Ceres

Ceres is the largest body in the asteroid belt — and the only dwarf planet inside Neptune's orbit. Discovered by Giuseppe Piazzi on January 1, 1801, it was briefly considered the eighth planet before astronomers realised it shared its orbit with thousands of similar objects. NASA's Dawn mission settled into orbit around Ceres in March 2015, returning detailed maps and discovering bright salt deposits, possible cryovolcanic features, and evidence for a subsurface brine reservoir. Ceres holds roughly a quarter of the entire asteroid belt's mass.

A rocky core wrapped in a thick mantle of water ice and hydrated minerals, possibly with a remaining liquid-brine layer just beneath the surface. About 25% of Ceres by mass is water — more fresh-water ice than all of Earth's. The crust is a complex mixture of clays, carbonates, and salts. Ceres is partially differentiated: dense rocky material settled toward the centre but not enough to form a metallic core like Earth's.

Ceres has an extremely tenuous water-vapour exosphere, detected by the Herschel Space Observatory in 2014. The vapour likely originates from sublimation of surface ice and possible cryovolcanic outgassing rather than a sustained atmosphere. ESA observed plumes coming from two specific surface points, supporting the idea that Ceres is geologically active even today.

Ceres is heavily cratered but missing the giant impact basins seen on similar-sized icy moons — its weak surface relaxes over geological time. The Occator crater, 92 km wide, contains the most famous "bright spots": sodium-carbonate salt deposits left behind when subsurface brine reached the surface and the water sublimated away. The Ahuna Mons cryovolcano stands 4 km tall — the only confirmed cryovolcano in the asteroid belt. Surface temperatures range from -105 °C average to lows below -130 °C in permanently shadowed crater floors.

NASA's Dawn spacecraft, having already orbited the asteroid Vesta from 2011-2012, arrived at Ceres in March 2015 and remained until its hydrazine ran out in October 2018. Dawn used ion propulsion — the first spacecraft ever to orbit two distinct extra-terrestrial bodies. It mapped the entire surface at multiple altitudes (the lowest at just 35 km), characterised the bright spots and cryovolcanic features, and confirmed evidence for a subsurface brine layer. Dawn now sits in a stable orbit around Ceres as a derelict.