Before the arrival of NASA'sDawn 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 the 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' 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]
Dawn was launched in September 2007 with the mission of studying Ceres and the asteroid 4 Vesta. The spacecraft entered orbit around Vesta on July 16, 2011, and completed a 14-month survey mission before leaving for Ceres in late 2012. It went into orbit around Ceres on March 6, 2015. Dawn performed near-global geological, chemical, and geophysical mapping of Ceres[8] until its hydrazine fuel was depleted on October 31, 2018.
Dawn's scientific payload consisted of two redundant multispectral Framing Cameras (FCs),[15] a visible and infrared mapping spectrometer (VIR),[16] and a Gamma-Ray and Neutron Detector spectrometer (GRaND).[17] The radio communications subsystem was used to map Ceres' gravity field.[18] A magnetometer was originally selected for the mission but was deleted by NASA during development of the payload.[5]
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.[19] 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 minerology and chemical composition of the cerean surface. [7]
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.[11] 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.[7] The spatial resolution of the spectral images was high enough (100 meters to several kilometers per pixel)[20] to allow associations to be made between mineralogy and surface morphology, linking chemistry with geology.[19]
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.[5]
Lastly, the radio science investigation used X-bandDoppler 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.[7]
Global physical properties
Tracking of the Dawn spacecraft's orbit using radiometric Doppler and range data[21] and optical surface landmark tracking by stereophotoclinometry[22] have enabled high precision measurement of Ceres' bulk properties.[5] 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).[22]
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 differentiation[5][23] (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).
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.[24]
This 100-kilometer-thick mantle (23%–28% of Ceres by mass; 50% by volume)[25] contains up to 200million cubic kilometers of water, which would be more than the amount of fresh water on Earth.[26] 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.[27]
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.[28][29] Measurements taken by Dawn confirm that Ceres is partially differentiated and has a shape in hydrostatic equilibrium, the smallest equilibrium body known.[30] In 2020, researchers reported evidence suggesting Ceres has a brine reservoir beneath its surface, pointing to possible subsurface brine oceans.[31]
Ceres has an axial tilt of about 4°,[33] a small part of its pole is currently not observable to Dawn. Ceres rotates once every 9 hours 4 minutes in a progradewest to east direction.
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.[34]
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
Several bright surface features were discovered on the dwarf planet Ceres by the Dawn spacecraft in 2015.[35] 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.[36] 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.[37]
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 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.[34]
Montes
Notable geological features on Ceres
The most prominent mountain on Ceres is Ahuna Mons,[38] a possible cryovolcanic dome[39] 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,[40][41] and indeed models of plastic relaxation of ice at the latitude of Ahuna Mons are consistent with that age.[39]
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.[39]Yamor Mons (previously named Ysolo Mons), near the north pole, has a diameter of 16km[42] 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.[39]Liberalia Mons is near the equator and has a diameter of 90km.[43]
Related Research Articles
An asteroid is a minor planet—an object that is neither a true planet nor an identified comet— that orbits within the inner Solar System. They are rocky, metallic, or icy bodies with no atmosphere, classified as C-type (carbonaceous), M-type (metallic), or S-type (silicaceous). The size and shape of asteroids vary significantly, ranging from small rubble piles under a kilometer across and larger than meteoroids, to Ceres, a dwarf planet almost 1000 km in diameter. A body is classified as a comet, not an asteroid, if it shows a coma (tail) when warmed by solar radiation, although recent observations suggest a continuum between these types of bodies.
Vesta is one of the largest objects in the asteroid belt, with a mean diameter of 525 kilometres (326 mi). It was discovered by the German astronomer Heinrich Wilhelm Matthias Olbers on 29 March 1807 and is named after Vesta, the virgin goddess of home and hearth from Roman mythology.
Pallas is the third-largest asteroid in the Solar System by volume and mass. It is the second asteroid to have been discovered, after Ceres, and is likely a remnant protoplanet. Like Ceres, it is believed to have a mineral composition similar to carbonaceous chondrite meteorites, though significantly less hydrated than Ceres. It is 79% the mass of Vesta and 22% the mass of Ceres, constituting an estimated 7% of the mass of the asteroid belt. Its estimated volume is equivalent to a sphere 507 to 515 kilometers in diameter, 90–95% the volume of Vesta.
A terrestrial planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate, rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to the Sun: Mercury, Venus, Earth and Mars. Among astronomers who use the geophysical definition of a planet, two or three planetary-mass satellites – Earth's Moon, Io, and sometimes Europa – may also be considered terrestrial planets. The large rocky asteroids Pallas and Vesta are sometimes included as well, albeit rarely. The terms "terrestrial planet" and "telluric planet" are derived from Latin words for Earth, as these planets are, in terms of structure, Earth-like. Terrestrial planets are generally studied by geologists, astronomers, and geophysicists.
