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Geology of Ceres

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Dawn spacecraft view of Occator Crater on Ceres in enhanced color, this image was taken on 4 May 2015.[1]

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
Ceres (bottom left), the Moon and Earth, shown to scale
Relative sizes of the four largest asteroids. Ceres is furthest left.
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 reminant 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]

Internal structure

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Diagram showing a possible internal structure of Ceres

Ceres's oblateness is consistent with a differentiated body, a rocky core overlain with an icy mantle.[15]

This 100-kilometer-thick mantle (23%–28% of Ceres by mass; 50% by volume)[16] contains up to 200 million cubic kilometers of water, which would be more than the amount of fresh water on Earth.[17] Also, some characteristics of its surface and history (such as its distance from the Sun, which weakened solar radiation enough to allow some fairly low-freezing-point components to be incorporated during its formation), point to the presence of volatile materials in the interior of Ceres.[18]

It has been suggested that a remnant layer of liquid water (or muddy ocean) may have survived to the present under a layer of ice.[19][20] Measurements taken by Dawn confirm that Ceres is partially differentiated and has a shape in hydrostatic equilibrium, the smallest equilibrium body known.[21] In 2020, researchers reported evidence suggesting Ceres has a brine reservoir beneath its surface, pointing to possible subsurface brine oceans.[22]

Ceres has a rocky, dusty crust with large deposits of salts such as sodium carbonate and ammonium chloride.[23]

Internal structure of Ceres.

Orientation

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Ceres has an axial tilt of about 4°,[24] a small part of its pole is currently not observable to Dawn. Ceres rotates once every 9 hours 4 minutes in a prograde west to east direction.

Craters

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See also: List of geological features on Ceres § Craters, and List of craters on minor planets § Ceres

Impact craters on Ceres exhibit a wide range of appearances. A large number of Cererian craters have central peaks. By correlating the presence or absence of central peaks with the sizes of the craters, scientists can infer the properties of Ceres’s crust, such as how strong it is. Rather than a peak at the center, some craters contain large pits, depressions that may be a result of gases escaping after the impact.[25]

The surface of Ceres has a large number of craters with low relief, indicating that they lie over a relatively soft surface, probably of water ice. Kerwan crater is extremely low relief, with a diameter of 283.88 kilometers, reminiscent of large, flat craters on Tethys and Iapetus. It is distinctly shallow for its size, and lacks a central peak, which may have been destroyed by a 15-kilometer-wide crater at the center. The crater is likely to be old relative to the rest of Ceres's surface, because it is overlapped by nearly every other feature in the area.[citation needed]

Faculae

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Several bright surface features were discovered on the dwarf planet Ceres by the Dawn spacecraft in 2015.[26] The brightest spot is located in the middle of Occator crater, and is called "bright spot 5". There are 130 bright areas that have been discovered on Ceres, which are thought to be salt or ammonia-rich clays.[27] Scientists reported that the bright spots on Ceres may be related to a type of salt in 2015, particularly a form of brine containing magnesium sulfate hexahydrate (MgSO4·6H2O); the spots were also found to be associated with ammonia-rich clays.[28]

Canyons

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Several long canyons are evident in this view. The large crater that extends off the bottom of the picture is in the center of the picture above. Also notice the bright spots, just visible on the limb at upper left. The first picture above shows them from overhead.

Many long, straight or gently curved canyons have been found by Dawn. Geologists have yet to determine how they formed, and it is likely that several different mechanisms are responsible. Some of these might turn out to be the result of the crust of Ceres shrinking as the heat and other energy accumulated upon formation gradually radiated into space. When the behemoth slowly cooled, stresses could have fractured the rocky, icy ground. Others might have been produced when being struck by other objects, rupturing the terrain.[25]

Montes

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Notable geological features on Ceres

The most prominent mountain on Ceres is Ahuna Mons,[29] a possible cryovolcanic dome[30] about 6 kilometers high and 15 kilometers wide at the base. It was discovered on images taken by the Dawn spacecraft in orbit around Ceres in 2015.

Bright streaks run top to bottom on its slopes; these streaks are thought to contain salts, similar to the better known Cererian bright spots. The low crater count on Ahuna Mons's edifice suggests that the cryovolcano could be no older than 200 million years,[31][32] and indeed models of plastic relaxation of ice at the latitude of Ahuna Mons are consistent with that age.[30]

There are twenty-two identified montes on Ceres. Most of these have relaxed substantially over time, and it was only after modeling the expected shapes of old cryovolcanoes that they were identified. It has been calculated that Ceres averages one such cryovolcano every 50 million years.[30] Yamor Mons (previously named Ysolo Mons), near the north pole, has a diameter of 16 km[33] and is the only other Cererian mountain with the shape of Ahuna Mons, though old and battered, the cold temperatures at the pole have preserved its shape.[30] Liberalia Mons is near the equator and has a diameter of 90 km.[34]

