Vesta and Ceres
- Kevin RighterKevin RighterNASA, Astromaterials Research & Exploration Science
Asteroids 1 Ceres and 4 Vesta are the two most massive asteroids in the asteroid belt, with mean diameters of 946 km and 525 km, respectively. Ceres was reclassified as a dwarf planet by the International Astronomical Union as a result of its new dwarf planet definition which is a body that (a) orbits the sun, (b) has enough mass to assume a nearly round shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a moon. Scientists’ understanding of these two bodies has been revolutionized in the past decade by the success of the Dawn mission that visited both bodies. Vesta is an example of a small body that has been heated substantially and differentiated into a metallic core, silicate mantle, and basaltic crust. Ceres is a volatile-rich rocky body that experienced less heating than Vesta and has differentiated into rock and ice. These two contrasting bodies have been instrumental in learning how inner solar system material formed and evolved.
1 Ceres was discovered by Giuseppe Piazzi, a Catholic priest, in Palermo, Italy on January 1, 1801. Ceres is named for the Roman goddess of agriculture, Cerere (in Italian; Lebofsky, 1997). 4 Vesta was discovered by Heinrich Wilhelm Olbers, a German astronomer, on March 29, 1807, and named after Vesta, the virgin goddess of home and hearth from Roman mythology (Swindle, 1997). When Ceres and Vesta were first discovered, they were originally called planets, when there were just a few large asteroids. But eventually so many such bodies were discovered that they became known as asteroids and were numbered according to the order of discovery (1 Ceres was the first, 2 Pallas the second, 3 Juno the third, 4 Vesta the fourth, and so on). Today there are >700,000 known minor planets (or asteroids) (Minor Planet Center, n.d.).
Orbital characteristics for Vesta and Ceres were determined by years of observations in the 1800s and early 1900s (Fig. 1). Vesta’s orbit, diameter, rotation, density, mass, and shape were all the focus of early observations (Table 1). Vesta’s mass was determined by gravitational perturbations caused by the close passing of asteroid 197 Arete to Vesta every 18 years (Hertz, 1968). By 1966 its mass was determined to be 1.31 × 10–10 solar masses (2.605 × 1020 kg); Dawn determined the mass to much greater precision at 2.59076(1) × 1020 (Russell et al., 2012). Together with the volume of Vesta, this demonstrated that Vesta is among the densest of all the asteroids (Swindle, 1997). In comparison, little was known about Ceres aside from its orbital elements, calculated very early by Carl Friedrich Gauss. Its diameter, mass, density, and rotation, for example, remained uncertain into the 20th century (Lebofsky, 1997).
Table 1. Comparison of Physical Properties of Ceres and Vesta
9.393 × 1020
2.59076 × 1020
Mean radius (km)
965.2 × 961.2 × 891.2
572.6 × 557.2 × 446.4
Mean density (g/cm3)
Escape velocity (km/s)
Moment of inertia factor
Orbital period (yrs)
Rotation period (days)
Note. Ceres information from Li, McFadden, & Parker (2006); Minor Planet Center; Park et al. (2016); Russell et al. (2016); and Rivkin et al. (2006). Vesta information from Minor Planet Center and Russell et al. (2012).
Vesta’s infrared (IR) spectrum has unique features that include a reflectivity peak near 0.75 microns and deep troughs near 0.9 and 2.0 microns that are due to pyroxene. It was recognized in the 1960s that Vesta’s IR spectrum is similar to that of the howardite-eucrite-diogenite (HED) clan of meteorites (e.g., McCord, Adams, & Johnson, 1970; Fig. 2A). These meteorites are described in more detail later but include dominant basaltic and pyroxenitic rocks and associated breccias, as well as minor ultramafic rocks (harzburgite and dunite), all of which come from the same body (Beck & McSween, 2010; Beck et al., 2011; Hewins & Newsom, 1988). The exciting discovery of McCord et al. (1970) meant that hundreds (>1,000 today) of meteorite samples in worldwide collections may originate from this large asteroid. This spectral link was strengthened with the discovery of a dynamic physical link between 4 Vesta and a family of asteroids, called vestoids, that can be connected to Earth’s orbit via resonances, such as the 3:1 mean motion resonance with Jupiter and the v6 secular resonance between Jupiter and Saturn (Binzel & Xu, 1993; Marzari et al., 1996; Migliorini et al., 1997; Moscovitz et al., 2008). The connection between Vesta and the HEDs was dramatically strengthened by Hubble Space Telescope images that revealed asymmetry and a potentially large 400 km crater in the south pole of 4 Vesta. The large crater still present at the south polar region (Zellner et al., 1996, 1997) led to the realization that this crater on Vesta may be the source of the many HED meteorites recovered on Earth (Drake, 2001).