Dawn is a retired space probe that was launched by NASA in September 2007 with the mission of studying two of the three known protoplanets of the asteroid belt: Vesta and Ceres. In the fulfillment of that mission—the ninth in NASA's Discovery Program—Dawn entered orbit around Vesta on July 16, 2011, and completed a 14-month survey mission before leaving for Ceres in late 2012. It entered orbit around Ceres on March 6, 2015. In 2017, NASA announced that the planned nine-year mission would be extended until the probe's hydrazine fuel supply was depleted. On November 1, 2018, NASA announced that Dawn had depleted its hydrazine, and the mission was ended. The derelict probe remains in a stable orbit around Ceres.
A cryovolcano is a type of volcano that erupts gases and volatile material such as liquid water, ammonia, and hydrocarbons. The erupted material is collectively referred to as cryolava; it originates from a reservoir of subsurface cryomagma. Cryovolcanic eruptions can take many forms, such as fissure and curtain eruptions, effusive cryolava flows, and large-scale resurfacing, and can vary greatly in output volumes. Immediately after an eruption, cryolava quickly freezes, constructing geological features and altering the surface.
Ceres is a dwarf planet in the middle main asteroid belt between the orbits of Mars and Jupiter. It was the first known asteroid, discovered on 1 January 1801 by Giuseppe Piazzi at Palermo Astronomical Observatory in Sicily, and announced as a new planet. Ceres was later classified as an asteroid and then a dwarf planet, the only one not beyond Neptune's orbit.
A dwarf planet is a small planetary-mass object that is in direct orbit around the Sun, massive enough to be gravitationally rounded, but insufficient to achieve orbital dominance like the eight classical planets of the Solar System. The prototypical dwarf planet is Pluto, which for decades was regarded as a planet before the "dwarf" concept was adopted in 2006.
Extraterrestrial liquid water is water in its liquid state that naturally occurs outside Earth. It is a subject of wide interest because it is recognized as one of the key prerequisites for life as we know it and is thus surmised to be essential for extraterrestrial life.
The geology of solar terrestrial planets mainly deals with the geological aspects of the four terrestrial planets of the Solar System – Mercury, Venus, Earth, and Mars – and one terrestrial dwarf planet: Ceres. Earth is the only terrestrial planet known to have an active hydrosphere.
Rheasilvia is the largest impact crater on the asteroid Vesta. It is 505 km (314 mi) in diameter, which is 90% the diameter of Vesta itself, and is 95% the mean diameter of Vesta, 529 km (329 mi). However, the mean is affected by the crater itself. It is 89% the mean equatorial diameter of 569 km (354 mi), making it one of the largest craters in the Solar System, and at 75°S latitude, covers most of the southern hemisphere. The peak in the center of the crater is 200 km (120 mi) in diameter, and rises 22.5 km from its base, making it one of the tallest mountains known in the Solar System.
A planetary surface is where the solid or liquid material of certain types of astronomical objects contacts the atmosphere or outer space. Planetary surfaces are found on solid objects of planetary mass, including terrestrial planets, dwarf planets, natural satellites, planetesimals and many other small Solar System bodies (SSSBs). The study of planetary surfaces is a field of planetary geology known as surface geology, but also a focus on a number of fields including planetary cartography, topography, geomorphology, atmospheric sciences, and astronomy. Land is the term given to non-liquid planetary surfaces. The term landing is used to describe the collision of an object with a planetary surface and is usually at a velocity in which the object can remain intact and remain attached.
Comparative planetary science or comparative planetology is a branch of space science and planetary science in which different natural processes and systems are studied by their effects and phenomena on and between multiple bodies. The planetary processes in question include geology, hydrology, atmospheric physics, and interactions such as impact cratering, space weathering, and magnetospheric physics in the solar wind, and possibly biology, via astrobiology.
Several bright surface features were discovered on the dwarf planet Ceres by the Dawn spacecraft in 2015.
Ahuna Mons is the largest mountain on the dwarf planet and asteroid Ceres. It protrudes above the cratered terrain, is not an impact feature, and is the only mountain of its kind on Ceres. Bright streaks run top to bottom on its slopes which are thought to be salt, similar to the better known Cererian bright spots, and likely resulted from cryovolcanic activity from Ceres's interior. It is named after the traditional post-harvest festival Ahuna of the Sümi Naga people of India. In July 2018, NASA released a comparison of physical features, including Ahuna Mons, found on Ceres with similar ones present on Earth.
Kerwan is the largest confirmed impact basin and one of the largest geological features on the dwarf planet Ceres. It was discovered on February 19, 2015 from Dawn images as it approached Ceres. The crater is distinctly shallow for its size, and lacks a central peak. A central peak might have been destroyed by a 15-kilometer-wide crater at the center of Kerwan. The crater is likely to be young relative to the rest of Ceres's surface, as Kerwan has largely obliterated the cratering in the southern part of Vendimia Planitia.
Liberalia Mons is a mountain on the surface of the dwarf-planet Ceres.
↑ 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.
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1 2 Castillo-Rogez, J.C. et al. (2020). Ceres: Astrobiological target and possible ocean world. Astrobiology, 20(2).
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1 2 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.
↑ 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.
↑ Konoplev, 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.
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