Mercury is the smallest, innermost, most-cratered planet in the Solar System.

Mercury's gravity is too weak to hold a permanent atmosphere, so when atoms are evaporated from the planet's surface by solar photons or other processes, some of the atoms form a tail that points away from the sun.

Because Mercury is so close to the sun, its surface temperature can reach a scorching 840 degrees Fahrenheit (450 degrees Celsius). However, since this planet doesn't have any real atmosphere to trap 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 largest in the solar system.

The earliest known recorded observations of Mercury are from the Mul.Apin tablets. These observations were most likely made by an Assyrian astronomer around the 14th century BC. The cuneiform name used to designate Mercury on the Mul.Apin tablets can be translated as "the jumping planet".

Mercury's iron core is about the size of the moon. In fact, iron accounts for about 70% of Mercury's total weight, making Mercury the most iron-rich planet in the Solar System.

It takes Mercury 87.969 days, or the equivalent of 0.24 Earth years, to complete a single orbit of the Sun.

The planet has no known natural satellites.

Mariner 10 flew past Mercury three times. The first encounter took place on March 29, 1974, at a range of 703 kilometers.

If you weigh 100 pounds on Earth, you would weigh only 38 pounds on Mercury.

Unexpected changes in the orbit of Mercury helped confirm Einstein's theory of general relativity: The orbit of Mercury is shifting very gradually over time, due to the curvature of space-time around the massive sun. In a few billion years, it could even collide with Earth.