Mercury ☿

Mercury possesses only a thin exosphere composed primarily of oxygen, sodium, hydrogen, helium, and potassium atoms blasted off its surface by the solar wind and micrometeoroids. This wispy envelope cannot retain heat, resulting in the most extreme temperature swings of any planet -- from roughly 430 degrees Celsius on the sunlit side to -180 degrees Celsius at night.

Mercury's heavily cratered surface closely resembles Earth's Moon. The Caloris Basin, one of the largest impact craters in the solar system at 1,550 km across, was created by a massive asteroid collision about 3.9 billion years ago. The planet also has extensive cliff-like landforms called lobate scarps, some hundreds of kilometers long, formed as the planet's interior cooled and contracted.

Only two spacecraft have visited Mercury. Mariner 10 made three flybys in 1974-1975, mapping about 45 percent of the surface. NASA's MESSENGER orbited Mercury from 2011 to 2015, mapping the entire planet in detail and discovering water ice in permanently shadowed craters near the poles. The ESA-JAXA BepiColombo mission, launched in 2018, is currently en route and will enter orbit around Mercury to study its magnetosphere, surface, and interior.