Solar System information #3: Mercury

Mercury - closest plant to the Sun

Mercury is the closest planet to the Sun but not the hottest (Venus is the hottest). It is a small rocky planet that looks similar to our Moon because its surface is heavily cratered. However, Mercury's craters are slightly flatter than the Moon's - its stronger gravity may have caused the crater rims to slump more; or erosion may have occurred long ago, if Mercury had a thicker atmosphere in the past. The craters are believed to be mainly from impacts of interplanetary debris during a much earlier stage in the life of our solar system.

Currently, Mercury has an almost negligible atmosphere - only a few billionths of the density of Earth's. The atmosphere consists mainly of sodium vapour, perhaps ejected from the surface by meteorite impacts. There are also small amounts of helium and hydrogen from the solar wind. As for the body of Mercury itself, data from the Mariner 10 spacecraft indicate that this small planet has an unexpectedly large metallic core (probably based on iron) and a rocky crust.

Mercury is much nearer to the Sun than the Earth is, and travels in an oval-shaped orbit. It is normally too close to the Sun to be easily seen from Earth. Even when it is visible, it is not far above the horizon - so the light coming from Mercury has to pass obliquely through the Earth's atmosphere and it is near impossible to obtain a clear image. Also, Mercury's surface is made of relatively dark material and reflects only 11% of the sunlight that falls on it.

Most of the knowledge about mercury comes from the Mariner 10 spacecraft. It passed as close as 10,000km above Mercury in 1974 and 1975, and the on-board TV cameras took some 4000 pictures which were then transmitted to Earth. The Mariner pictures show a large meteorite impact crater called the Caloris Basin (1300km diameter), covering nearly one-tenth of the planet's surface. Another feature that stands out is lines of cliffs (scarps) hundreds of kilometres long. In 2004, NASA launched Messenger to Mercury, and it has already made three close ‘fly-by’ passes, preparing to enter orbit around Mercury in 2011. The European Space Agency (ESA) hopes to launch a mission to Mercury in 2012.