Mercury
Mercury is the smallest planet and the closest to the Sun, heavily cratered and with a huge iron core.
Mercury is the innermost planet, orbiting the Sun every 88 days. It is about 4,880 kilometres across, only a little larger than our Moon, and has almost no atmosphere. Days are scorching, around 430 °C, and nights fall to about −180 °C.
Mercury's surface is heavily cratered, much like the Moon's, with the giant Caloris impact basin about 1,550 kilometres across. Its iron core is enormous, making up about 85 per cent of its radius. One idea is that a giant impact stripped away much of its rocky mantle.
No meteorite has been confirmed as coming from Mercury, partly because rock blasted off Mercury would tend to fall inwards towards the Sun. Some researchers have suggested that unusual meteorites such as certain aubrites or the ungrouped meteorite NWA 7325 might be Mercurian, but these ideas are not widely accepted. The European–Japanese BepiColombo mission will reach Mercury orbit in late 2026.
Related terms
Level: Beginner Ages 7 and up