Mercury

The smallest planet in the solar system, Mercury is also the closest to the sun. Mercury bakes by day, but it freezes at night! It is the fastest planet in the Solar System. Mercury's year is the shortest, too. It zips around the Sun every 88 Earth-days.

Image at right: Mercury (courtesy of NASA).

Comets and meteoroids crashed into Mercury's surface when it formed about 4.6 billion years ago, so Mercury has many craters. Its surface is reminiscent of Earth's Moon. On this small, nearly airless world, you would also find rocks, flat plains, deep valleys, and steep cliffs. With no wind or water to change its surface, Mercury has remained much the same since it formed.

Features

: Mercury's Caloris Basin (Courtesy of NASA, Viking Project)

Caloris Basin (Basin of Heat) is a huge crater on Mercury. It is about 800 miles (1,300 kilometers) wide - about as wide as Texas! Smaller and more recent impact craters dot the floor of the crater. A ring of mountains surrounds it. Many ridges, wrinkles, and cracks also formed on Mercury as the young planet cooled and shrank.

Mercury has very little atmosphere. There is no protection against the Sun's heat by day, and no blanket to keep the surface warm at night. Mercury travels very quickly around the Sun in 88 Earth-days. But the planet spins only once on its axis in about 59 Earth-days - 2/3 of the time it takes Mercury to complete an entire orbit. Because of its quick orbit and slow rotation, there are 176 Earth days between sunrises on this little planet.

Explore more about Mercury's features on NASA's Solar System Exploration website.

Missions

: An artist's impression of MESSENGER orbiting Mercury (NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

Understanding Mercury helps us understand all of the terrestrial planets better. Mariner 10 made three flybys past Mercury in 1974 and 1975. Scientists learned that the planet had a thin atmosphere and a magnetic field. The probe mapped half of the planet's surface and took 10,000 pictures.

MESSENGER (NASA's Mercury Surface, Space Environment, Geochemistry and Ranging orbiter) launched in 2004. Flybys set for January 2008, October 2008 and September 2009 will send back the first new data from Mercury in over 30 years. MESSENGER will map the planet in color and measure the composition of the surface and atmosphere.

Learn more about missions to Mercury at NASA's Solar System Exploration website.

Myths, Stories, and More

The Roman god of games, business, and story telling lends his name to Mercury. Mercury (also called Hermes by the Greeks) was the messenger of the gods because he was so fast. Artists usually draw Mercury wearing a hat and sandals with wings on them as a reminder of his special speed.

Learn more about Mercury myths and lore at Windows to the Universe.

Earth Matters

: Earth is about 2.6 times bigger than Mercury (Courtesy of NASA)

Mercury's orbit is so close to the Sun that it is difficult to see from Earth. As a result, some early astronomers never observed the planet. Because of the glare of the Sun, you can see Mercury only at twilight. The Sun's rays are about 10 times stronger on Mercury than on Earth, and the Sun looks about 3 times as big.

Mercury's interior seems to be like Earth's. Many scientists think that both planets have a core mostly made of iron because Mariner 10 discovered a magnetic field around Mercury.

Find out more about why we study Mercury on NASA's website.