Mercury

Mariner 10 photo mosaic of Mercury from 37,000 miles (NASA/JPL/Northwestern University)		Mariner 10 photo mosaic of Mercury's Southern hemisphere (NASA/JPL)		Mariner 10 closeup view of Antoniadi Ridge on Mercury (NASA/JPL/Northwestern University)

Messenger of the Gods

The next stop on our tour is a small, barren world called Mercury. It is the closest planet to the Sun and the second smallest planet in the Solar System. Mercury was named after the ancient Roman messenger of the gods because it moved across the sky much faster than the other planets. In fact, Mercury completes one orbit around the Sun in only 88 days! The planet has a very slow rotational period. It takes 58 days for Mercury to make a complete rotation on its axis. Because of its close proximity to the Sun, it can only be observed in the sky in the east just before sunrise or in the west just after sunset. It never appears more than 27 degrees away from the Sun.

Mariner 10 closeup view of large double-rimmed basin on Mercury (NASA/JPL/Northwestern University)		Mariner 10 close-up Mercury showing the Caloris Basin (NASA/JPL/Northwestern University)		Close-up of craters on Mercury as seen by Mariner 10 (NASA/JPL/Northwestern University)

A Hot and Cold Barren World

Mercury has almost no atmosphere. Scientists believe that a thicker atmosphere may have existed at one time, but was stripped away by the strong solar winds. The atmosphere on Earth helps to maintain a relatively uniform temperature around the planet. By contrast, the temperature on Mercury ranges from 800° F (427° C) during the day, to -279° F (-173° C) at night. It is indeed a very inhospitable world.

Features of Mercury

Mercury was visited by NASA's Mariner 10 space probe in 1974. In a series of three flybys, the probe managed to map most of the planet's surface. When the photos were received on Earth, the features might easily have been mistaken for the Moon. Mercury's surface, however, is almost completely covered with craters. The moon has more open areas known as maria, which are mostly crater-free. Mercury has only one major open area known as the Caloris Basin. These open areas are thought to have been formed by more recent volcanic flows, which may have covered the older craters. The surface is also marked by steep cliffs, which rise as much as 6300 feet (1920 m) above the valley floors.