Ceres

Italian astronomer Giuseppe Piazzi discovered Ceres in 1801. It was initially regarded as a planet, then reclassified as an asteroid as additional bodies were found in the region between Mars and Jupiter. In 2006, the International Astronomical Union designated Ceres a dwarf planet. At about 940 kilometres across, it is the largest object in the asteroid belt.

Before the Dawn spacecraft arrived, Ceres' relatively low density and observations of water vapour suggested that it contained substantial water. Dawn's measurements of shape, gravity, topography and surface composition support a differentiated interior: denser rocky material is concentrated at depth, beneath a water-rich crust containing hydrated salts and rock. Surface ice is especially evident at high latitudes. These findings make Ceres a distinctive icy world within the asteroid belt, rather than simply a large rocky asteroid.

Dawn found many bright deposits on Ceres, often associated with impact craters. These reflective areas consist largely of salts, including sodium carbonate. In Occator Crater, analysis of gravity and surface data indicates that brine from a deep reservoir contributed to the deposits. The crater's bright regions include Cerealia Facula and Vinalia Faculae. Researchers infer that impact heating and fractures helped bring salty liquid toward the surface; young salt deposits and hydrated salts suggest that geological processes there occurred relatively recently and may have persisted after the impact.

NASA's Dawn spacecraft entered orbit around Ceres in March 2015 and concluded its mission in November 2018 after completing its planned observations and extended missions. Dawn mapped the dwarf planet's surface, composition and gravity, revealing an internally differentiated, water-rich body with signs of activity. The mission showed that geological processes sustained by brines can occur on a small icy world without the strong tidal heating experienced by some large planetary moons. This has informed broader study of icy bodies, while the exact history and present activity of Ceres remain subjects of scientific investigation.