On Mercury, a single sunrise-to-sunrise day lasts two of its years
Mercury spins three times for every two trips around the Sun, a lockstep that stretches one solar day to about 176 Earth days. Any spot on the surface bakes in sunlight for 88 days, reaching 420 degrees Celsius at the equator, then freezes in darkness for the next 88, dropping to minus 170.
The smallest planet is only 4,880 kilometres across, smaller than the moons Ganymede and Titan, though heavier than either. Its year lasts 88 Earth days. Being the planet closest to the Sun, it always appears near it in our sky as a morning or evening star; Greeks had worked out by about 350 BC that the two were one object. The Romans named it for their swift messenger god because it crosses the sky faster than any other planet. Despite its position, it is only the second-hottest planet, since it has no real atmosphere to trap heat, and craters at its poles lie in permanent shadow, where water ice may survive.
Inside, Mercury is mostly metal: roughly 70 percent metallic and 30 percent rock by composition, with an iron-rich core spanning about 2,020 kilometres in radius, some 57 percent of the planet's volume compared with 17 percent for Earth. Why it is so metal-heavy is an open puzzle. One idea is that a giant impact stripped away much of an originally larger rocky mantle; a rival idea holds that heat from the young Sun vaporized surface rock; a third is that drag in the solar nebula sorted out lighter particles. The MESSENGER orbiter, which ended in 2015, found surprising amounts of potassium and sulfur, which argue against the two violent scenarios.
The surface resembles the Moon's, with plains and dense cratering, but also long cliffs thought to have wrinkled up as the interior cooled and shrank after the crust had hardened. The largest impact basin, Caloris Planitia, spans 1,550 kilometres, about a third of the planet's width. A crater called Apollodorus, nicknamed the Spider, sends troughs radiating from its centre. Darker regions are rich in carbon, probably graphite.
Feature names follow themed rules. Craters honour artists, musicians and writers whose fame has lasted over half a century and who died at least three years earlier; escarpments take the names of ships of scientific expeditions, and mountains are named after the word hot in various languages. Mariner 10 made the first flyby in 1974, and the BepiColombo mission was scheduled to reach the planet in 2026.
Source: Mercury (planet)
