The Planets- Mercury 2

The Caloris Basin, one of the largest features on Mercury, is about 1,300 km in diameter. It was the result of an asteroid impact on the planet's surface early in the solar system's history, the probable cause of the strange surfaces on the opposite side of the planet. Over the next half-billion years, Mercury actually shrank in radius from 2 to 4 km as the planet cooled from its formation.

The outer crust, called the lithosphere, was compressed and grew strong enough to prevent the planet's magma from reaching the surface, effectively ending the planet's period of geologic activity. Evidence of Mercury's active past is seen in the smooth plains in the Caloris basin.

Mercury is the second smallest planet in the solar system, larger only than Pluto, the most distant planet in our solar system. If Earth were the size of a baseball, Mercury would be the size of a golf ball. Viewed from Mercury, the Sun would look almost three times as large as it does from Earth.

Mercury is the second densest body in the solar system after Earth, with an interior made of a large iron core with a radius of 1,800 to 1,900 km, nearly 75 percent of the planet's diameter and nearly the size of Earth's Moon. Mercury's outer shell, comparable to Earth's outer shell (called the mantle) is only 500 to 600 km thick.

Only one spacecraft has ever visited Mercury: Mariner 10 in 1974-75. Mariner 10's discovery that Mercury has a very weak magnetic field, similar to but weaker than Earth's, was a major surprise. In 1991, astronomers using radar observations showed that Mercury may have water ice at its north and south poles. The ice exists inside deep craters. The floors of these craters remain in perpetual shadow, so the Sun cannot melt the ice.

Next- Planet Mercury facts and figures part3