Geology of Ceres

The geology of Ceres is the scientific study of the surface, crust, and interior of the dwarf planet Ceres. It seeks to understand and describe Ceres' composition, landforms, evolution, and physical properties and processes. The study draws on fields such as geophysics, remote sensing, geochemistry, geodesy, and cartography (see Planetary geology).

Before the arrival of NASA's Dawn spacecraft in 2015, knowledge of Ceres' geology was limited to spectroscopic studies from earth-orbital and ground-based telescopes, which tentatively identified the dwarf planet's overall surface composition.^[2][3] Thermodynamic models of Ceres’ interior and evolution were also constructed based on properties such as its shape and bulk density.^[4] Data from the Dawn mission not only confirmed many of the results of earlier studies, but dramatically increased our understanding of Ceres’ composition and evolution,^[5] moving it from a largely astronomical object to a geological one.

At a diameter of 964 km, Ceres is the largest object in the main asteroid belt and comprises about one-third of the belt's total mass. Ceres possesses sufficient gravity to form a rounded, ellipsoid shape, suggesting that it is close to being in hydrostatic equilibrium^[6]—one of the conditions for defining a dwarf planet according to the International Astronomical Union (IAU).

Though large relative to asteroids, Ceres is small compared with many other solid bodies in the solar system. For example, it is only 28% the size of Earth's moon and 41% that of Pluto, another dwarf planet. It is comparable in size to Saturn's moons Tethys and Dione. Ceres’ small size means that it cooled much faster than full-sized planets and larger moons, limiting its degree of thermal evolution.^[7]

Ceres (bottom left), the Moon and Earth, shown to scale

Relative mean diameters of the four largest minor planets in the asteroid belt (dwarf planet Ceres at left)

Ceres’ mean density of 2,162 kg/m³ is midway between rock (~3,000 kg/m³) and ice (~1,000 kg/m³). This implies that water in some form makes up 17–27% of its total mass.^[4] The water is present both as ice and in hydrated and hydroxylated minerals. Being the most water-rich body in the inner solar system after Earth, Ceres is believed to have once hosted a subsurface ocean,^[8] the remnant of which may still exist as a global reservoir or as pockets of brines (salty water) at depth.^[5] The presence of liquid water has astrobiological significance as any extant water may provide a habitat for life.

Ceres orbits the sun at a mean distance of 2.77 astronomical units (AU), near the center of the asteroid belt. It receives only 15% of the solar energy as Earth and has a maximum daytime temperature at the equator of 235 K (−38° C).^[9] This temperature is still high enough that surface ice is not stable and tends to sublimate away over geologic timescales.^[10]

Ceres is a dark object, having a geometric albedo of 0.094,^[11] meaning that on average its surface reflects only 9% of the sunlight striking it. The composition of the material contributing to the low albedo remains uncertain, but graphitized carbon compounds or the mineral magnetite have been suggested.^[5]

Ceres has spectral similarities to C-type asteroids,^[3] which are rich in volatiles and carbonaceous compounds. Ceres is also sometimes classified as a G-type asteroid,^[12][13] which is a subtype of the Tholen C-class and characterized by abundant phyllosilicates, such as clay minerals. Ceres is not associated with any asteroid family or known meteorites.^[14]