Geology of Ceres

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

The geology of Ceres consists of the characteristics of the surface, the crust and the interior of dwarf planet Ceres. The spectrum of Ceres is similar to that of C-type asteroids,[2] but with spectral features of carbonates and clay minerals, which are usually absent in the spectra of other C-type asteroids. Ceres is sometimes classified as a G-type asteroid. The surface of Ceres is comparable to the surfaces of Saturn's moon Rhea and Tethys, and Uranus's moon Umbriel and Oberon.

With an albedo of 0.09, Ceres's surface is quite dark compared to the moons in the outer Solar System. This might be a result of the relatively high temperature of Ceres's surface, the maximum temperature with the Sun overhead was estimated from measurements to be 235 K (approximately −38 °C, −36 °F) on 5 May 1991.[3] In a vacuum, ice is unstable at this temperature. Material left behind by the sublimation of surface ice could explain the dark surface of Ceres compared to the icy moons of the outer Solar System.

Internal structure

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.[4] This 100-kilometer-thick mantle (23%–28% of Ceres by mass; 50% by volume)[5] contains up to 200 million cubic kilometers of water, which would be more than the amount of fresh water on Earth.[6] This result is supported by the observations made by the Keck telescope in 2002 and by evolutionary modeling.[7][8] 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.[8] 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.[7][9] Measurements taken by Dawn confirm that Ceres is both differentiated and has a shape consistent with hydrostatic equilibrium,[10] which makes Ceres the smallest object confirmed to be in hydrostatic equilibrium, being 600 km smaller and less than half the mass of Saturn's moon Rhea, the next smallest such object.[11]

Internal structure of Ceres.

Orientation

Ceres has an axial tilt of about 4°,[12] 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

The craters exhibit a wide range of appearances, not only in size but also in how sharp and fresh or how soft and aged they look. Large number of Cererian craters have central pits, and many have central peaks. The central peak is like a snapshot, preserving a violent moment in the formation of the crater. By correlating the presence or absence of central peaks with the sizes of the craters, scientists can infer properties of Ceres’ 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.[13]

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.

Faculae

Several bright surface features were discovered on the dwarf planet Ceres by the Dawn spacecraft in 2015.[14] The brightest spot is located in the middle of Occator crater, and is called as "bright spot 5". 130 bright areas have been discovered on Ceres, which are thought to be salt or ammonia-rich clays.[15]

Scientists reported that the bright spots on Ceres may be related to a type of salt on 9 December 2015, particularly a form of brine containing magnesium sulfate hexahydrite (MgSO4·6H2O); the spots were also found to be associated with ammonia-rich clays.[16]

Canyons

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 has 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 as part of the devastation when a space rock crashed, rupturing the terrain.[13]

Montes

True-color image of Ceres

The best known montes on Ceres is Ahuna Mons,[17] it was discovered on images taken by the Dawn spacecraft in orbit around Ceres in 2015. Ahuna Mons is the first known montes on Ceres. The peak of Ahuna Mons is about 6 kilometers high and 15 kilometers wide at the base.

Its nature is unknown: it is not an impact feature and it appears to be the only mountain of its kind on Ceres, the tech website Cnet.com called the feature "odd."[18] Bright streaks run top to bottom on its slopes; these streaks are thought to be salt, similar to the better known Cererian bright spots, and likely resulted from cryovolcanic activity from Ceres's interior; the low crater count on Ahuna mons suggests that the cryovolcano could be no older than 200 million years. [19][20]

Another two known mons on Ceres are Liberalia Mons and Ysolo Mons. Their diameter are about 90 and 17 kilometers. Ysolo Mons is a mountain located at near Ceres' north pole.[21]

Maps

References

  1. ↑ "Dawn data from Ceres publicly released: Finally, color global portraits!". www.planetary.org. Retrieved 2016-02-04.
  2. ↑ http://irtfweb.ifa.hawaii.edu/~elv/icarus185.563.pdf
  3. ↑ Saint-Pe, O.; Combes, M.; Rigaut, F. (1993-10-01). "Ceres surface properties by high-resolution imaging from earth". Icarus. 105: 271. doi:10.1006/icar.1993.1125. ISSN 0019-1035.
  4. ↑ 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.
  5. ↑ 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 Archived October 5, 2011, at WebCite
  6. ↑ Carey, Bjorn (7 September 2005). "Largest Asteroid Might Contain More Fresh Water than Earth". SPACE.com. Archived from the original on 5 October 2011. Retrieved 16 August 2006.
  7. 1 2 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. Retrieved 7 March 2015.
  8. 1 2 Carry, Benoit; et al. (2007). "Near-Infrared Mapping and Physical Properties of the Dwarf-Planet Ceres" (PDF). Astronomy & Astrophysics. 478 (1): 235–244. arXiv:0711.1152Freely accessible. Bibcode:2008A&A...478..235C. doi:10.1051/0004-6361:20078166. Archived from the original (PDF) on 30 May 2008.
  9. ↑ 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.
  10. ↑ "DPS 2015: First reconnaissance of Ceres by Dawn".
  11. ↑
  12. ↑ http://naif.jpl.nasa.gov/pub/naif/DAWN/kernels/pck/dawn_ceres_v05.tpc
  13. 1 2 "Dawn Journal: Ceres' Intriguing Geology". www.planetary.org. Retrieved 2016-03-10.
  14. ↑ "Mysterious Bright Spots Shine on Dwarf Planet Ceres (Photos)". Space.com. Retrieved 2016-02-05.
  15. ↑ "Dawn And Ceres: A Dwarf Planet Revealed [Infographic]". Forbes. Retrieved 2016-03-27.
  16. ↑ "New Clues to Ceres' Bright Spots and Origins". NASA/JPL. Retrieved 2016-03-13.
  17. ↑ "Planetary Names: Mons, montes: Ahuna Mons on Ceres". planetarynames.wr.usgs.gov. Retrieved 2016-03-09.
  18. ↑ "NASA spies 3-mile-tall 'pyramid,' more bright spots on Ceres". CNET. Retrieved 2016-03-14.
  19. ↑ "Deep freeze puts the squeeze on dwarf planet Ceres". ASU Now: Access, Excellence, Impact. Retrieved 2016-03-09.
  20. ↑ http://www.space.com/33934-dwarf-planet-ceres-ice-volcano-discoveries.html
  21. ↑ "Dawn Team Shares New Maps and Insights about Ceres". NASA/JPL. Retrieved 2016-03-14.
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