense magnetic tornadoes that funnel the fast, hot solar wind plasma down to the surface. When these ions strike the surface, they knock off neutral atoms and send them on a loop high into the sky where other processes may fling them back to the surface or accelerate them away from Mercury.

Mercury's surface resembles that of Earth's Moon, scarred by many impact craters resulting from collisions with meteoroids and comets. While there are areas of smooth terrain, there are also lobe-shaped scarps or cliffs, some hundreds of miles long and soaring up to a mile high, formed by contraction of the crust. The Caloris Basin, one of the largest features on Mercury, is about 1,550 km (960 miles) in diameter. It was the result of an asteroid impact on the planet's surface early in the solar system's history. Over the next several billion years, Mercury shrank in radius about 1 to 2 km (0.6 to 1.2 miles) as the planet cooled after its formation. The outer crust contracted and grew strong enough to prevent magma from reaching the surface, ending the period of volcanic activity.

Mercury is the second densest planet after Earth, with a large metallic core having a radius of 1,800 to 1,900 km (1,100 to 1,200 miles), about 75 percent of the planet's radius. In 2007, researchers using ground-based radars to study the core found evidence that it is molten (liquid). Mercury's outer shell, comparable to Earth's outer shell (called the mantle), is only 500 to 600 km (300 to 400 miles) thick.

The first spacecraft to visit Mercury was Mariner 10, which imaged about 45 percent of the surface. In 1991, astronomers on Earth using radar observations showed that Mercury may have water ice at its north and south poles inside deep craters that are perpetually cold. Infalling comets or meteorites might have brought ice to these regions of Mercury, or water vapor might have outgassed from the interior and frozen out at the poles.

In 2008 and 2009, NASA's MESSENGER mission performed two close flybys of Mercury. By the second flyby, the spacecraft had imaged about 80 percent of the surface at useful resolution and made discoveries about the magnetic field and how Mercury's crust was formed. The flybys employed Mercury's gravity to help ease the spacecraft into orbit in March 2011. The spacecraft is studying and imaging Mercury from orbit and will map nearly the entire planet in color. MESSENGER is the first spacecraft to orbit Mercury.