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The planet Mercury

Mercury is the closest planet to the sun and the smallest planet in our solar system; it is a bit over a third the diameter of Earth.

It is named after the Roman messenger of the gods.

Mercury is one of only two planets that does not have any natural satellites (the other being Venus)

Mercury is the most cratered planet in our solar system.

Mercury's atmosphere contains oxygen, hydrogen, sodium, potassium and helium while its surface is made up of plains and craters. Its surface is similar to that of the moon. Mercury is so small that it cannot retain an atmosphere* and it is geologically inactive because it has lost much of its internal heat.

*Technically, Mercury does have an atmosphere but it is constantly blown away by solar wind. However, the gases that escape are constantly replenished by the same solar winds and radioactive decay.