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. Thermodynamic models of Ceres' interior and evolution were also constructed based on properties such as its shape and bulk density. Data from the Dawn mission not only confirmed many of the results of earlier studies, but dramatically increased the understanding of Ceres' composition and evolution, 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 equilibriumVernazza, P.; Usue, F.; Hasegawa, S. (2022). "Remote Observation of the Main Belt" in Vesta and Ceres: Insights from the Dawn Mission for the Origin of the Solar System, Marchi, S., Raymond, C.A., Russell, C.T., eds., Cambridge University Press: Cambridge U.K., 266 pp.—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.Li, J-Y. and Castillo-Rogez, J.C. (2022). Chapter 3: "Dawn Mission Overview" in Ceres: An Ice-rich World in the Inner Solar System (Advances in Planetary Science Volume 6), World Scientific Publishing, 256 pp.
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. The water is present both as ice and in hydrated minerals. Being the most water-rich body in the inner Solar System after Earth, Ceres is believed to have once hosted a subsurface ocean,Castillo-Rogez, J.C. et al. (2020). Ceres: Astrobiological target and possible ocean world. Astrobiology, 20(2). the remnant of which may still exist as a global reservoir or as pockets of brines (salty water) at depth. 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 243 K (−30 °C). This temperature is still high enough that surface ice is not stable and tends to sublimate away over geologic timescales.Hayne, P.O. and Aharonson, O. (2015). Thermal stability of ice on Ceres with rough topography. Journal of Geophysical Research: Planets, 120, 1567–1584, doi:10.1002/2015JE004887.
Ceres is a dark object, having a geometric albedo of 0.094,Ciarniello, M. et al. (2017). "Spectrophotometric properties of dwarf planet Ceres from the VIR spectrometer on board the Dawn mission", Astronomy & Astrophysics, 598, A130. 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.
Ceres has spectral similarities to C-type asteroids, which are rich in volatiles and carbonaceous compounds. Ceres is also sometimes classified as a G-type asteroid,Germann, J.T.; Fieber-Beyer, S.K.; Gaffey, M.J. (2022). Evidence for hydrated minerals in the VNIR spectra of G-class asteroids: A first look. Icarus, 377, 114916.Burbine, T.H. (1998). Could G-class asteroids be the parent bodies of the CM chondrites? Meteoritics & Planetary Science, 33, 253–258. 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.Russell, C.T. et al. (2016). Dawn arrives at Ceres: Exploration of a small, volatile-rich world. Science, 353(6303).
Dawn mission
Dawn was launched in September 2007 with the mission of studying Ceres and the asteroid 4 Vesta. The spacecraft entered orbit around Vesta on 16 July 2011, and completed a 14-month survey mission before leaving for Ceres in late 2012. It went into orbit around Ceres on 6 March 2015. Dawn performed near-global geological, chemical, and geophysical mapping of Ceres until its hydrazine fuel was depleted on 31 October 2018.
Dawns scientific payload consisted of two redundant multispectral Framing Cameras (FCs),Sierks, H. et al. (2011). The Dawn Framing Camera. Space Science Reviews, 163, 263–327, doi 10.1007/s11214-011-9745-4. a visible and infrared mapping spectrometer (VIR),De Sanctis, M.C. et al. (2011). The VIR Spectrometer. Space Science Reviews, 163, 329–369 and a Gamma-Ray and Neutron Detector spectrometer (GRaND).Prettyman, T.H. et al. (2011). Dawn's Gamma Ray and Neutron Detector. Space Science Reviews, 163, 371–459 The radio communications subsystem was used to map Ceres' gravity field.Konoplev, A.S. et al. (2011). The Dawn gravity investigation at Vesta and Ceres. Space Science Reviews, 163, 461–486. A magnetometer was originally selected for the mission but was deleted by NASA during development of the payload.
During the primary mission, the FC mapped nearly the entire surface of Ceres at a spatial resolution of 35 m/pixel in the visible channel and 135 m/pixel in color.McCord, T.B. and Castillo-Rogez, J.C. (2018). Ceres's internal evolution: The view after Dawn. Meteoritics & Planetary Science, 53(9), 1778–1792, doi: 10.1111/maps.13135. An 8-position filter wheel permitted panchromatic (clear filter) and spectrally selective imaging (7 narrow band color filters). The broadest filter allowed imaging at wavelengths from 400 to 1050 nm. The main science objectives of the FC were to determine Ceres' physical properties, such as rotational state and global shape, to image surface geomorphology, and to produce high-resolution digital terrain models. Multicolor imaging, in conjunction with the VIR (below), helped in mapping the mineralogy and chemical composition of the Cerean surface.