References

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  1. ^ "Dawn data from Ceres publicly released: Finally, color global portraits!". www.planetary.org. Retrieved 4 February 2016.
  2. ^ King, T. V. V.; Clark, R. N.; Calvin, W. M.; Sherman, D. M.; Brown, R. H. (20 March 1992). "Evidence for ammonium-bearing minerals on Ceres". Science. 255 (5051): 1551–1553. Bibcode:1992Sci...255.1551K. doi:10.1126/science.255.5051.1551.
  3. ^ a b Rivkin, A. S.; Volquardsen, E. L.; Clark, B. E. (December 2006). "The surface composition of Ceres: Discovery of carbonates and iron-rich clays" (PDF). Icarus. 185 (2): 563–567. Bibcode:2006Icar..185..563R. doi:10.1016/j.icarus.2006.08.022.
  4. ^ a b McCord, T. B.; Sotin, C. (21 May 2005). "Ceres: Evolution and current state". Journal of Geophysical Research. 110 (E5). Bibcode:2005JGRE..110.5009M. doi:10.1029/2004JE002244.
  5. ^ a b c McCord, T. B.; Combe, J-P; Castillo-Rogez, J. C.; McSween, H. Y.; Prettyman, T. H. (May 2022). "Ceres, a wet planet: The view after Dawn". Geochemistry. 82 (2). Bibcode:2022ChEG...82l5745M. doi:10.1016/j.chemer.2021.125745.
  6. ^ Vernazza, 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.
  7. ^ 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.
  8. ^ Castillo-Rogez, J.C. et al. (2020). Ceres: Astrobiological target and possible ocean world. Astrobiology, 20(2).
  9. ^ Tosi, F. et al. (2015). Surface temperature of dwarf planet ceres: Preliminary results from Dawn. 46th Lunar and Planetary Science Conference, Abstract #1745
  10. ^ 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.
  11. ^ Ciarniello, M. et al. (2017). Spectrophotometric properties of dwarf planet Ceres from the VIR spectrometer on board the Dawn mission, Astronomy & Astrophysics, 598, A130.
  12. ^ 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.
  13. ^ Burbine, T.H. (1998). Could G-class asteroids be the parent bodies of the CM chondrites? Meteoritics & Planetary Science, 33, 253–258.
  14. ^ Russell, C.T. et al. (2016). Dawn arrives at Ceres: Exploration of a small, volatile-rich world. Science, 353(6303).
  15. ^ Thomas, P. C.; Parker, J. Wm.; McFadden, L. A.; et al. (2005). "Differentiation of the asteroid Ceres as revealed by its shape". Nature. 437 (7056): 224–226. Bibcode:2005Natur.437..224T. doi:10.1038/nature03938. PMID 16148926. S2CID 17758979.
  16. ^ 0.72–0.77 anhydrous rock by mass, per William B. McKinnon (2008) "On The Possibility Of Large KBOs Being Injected Into The Outer Asteroid Belt". American Astronomical Society, DPS meeting No. 40, #38.03 Bibcode:2008DPS....40.3803M
  17. ^ Carey, Bjorn (7 September 2005). "Largest Asteroid Might Contain More Fresh Water than Earth". SPACE.com. Archived from the original on 18 December 2010. Retrieved 16 August 2006.
  18. ^ Carry, Benoit; et al. (2007). "Near-Infrared Mapping and Physical Properties of the Dwarf-Planet Ceres" (PDF). Astronomy & Astrophysics. 478 (1): 235–244. arXiv:0711.1152. Bibcode:2008A&A...478..235C. doi:10.1051/0004-6361:20078166. S2CID 6723533. Archived from the original (PDF) on 30 May 2008.
  19. ^ McCord, T. B.; Sotin, C. (21 May 2005). "Ceres: Evolution and current state". Journal of Geophysical Research: Planets. 110 (E5): E05009. Bibcode:2005JGRE..110.5009M. doi:10.1029/2004JE002244.
  20. ^ O'Brien, D. P.; Travis, B. J.; Feldman, W. C.; Sykes, M. V.; Schenk, P. M.; Marchi, S.; Russell, C. T.; Raymond, C. A. (March 2015). "The Potential for Volcanism on Ceres due to Crustal Thickening and Pressurization of a Subsurface Ocean" (PDF). 46th Lunar and Planetary Science Conference. p. 2831. Retrieved 1 March 2015.
  21. ^ Lakdawalla, Emily (12 November 2015). "DPS 2015: First reconnaissance of Ceres by Dawn". The Planetary Society.
  22. ^ "NASA's Dawn Reveals Recent Changes in Ceres' Surface". NASA. 10 August 2020. Retrieved 17 July 2024.
  23. ^ "In Depth: Ceres". NASA Solar System Exploration. Retrieved 24 September 2023.
  24. ^ "Asteroid Ceres P_constants (PcK) SPICE kernel file" (txt). Retrieved 21 August 2023.
  25. ^ a b "Dawn Journal: Ceres' Intriguing Geology". www.planetary.org. Retrieved 10 March 2016.
  26. ^ "Mysterious Bright Spots Shine on Dwarf Planet Ceres (Photos)". Space.com. 18 February 2015. Retrieved 5 February 2016.
  27. ^ "Dawn And Ceres: A Dwarf Planet Revealed [Infographic]". Forbes. Retrieved 27 March 2016.
  28. ^ "New Clues to Ceres' Bright Spots and Origins". NASA/JPL. Retrieved 13 March 2016.
  29. ^ "Planetary Names: Mons, montes: Ahuna Mons on Ceres". planetarynames.wr.usgs.gov. Retrieved 9 March 2016.
  30. ^ a b c d Ceres takes life an ice volcano at a time, 2018-9-17
  31. ^ "Deep freeze puts the squeeze on dwarf planet Ceres". ASU Now: Access, Excellence, Impact. 15 December 2015. Retrieved 9 March 2016.
  32. ^ "Ice Volcanoes and More: Dwarf Planet Ceres Continues to Surprise". Space.com. September 2016.
  33. ^ "Yamor Mons". Gazetteer of Planetary Nomenclature. US Geological Survey. Retrieved 24 December 2016.
  34. ^ "Liberalia Mons". Gazetteer of Planetary Nomenclature. US Geological Survey. Retrieved 24 December 2016.

Further reading

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