The IR spectrum of Ceres, on the other hand, has reflectivity troughs at 2.7 and 3.1 microns but is otherwise featureless and flat. Ceres is not spectrally similar to any known large meteorite group in particular, but matches with a few meteorites have been suggested (Rivkin, Volquardsen, & Clark, 2006), and overall similarity to carbonaceous chondrites such as Murray, Nagoya, or QUE 97990 indicates that Ceres is likely a carbonaceous chondrite (Fig. 2B). Near IR features include 3.05 micron absorption attributed to water and other features attributed to carbonates and clays. Near IR and Hubble Space Telescope imaging revealed dark and bright spots on the surface of Ceres, of unclear origin, but Dawn spacecraft images were much more detailed and clear and allowed a more detailed identification (see later discussion; Nathues et al., 2015). Ceres may not look similar to any meteorite groups, but it appears to be a water ice bearing body that may be differentiated into rocky core and icy mantle and may have similarities to outer solar system moons or Kuiper belt objects. Spectral features and other observations, combined with available meteorite studies, are used to infer the history of Ceres for comparison to the available data for Vesta (see later discussion).
NASA’s Dawn mission revolutionized scientists’ understanding of both bodies, including documentation of morphologic features, geologic mapping, compositional and mineralogical variations at the surface, and geophysical structures (Russell et al., 2012; Russell et al., 2016). Dawn was launched in September 2007, received a Mars gravity assist in 2009, arrived at Vesta in 2011 and then at Ceres in 2015 (Russell et al., 2012; Russell et al., 2016; Fig. 3A). The Dawn spacecraft (Fig. 3B) carries three science instruments whose data were used in combination to characterize these bodies: a visible camera, a visible and IR mapping spectrometer, and a gamma ray and neutron spectrometer; the cameras alone returned numerous high resolution images of the entire surface of both bodies (Figure 4). In addition to these instruments, radiometric and optical navigation data provided data relating to the gravity field and thus bulk properties and internal structure of the two bodies.
Upon arrival at Vesta in August 2011, Dawn conducted a survey of the region around the asteroid for any possible natural satellites, dust, and debris. It then used ion propulsion to brake itself into a polar mapping orbit around Vesta. The spacecraft followed a series of circular near-polar orbits, allowing it to study nearly the entire surface of the asteroid. These different orbits varied in altitude (200 km to 2500 km) and orientation relative to the Sun to achieve the best positioning for the various observations planned. Dawn provided many new images of the surface of Vesta and spectral and elemental data for the surface (from visual and infrared spectrometer and Gamma Ray and Neutron Detector instrument [GRaND]) used to place new constraints on the formation of Vesta. For example, K/Th ratios measured by GRaND provided information about the material that grew to form the asteroids and how the composition of the solar nebula may have changed with heliocentric distance. GRaND also found hydrogen on a small area of Vesta (Prettyman et al., 2012); this is interpreted to be hydrous material from infall of carbonaceous chondrites rather than endogenous or solar wind implanted hydrogen (Prettyman et al., 2012). In addition, the large impact basin at the south pole of Vesta provided an opportunity to determine the composition of the interior of this planet, providing additional constraints on structure and thermal evolution. Surprisingly, although models of the impact basin clearly indicate it penetrated into the mantle (Jutzi, Asphaug, Gillet, Barrat, & Benz, 2013) where olivine is expected, very little olivine was observed (McSween et al., 2013). Dawn orbited Vesta for more than a year (July 2011 to September 2012) and among many other things confirmed that Vesta is the parent of the HED meteorites and differentiated into iron core, silicate mantle, and igneous crust.