The VIR was a hyperspectral spectrometer with imaging capability that obtained reflectance spectra of the surface at wavelengths between 0.25 and 5 μm. It used two channels covering the visible (VIS, 0.25–1.05 μm) and infrared (IR, 1–5.1 μm) ranges, with a spectral sampling of 1.8 nm and 9.5 nm, respectively. Its scientific objective was to determine the mineralogy of surface materials through the diagnostic absorption features of common rock-forming minerals. The diagnostic features of minerals expected on Ceres include the 1 μm and 2 μm mafic bands of pyroxene and olivine, the 3 μm water region of the hydration band, the 1.5, 2.0, and 3.0 μm bands of water ice, the 3.9 μm carbonate band, and the 3.2–3.6 μm C–H stretching bands of organic material. The spatial resolution of the spectral images was high enough (100 meters to several kilometers per pixel)De Sanctis, M.C. and Raponi, A. (2022). "Ceres' Surface Composition." in Vesta and Ceres: Insights from the Dawn Mission for the Origin of the Solar System, Marchi, S., Raymond, C.A., Russell, C.T., Eds., Cambridge University Press: Cambridge, U.K., 266 pp. to allow associations to be made between mineralogy and surface morphology, linking chemistry with geology.
The GRaND spectrometer measured elemental abundances on a regional to global scale by detecting an element's characteristic gamma ray emissions and neutron radiation activated by high energy cosmic rays. Elements measured included carbon, iron, hydrogen (a proxy for water), potassium, and other silicate-forming elements occurring within approximately the upper meter of the surface. Although spatial resolution was limited, the GRaND instrument's elemental abundances proved complementary with the VIR-derived mineralogy because it was able to detect elements in the shallow subsurface and in polar areas where sunlight was insufficient for the VIR spectrometer.
Lastly, the radio science investigation used X-band Doppler tracking and landscape tracking from optical instruments to determine Ceres' gravity field to high precision. Gravity field measurements, along with a shape model, allowed for estimations of internal mass distribution.
Global physical properties Tracking of the Dawn spacecraft's orbit using radiometric Doppler and range dataKonoplev, A.S. et al. (2018). The Ceres gravity field, spin pole, rotation period and orbit from the Dawn radiometric tracking and optical data. Icarus, 299, 411–429. and optical surface landmark tracking by stereophotoclinometryPark, R.S. et al. (2019). High-resolution shape model of Ceres from stereophotoclinometry using Dawn Imaging Data. Icarus, 319, 812–827. have enabled high precision measurement of Ceres' bulk properties. These properties include a total mass of 9.3833599×1020 kg and a rotational rate of 952.1532635 deg/day (period of rotation = 9.074170 hours).
Ceres' shape is controlled mainly by gravity and spin, with only a 3% departure from hydrostatic equilibrium. Its best-fit shape is a triaxial ellipsoid with dimensions a = 483.1 km, b = 481.0, km and c = 445.9 km, with c being the north-south axis and a and b the semimajor and semiminor equatorial axes. Combining total mass with volume gives a bulk density of 2,162 kg/m3. Gravity data suggests that Ceres has a mean dimensionless moment of inertia (I/MR2) value of about 0.37 indicating some amount of internal differentiationPark, R.S. et al. (2016). A partially differentiated interior for (1) Ceres deduced from its gravity field and shape. Nature, 537, 515–517. (a spherical body with a uniform density throughout has a moment of inertia of 0.40 while a body whose mass is mostly concentrated near the center has a moment of inertia closer to 0.30).
Surface composition During its primary mission, Dawn identified and mapped the mineralogy of almost the entire surface of Ceres. The key instrument for this effort was the VIR imaging spectrometer, but multicolor information from the FC and elemental composition derived from the GRaND instrument played complementary roles.
A sharp absorption feature in the infrared reflectance spectrum at 2.72 μm is characteristic of minerals containing hydroxyl (OH). Comparisons with laboratory spectra indicate that this absorption feature is likely due to the presence of magnesium (Mg)-bearing phyllosilicates such as antigorite (Mg-serpentine) or saponite (Mg-smectite).De Sanctis, M.C. et al. (2018). Ceres’s global and localized mineralogical composition determined by Dawn’s visible and infrared spectrometer (VIR). Meteoritics & Planetary Science, 53(9), 1844–1865. Another prominent band at 3.06 μm is attributed to ammoniated (NH4+-bearing) phyllosilicates. The specific phyllosilicate containing the ammonium ion is uncertain but is likely some form of Mg-saponite.