Mercury, the smallest planet in our solar system, is heavily cratered and visible to the naked eye. Its first observation was made in 1631 by Galileo Galilei and Thomas Harriot, and it has continued to intrigue scientists since its first visit by NASA's Mariner 10 spacecraft in 1974.
Mercury has the largest temperature fluctuations of any planet in the solar system, with a temperature swing of about 1,100 F (600 C) due to its lack of a heat-trapping atmosphere.
Mercury is the smallest planet in the solar system, approximately the same size as the continental United States and only slightly larger than Earth's moon.
Mercury has a bizarre, highly elliptical orbit that takes it as close as 29 million miles and as far as 42 million miles from the sun, with a speed of nearly 29 miles per second, making it the fastest planet in the solar system.
Despite its proximity to the sun, Mercury has surface ice located near its poles, due to the planet's low axial tilt and the shadows cast by some craters that keep those areas in constant darkness.
Mercury has a massive iron core that makes up about 75% of the planet's diameter, the largest of any planet in the solar system.
Mercury has the thinnest atmosphere of any planet in the solar system, called an exosphere, consisting mostly of oxygen, sodium, hydrogen, helium, and potassium.
Mercury exhibits comet-like tails made of particles that slough off its surface. These tails are believed to be caused by sunlight exciting sodium molecules in the planet's exosphere and pushing them away from its surface.