While at Vesta, Dawn revealed the details of the giant impact basin at Vesta’s south pole, which is actually composed of two overlapping impact features. The younger one, Rheasilvia, is also the largest at 500 km diameter and 19 km deep, whereas the older one, Veneneia, is 400 km diameter. Both are 1 to 2 Ga in age, which is surprisingly young. Their excavated sufficient volumes of crustal material to have created the Vesta-related asteroids (Vestoids) and the HED meteorites (>1,000 recovered as of 2015; Mittlefehldt, 2015). The mineralogy of Vesta’s surface reflects the composition of the HED meteorites, with pyroxene and plagioclase feldspar-bearing eucrites, olivine and pyroxene-bearing diogenites, and howardites a brecciated mixture of the two. Dawn also constrained the size of Vesta’s metallic core to an average radius of 107 to 113 km, which suggests Vesta was molten in its early history, allowing molten metal to segregate to the core without significant interference from solid material (Russell et al., 2012). An iron metallic core of this size was also predicted by Righter and Drake (1997), who combined our chemical analyses of eucrites meteorites with knowledge of siderophile element (Ni, Co, Mo, W, and P, for example) partitioning in a model for the differentiation of Vesta and the HED parent body. The density of Vesta is nearly twice that of Ceres at 3456 kg/m3. Geologic maps of Vesta are dominated by the impact basins (small and large) the rock lithologies exposed therein and the regolith that has covered the surface of this asteroid (Fig. 5A; Williams et al., 2014).
After its departure from Vesta, Dawn entered orbit around Ceres in March 2015 and followed a similar schedule regarding its arrival at Vesta with a period of surveying orbits and detailed mapping orbits. Dawn discovered that Ceres, the only dwarf planet in the inner solar system, was never fully differentiated and is a volatile-rich world where water, ammonia, and associated silicates are abundant at the surface. While at Ceres, Dawn discovered that the surface is heavily cratered but lacks the larger crater population that might be expected (Hiesinger et al., 2016); preservation of larger craters may have been hindered by endogenous (e.g., relaxation of crust) or exogenous (e.g., variable impact flux) processes over geologic time. One of the largest and most interesting craters is the 92 km diameter Occator crater (Buzkowski et al., 2016). Within the Occator crater are bright spots that are Na-carbonate-rich material (as opposed to Mg-rich carbonate found over most of the surface), which are perhaps indicative of cryo-volcanism (Kargel, 1991; Nathues et al., 2015). Cryovolcanism includes salts and water ice and looks geologically young, suggesting present-day internal activity within Ceres (Ruesch et al., 2016). Ahuna Mons is a morphologically young feature suggestive of cryomagmatism—again evidence for young thermal and volcanic activity on Ceres (Ruesch et al., 2016). Lobate flows are found associated with crater edges and, together with the cryovolcanic, cryomagmatic, and other topographic features, suggest that the interior of Ceres is ice-rich (Buzkowski et al., 2016; Kargel, 1991). GRaND also mapped the distribution of water ice on Ceres (Prettyman et al., 2017). The density of Ceres was determined by Dawn to be 2180 kg/m3, again consistent with a less dense and ice-rich interior (Russell et al., 2016). The surface of Ceres seems to be comprised of carbonate-, serpentine-, and ammoniated clay-rich material, with magnetite, sulfide, and carbon as potential darkening agents (DeSanctis et al., 2015; McSween et al., 2017). These spectral features are not unique and in fact are similar to several other large C-class asteroids—Hygeia and Interamnia. Altogether the spectral data indicate that Ceres experienced extensive aqueous alteration (more than documented in meteorite groups with high levels of aqueous alteration; McSween et al., 2017). Geologic maps of Ceres are dominated by the relatively young impact craters, crater floors, and ejecta blankets (Fig. 5B; Mest et al., 2017).
Links to Meteorites
Eucrites are basaltic rocks comprised of pigeonite (low Ca clinopyroxene) and plagioclase feldspar, with minor phosphate, metal, troilite, silica, and ilmenite, and come in several different types based on their textures. First are basaltic textures such as coarse-grained ophitic and sub-ophitic (Fig. 6A), fine-grained aphyric, and some even vesicular. These are all referred to as the non-cumulate eucrites (e.g., Mayne, McSween, McCoy, & Gale, 2009; Takeda & Graham, 1991). Second are even coarser grained pigeonite and plagioclase rocks that have adcumulate or cumulate textures—these are classified as the cumulate eucrites. Eucrites can be unbrecciated, but most commonly they are brecciated and can be of one lithology (monomict) or many different lithologies (polymict).
Diogenites are coarse-grained orthopyroxene-rich rocks, generally ~90% orthopyroxene, and contain minor amounts of olivine, chromite, plagioclase, clinopyroxene, and opaque minerals such as troilite and metal (Mittlefehldt, McCoy, Goodrich, & Kracher, 1998). Diogenites are commonly brecciated (Fig. 6B), but there are unbrecciated samples as well (e.g., GRO 95555). Although olivine is typically <10%, there is a growing group of olivine-rich diogenites that contain up to 50% olivine and even a few dunites (Beck et al., 2011). These are of great interest to HED meteorite specialists because they may offer insight into the HED mantle (e.g., Hahn, Lunning, McSween, Bodnar, & Taylor, 2018; Lunning, McSween, Tenner, Kita, & Bodnar, 2015) or to one end-member of magmatic evolution that had olivine and orthopyroxene crystallizing together (Beck & McSween, 2010; Mittlefehldt, Beck, Lee, McSween, & Buchanan, 2012).
