Mercury (Courtesy to Microsoft Encarta for this photograph.)

Mercury is the closest planet to the Sun. That is the main reason why astronomers find it hard to study Mercury. Only every so often does this planet get out of the Sun's glare at sunrise and sunset. Even when they saw Mercury, it was only a fuzzy ball. One of the reasons is that Mercury has a surface made of rough, dark-colored rock. This makes Mercury an insufficient reflector of light.

In 1974, Mariner 10, a space probe, flew over Mercury and gave the first real pictures of the planet Mercury. Scientists got many surprises from the pictures and the information gathered from Mariner 10. The pictures showed that Mercury was very similar to our Moon and that it had many craters with a rough and rocky terrain. Mariner 10 also confirmed that Mercury had a thin atmosphere and a weak magnetic field. Scientists think that the thin atmosphere is caused by the Sun boiling away the atmosphere. The weak magnetic field had scientists stumped the most. In the end it seemed that Mercury had an iron core. They also calculated that the core stretched 3/4 of the way out from the center of the planet with a 400-mile mantle of rock.

Unlike the Moon, Mercury has many hills, mountains, and escarpments. Scientists think that there was once in Mercury's lifetime, its surface contracted because of the iron core shrinking and expanding. Another thing that might have caused some of these formations is that a large rock (e.g. asteroid, meteor, etc.) hit Mercury and broke through the crust. Lava then came up and then cooled and hardened. Those shock waves stood frozen in place. Shock waves also ran through the core and must have shaken the other side of Mercury too. Scientists think that those waves caused mountains and ridges to pop up in seconds. That collision thus created the Caloris Basin, an 860-mile wide basin.

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