Mercury was visited by the U. S. spacecraft Mariner 10 in 1974.

Mercury is heavily cratered, but not as cratered as the highlands of the Moon.

There are no signs of clouds, rivers, dust storms, or any other weather.

Mercury’s gravity is 2/5 that of Earth. (Moon was 1/6.)

Mercury has extensive inter-crater plains. (These are not like maria on the Moon. The plains are not as flat, and are lighter in color.)

Mercury has scarps, which are cliffs. (These are not found on the Moon.)

These scarps cut across craters, meaning they were formed after the craters.

Scarps were probably formed by cooling and shrinking of the crust.

Mercury also has “double-ringed” craters. The conditions that produced these are not known.

The Caloris Basin is a huge crater caused by asteroid impact.

The opposite side of the planet from the Caloris Basin has rippled, wavy surface features called the “Weird Terrain”.

The Weird Terrain was formed by shock waves from the impact that formed the Caloris Basin.

The daytime temperature on Mercury is about 700K. The nighttime temperature is about 100K.

This 600K variation in temperature from day to night is the largest of any planet or moon in the solar system.

Mercury has a magnetic field that is about 100 times weaker than Earth’s.

The presence of a magnetic field is surprising since the Moon, Venus and Mars have no magnetic field.

There is a question as to whether Mercury has the molten core needed to cause a magnetic field.

If there is a molten core, there has been no recent geological activity.

The ratio of core volume to total planet volume is greater than for any other object in the solar system.

Mercury does appear to be deficient in lighter rocky material.