Many eucrites and diogenites have been metamorphosed such that their pyroxenes and plagioclases have been equilibrated and lost any compositional record of the original igneous zoning (Reid & Barnard, 1979; Takeda & Graham, 1991). These include basaltic rocks that have been recrystallized into fine-grained granulitic textures and represent metamorphosed basalts (Yamaguchi, Taylor, & Keil, 1996, 1997). Diogenites have also experienced thermal metamorphism (e.g., Mori & Takeda, 1981; Yamaguchi, Barrat, Ito, & Bohn, 2011), and this must be kept in mind when considering igneous formation models.
Finally, howardites are brecciated mixtures of diogenite and eucrite material. As one might imagine, there is some difficulty in drawing the line between a howardite, a polymict eucrite, and a brecciated diogenite with some eucritic material. As such, there are many cases in the literature of samples being characterized as “howardite or diogenite” or “howardite or polymict eucrites.” Howardites are defined as having >10% diogenite or eucrite material (Fig. 6C), and they are also known to contain foreign material such as carbonaceous chondrite clasts (Buchanan, Lindstrom, & Mittlefehldt, 2000; Buchanan & Mittlefehldt, 2003; Buchanan & Reid, 1991; Buchanan, Reid, & Schwarz, 1990; Buchanan, Zolensky, & Reid, 1993; Metzler, Bobe, Palme, Spettel, & Stöffler, 1995; Mittlefehldt & Lindstrom, 1991). These clasts may represent impact of a carbonaceous chondrite body onto Vesta, which is also supported by the Dawn GRaND hydrogen data (Prettyman et al., 2012). Some large howardite pairing groups have added to the understanding of the diversity of material found in howardites (e.g., Lunning, Welten, McSween, Caffee, & Beck, 2015; Fig. 7).
Mesosiderites are stony-iron meteorites that are mixtures of metal and brecciated silicates, with the silicates having overall similarities to HED meteorite components. Mesosiderites had in the past been linked to IIIAB irons and pallasites due to similarity of metal composition and also oxygen isotopes (Clayton & Mayeda, 1983). However, with higher resolution oxygen measurements, a distinction between mesosiderites and pallasites has been made (Greenwood, Franchi, Jambon, & Buchanan, 2005). On the other hand, O isotopes for mesosiderites overlap completely with the HED meteorites, leading some to propose that they are from the same parent body and perhaps 4 Vesta. This possibility was also supported by the occurrence of mesosiderite clasts in howardites (Rosing & Haack, 2004), but Dawn found no evidence to substantiate a link between mesosiderites and Vesta.
Some interesting and important compositional features of the HEDs include their oxygen isotopic and major element compositions. Oxygen isotopic measurements have shown that HED meteorites are offset slightly from the Earth’s “terrestrial fractionation line” that defines samples from the Earth and Moon (e.g., Clayton & Mayeda, 1983; Wiechert, Halliday, Palme, & Rumble, 2004; Fig. 8A). This indicates that Vesta accreted from nebular material that has a distinctly different composition from that which formed the Earth and Moon. Also, major elements Ti and Mg have been used to distinguish two different trends among the eucrites—the Stannern, or partial melting, trend, and the Nuevo Laredo, or fractionation, trend (Barrat et al., 2007; Stolper, 1977; Fig. 8B). These two trends can also be defined using different trace elements (e.g., Sm; Warren & Jerde, 1987).
How Did They Get to Earth?
Space exposure ages of HES meteorites have been measured using production rates of 3He, 21Ne, and 38Ar, and the exposure ages range from 5 million to 80 million years (Welten et al., 1997). This is very young compared to the age of the craters estimated from morphology and surface studies of the Dawn images. If the enormous south pole crater is truly the source of the many HEDs that have made their way to Earth, the exposure ages must reflect multiple younger events that are associated with more localized impacts that occurred during the vestoids’ dynamical journey from Vesta to an Earth-crossing orbit.
How Old Are the HEDs?
Much detailed chronology of HED meteorites has been carried out and can be coupled with the detailed geologic maps of Vesta to understand the chronology of Vesta lithologies and geologic events. Based on the analysis of both long-lived and short-lived isotopic systems, it is thought that the differentiation of the HED parent body occurred only a few million years after the formation of the CAIs and chondrules, which are thought to be the oldest solids in the solar system.
There are several parent-daughter isotope pairs with parent isotopes having long half-lives that offer useful constraints on the age of solar system materials, such as 87Rb-87Sr, 147Sm-143Nd, 176Lu-176Hf, 187Re-187Os, and the multiple pairs in the U-Th-Pb system (Carlson & Lugmair, 2000). Both cumulate and non-cumulate eucrites yield ancient ages: a Lu-Hf isochron for 18 eucrites yields an age of 4.464 Ga (Blichert-Toft, Boyet, Telouk, & Albarede, 2002), and U-Pb ages obtained on zircons yield ages from 7 to 20 Ma after T0 (Iizuka et al., 2015; Misawa, Yamaguchi, & Kaiden, 2005), where T0 is the age of the oldest solid in the solar system and thus represents the age of the solar system (T0 = 4.567.18 Ga; Amelin et al., 2010). It is generally accepted that many younger ages (<4.4 Ga) may reflect resetting due to impact or thermal metamorphism (Bogard, 1995). A clustering of Ar-Ar ages for eucrites between 4 and 3.5 Gyr is highly suggestive of a period of heavy bombardment similar to the late heavy bombardment suggested for the Moon (Ryder, 2002).
Several short-lived isotopes also offer important constraints on early differentiation events, such as the 26Al-26Mg (0.73 Ma half-life), 53Mn-53Cr (3.7 Ma half-life), 60Fe-60Ni (1.5 Ma half-life), 107Pd-107Ag 6.5 Ma half-life), and 182Hf-182W (9 Ma half-life) systems (Carlson & Lugmair, 2000). Early igneous differentiation on the HED parent body is also supported by analysis of the Al-Mg, Mn-Cr, and Hf-W systems. An Al-Mg study of the Piplia Kalan basaltic eucrite suggests possible formation 5 Myr after CAIs (Srinivasan, Goswami, & Bhandari, 1999). Hf-W systematics of eucrites demonstrate that the eucrites and diogenites formed within 2 Ma after T0—very early and quickly (Lee & Halliday, 1996; Touboul, Sprung, Aciego, Bourdon, & Kleine, 2015). Bulk rock Mn-Cr systematics of eucrites and diogenite samples define a good correlation line, indicating that the source regions for these HED meteorites were formed contemporaneously (Lugmair & Shukolyukov, 1998); they obtain a Mn-Cr model age of 4564.8 ± 0.9 Myr. Finally, 60Fe, with a half-life of 1.5 Myr, has been shown to have been extant at the time of differentiation and basalt formation on the HED parent body (Shukolyukov & Lugmair, 1993). All of these ages indicate that igneous differentiation could have occurred very early on the HED parent body. This is important as it may add validity to previous suggestions that decay of 26Al was an important heat source for melting and differentiation on asteroids.
The chronologic information obtained from both the long- and short-lived chronometers can be summarized as follows. The differentiation of the HED parent body was completed within 3 to 5 Ma after T0. Some crystallization ages of basaltic meteorites are as young as 10 to 20 Ma after T0, but ages younger than this can be attributed to resetting of various isotopic systems during impact or thermal metamorphism. Impact craters reworked the surface during and after formation of the primary zones, rearranging and mixing components, and produced many rocks with younger ages. Deposits from the largest (and perhaps earliest) craters could have buried samples deeply enough to cause heating and moderate metamorphism.
The lack of a meteorite family connection makes detailed knowledge of Ceres more of a challenge. However, the Dawn observations, together with detailed studies of carbonaceous chondrites, can lead to some important implications for Ceres. For example, the Dawn observations of ammoniated silicates, carbonates, and organics have some loose connections to mineralogy and geochemistry documented in certain carbonaceous chondrite meteorites. Carbonaceous chondrites are a large group of meteorites, all of which have high C contents. Within this group there is much diversity from primitive (unmetamorphosed) types such as CV3, CO3, CH3, CB3, and CR3, to aqueously altered types such as CM1, CM2, CI1, CR1, CR2, to thermally metamorphosed types such as CK4, CK5, and CK6 (Fig. 9). There are also breccias within the carbonaceous chondrite groups that represent surface material developed over billions of years of impacts into asteroid regoliths (Bischoff, Scott, Metzler, & Goodrich, 2006). The features observed at Ceres have connections to several carbonaceous chondrite groups.
Ammonia has been noted in carbonaceous chondrites (e.g., DuFresne & Anders, 1962; Kerridge, 1991) measured in CR chondrites and in several samples in quite large concentrations (e.g., Pizzarello & Williams, 2012; up to 18 μm/g). Its presence, along with other organic compounds such as amino and other acids, has been of intense interest among organic geochemists and in the search for understanding the origin of life. For example, one reaction pathway that can produce amino acids is called Strecker synthesis, and the presence of ammonia is required for the reaction to take place (Elsila et al., 2016). The presence in CR chondrites may help to elucidate the origin of the ammoniated silicates found at Ceres.
Similarly, CM chondrites contain carbonates, which are associated with aqueous alteration on the parent bodies of likely many parent bodies. The formation of carbonates of specific composition has been used to constrain the formation conditions—pressure, temperature, H2 and H2O fugacities—on parent asteroids (Zolotov, Mironenko, & Shock, 2006). In addition, carbonates contain Mn, and application of the Mn-Cr isotopic system has placed constraints on the age of formation of the carbonates and thus the alteration events that formed them. Ages determined using this approach are very old—15 to 20 Ma after T0—and demonstrate that such carbonates can form early on asteroid parent bodies (de Leuw, Rubin, Schmitt, & Wasson, 2009). The Mg-carbonates on Ceres may have formed this way, or they may be associated with younger geologic events. But the constraints provided from the meteorite studies are directly relatable to interpreting Ceres geology and can be very useful.
Finally, serpentine and clays have been detected in the spectra of Ceres, and the extent of alteration of Ceres has been suggested to be more extreme than what has been documented and observed in meteorite groups (McSween et al., 2017). Therefore, studies of other types of altered material have been undertaken as well in order to understand the Ceres observations. For example, hydrated interplanetary dust particles (IDPs) have experienced an equal or even greater extent of aqueous alteration than present in meteorites, and so their properties have been compared to those of materials at the surface of Ceres. However, most hydrated IDPs contain only clays and especially smectite and do not contain serpentine phyllosilicates such as those detected on Ceres (Bradley, 2003; Keller, Thomas, & McKay, 1994). So even though there are some similarities, the detailed mineralogy of hydrated IDPs appears to be distinct from Ceres.
Models for Origin
The HED meteorite clan may represent a group of meteorites all related by differentiation and igneous events early in the solar system. Understanding relations between the eucrites and diogenites has offered a testing ground for petrologists and geochemists since the early 1970s. An early model advanced for explaining the eucrites or relations between the HED meteorites is an origin by fractional crystallization (Mason, 1963; Ruzicka, Snyder, & Taylor, 1997). Although this could not connect the diogenites and eucrites, it was a way of producing a range of eucritic liquids from a more magnesian (or chondritic) parent. Later models focused on the process of partial melting of chondritic material (Consolmagno & Drake, 1977; Stolper, 1977). The idea that eucrites represented partial melts of chondrites came from the observation that many eucrites plotted near the eutectic of a chondritic composition (where “eutectic” refers to the composition of the melt that first forms when melting chondritic material), as expected of a partial melt in a multicomponent system. Again, no clear connection was made between eucrites and diogenites in partial melting models, and usually they require separate formation and origin (e.g., Shearer, Fowler, & Papike, 1997). Attempts to relate eucrites and diogenites by a single crystallization sequence, as suggested by Delaney, Prinz, and Takeda (1983) and Hewins and Newsom (1988), were supported by melting experiments on eucrites and diogenitic parent liquid at elevated pressures by Bartels and Grove (1991, 1992). These experiments showed that elevated pressure is required for a simple crystallization sequence, but of course the central pressure of Vesta is ~1.5 kb so there is a limit to the leverage pressure can have on such a small asteroid sized body. Nonetheless, this pressure range makes olivine and pyroxene in a cotectic relation (crystallization path that involves the crystallization of both olivine and pyroxene together) rather than a peritectic relation (crystallization path that involves the reaction of olivine with the melt to form pyroxene instead) boundary in a relevant depth and has become an important concept in unraveling the history of these igneous rock types. Additional experimental work and geochemical considerations such as Fe/Mn and O isotopes indicated eucrites and diogenites are likely formed from mixtures of ordinary and carbonaceous chondritic starting materials (Boesenberg & Delaney, 1997; Dreibus & Wänke, 1980; Jurewicz, Mittlefehldt, & Jones, 1995; Righter & Drake, 1997; Toplis et al., 2013). The pioneering study of Dreibus and Wänke highlighted the fact that the eucrite parent body must be volatile element depleted, relative to the Earth and other differentiated bodies, and this constraint is still one of the most important and distinctive aspects of the bulk composition of Vesta (EPB).
An unresolved issue with many of these prior formation scenarios involved adequate heat sources for the multiple melting episodes required (e.g., Hewins & Newsom, 1988). For example, core formation, eucrites genesis, and diogenite genesis could all potentially require their own heat source since they may have occurred at three different times. Righter and Drake proposed a solution to this problem by having the chondritic HED parent body start completely molten to form a metallic core, crystallize to form the diogenites and eucrites during the cooling sequence (and taking advantage of the phase equilibria changes at elevated pressure), and finally allowing later fractionation in the crust to form more evolved (Nuevo Laredo trend) eucrites (Fig. 10). This solution formed the eucrites and diogenites in the same sequence, but diogenites are not directly formed from eucritic parent melts but rather from a diogenitic parent liquid that was consumed during the equilibrium crystallization process just before lock-up in the magma ocean. Mandler and Elkins-Tanton (2013) suggested a way to make several additional HED lithologies in a series of modeling efforts designed to test the Righter and Drake model and also explain lithologies discovered since the 1997 work.
The general model of Righter and Drake (1997) was not able to explain all compositional variation represented in HED meteorites in available collections, and some notable exceptions led Barrat et al. (2007) and Barrat, Yamaguchi, Zanda, Bollinger, and Bohn (2010) to propose that assimilation of crustal materials was important to both eucrites formation and diogenite parent liquids, perhaps energetically unavoidable in large magmatic systems that are undergoing fractionation as some point, even if it is in the final stages (e.g., Carmichael, Turner, & Verhoogen, 1974, p. 68). In addition, the scenarios described here can explain many primary igneous features of eucrites and diogenites, but many of these samples have been affected by later metamorphism due either to burial and thermal conditions or to shock. The layered crust model for the HED meteorites, proposed by Takeda (1979), explains many complicated features recorded in the meteorites such as the homogeneous Mg# (Mg/(Mg+Fe) molar) of the diogenites, pyroxene exsolution features in diogenites and eucrites, and some of the younger ages obtained on eucrites by isotopic studies (e.g., Bogard & Garrison, 2003).
Dawn observations, combined with physical constraints and topographic (isostatic) modeling, indicate that Ceres likely has a crust consisting of a mixture of water ice (~25%), carbonates and phyllosilicates (30%–40%), and salts or clathrate hydrates (30%–40%) and a deeper interior that is silicate rich (Fu et al., 2017). The uppermost portion of the mantle has a lower viscosity of ~1021 (Pa s), which helps to constrain the thermal history of the mantle, the upper part of which may not have experienced metamorphic dewatering and thus did not see temperatures >600ºC. These temperature constraints help to explain why the degree of differentiation of Ceres is small, as efficient segregation of liquid, solid silicates, and any sulfide or metal will be significantly enhanced when temperature is high enough to form fluids (Ermakov et al., 2017; Fu et al., 2017; Park et al., 2016). Isostasy models suggest densities for the icy crust and rocky mantle of ~1250 and ~2400 kg m-3, respectively.
Although this detailed structure has not been reported previously for other bodies, and may make Ceres seem unique, there are several other bodies that have the same spectral features as Ceres and suggest that there may be multiple large bodies of this type.
The formation history of Ceres is actively debated. Some argue that Ceres accreted in the Kuiper Belt and migrated inward (McKinnon, 2008). Larger planetesimals like Ceres may have been scattered at the tail end of accretion by giant planets cleared planetesimal reservoirs (Grazier, Castillo-Rogez, & Sharp, 2014). Such an outer solar system origin would be consistent with the presence of ammoniated phyllosilicates and sodium carbonate (Lodders, 2003).
Alternatively, Ceres may have accreted close to its current position. Some modeling suggests that the condensation fronts of ammonia ice and water ice are nearly coincident and therefore swept into the asteroid belt (Dodson-Robinson, Willacy, Bodenheimer, Turner, & Beichman, 2009).
Finally, the ammonia in Ceres may have formed through a parent body process rather than a nebular process. In that case, there may be a large number of bodies in this part of the belt that contain the ingredients for making ammonia-bearing organics from reaction between organic precursors and ammonia hydrate during aqueous alteration processes. The chondrites also have deuterium/hydrogen (D/H) ratios consistent with formation near the snow line within the asteroid belt (Alexander et al., 2012). Doyle et al. (2015) emphasized that observations from meteorites do not provide support for dynamical models that suggest they formed beyond the giant planets and were implanted into the main belt.
Clearly there are some basic unresolved issues that will help extend the understanding of Ceres, and the new data from Dawn will surely help enhance this knowledge.
Overall, Vesta has helped explain the process of high temperature differentiation—segregation of a chondritic or ancient undifferentiated body into a dense core (metallic liquid), rocky mantle, and crust formed by melting and crystallization of the mantle (Mandler & Elkins-Tanton, 2013; Righter & Drake, 1997; Russell et al., 2012; Fig. 11A). The metallic core may even have caused an early geodynamo and magnetic field for Vesta (Fu et al., 2012)—a very important additional clue to understanding this process for all the inner solar system bodies. Through the extensive knowledge provided by the Dawn mission, coupled with the detailed studies of HED meteorites, the differentiation of Vesta is well understood, but there remain some uncertain aspects (see later discussion). Ceres has helped increase understanding of the features of a relatively undifferentiated volatile-rich body (Fig. 11B). Because researchers have no (or very few) meteorite samples from Ceres, the detailed mineralogy and formation models have been relatively qualitative. With detailed knowledge of surficial mineralogy, and geophysical constraints on the interior structure, scientists’ understanding of Ceres has improved substantially.
Comparison to Each Other and to Other Asteroids
These two asteroids are significantly different from each other and give a glimpse into two types of protoplanetary end members—one that is differentiated, volatile-poor, and experienced heating such that it was molten in its early history (Vesta) and one that is less differentiated, volatile-rich, and did not experience such extreme heating (Ceres). Ceres is nearly spherical in geometry—perhaps a reflection of its massive size and icy outer layers. Large basins are absent or perhaps relaxed due to excavation of impacts into a soft layer below the 40 to 50 km outer shell. The lack of an obvious connection to a meteorite group may be related to Ceres’ icy outer shell, which would not produce coherent strong ejecta, but could also be due to the absence of distinctive spectral features that would allow a unique connection. Vesta, on the other hand, was likely originally spherical as well but is currently asymmetric in geometry with a large south polar impact basin and an associated large meteorite group that may have originated from the giant crater.
Although Vesta formation seems well understood, there are many fine details that remain unsolved and will be debated further. Unresolved issues include:
the origin of the volatile depletions in HEDs (and thus in Vesta).
the existence of an early deep or shallow magma ocean versus more regionalized serial magmatism.
a better understanding of the lack of olivine in impact basins—is it due to incomplete understanding of differentiation or just a weak IR signal for olivine (Beck et al., 2013)?
chronologic connections between the diogenites and eucrites—because diogenites have been difficult to date due to their dominantly orthopyroxene mineralogy, there are sparse diogenite ages that are of comparable quality to those measured from eucrites (e.g., Hublet, Debaille, Wimpenny, & Yin, 2017; Schiller, Baker, & Bizzarro, 2010).
These and other problems can be addressed, especially if new samples are obtained for study. The classic falls of the HED clan had an influence on early models for the origin of these meteorites. Meteorites such as Sioux County, Juvinas, Stannern, and Johnstown were readily available and helped define many of the key issues surrounding ideas for the origin of HEDs. Subsequently, the discovery of many new HEDs in Antarctica led to detailed studies of new sample suites and a realization that simple models may not suffice. After some time it was realized also that there were fundamental differences between the collection of falls and Antarctic meteorites (Takeda, 1991). For example, the standard Stannern and Nuevo Laredo eucrite trends have been verified to some extent, but there are recently collected samples defining new trends or falling off the main trends (e.g., Ruzicka et al., 1997). With the numerous new samples coming from the hot desert localities, the field has undergone a transformation because the new samples provided additional new insights into the compositional and chemical diversity among the HED meteorites.
The type of material, in detail, that comprises Ceres remains unknown but could be revealed by further recovery and analysis of carbonaceous chondrites. Now that researchers are more certain of the kinds of volatiles and volatile-bearing minerals present on the surface of Ceres (Zolotov, 2014), modeling may commence as to the origin of Ceres and whether it formed in the main asteroid belt with other carbonaceous chondrite or in the outer solar system and migrated inward to its current location. An additional question is the nature of the heat source that drives the geologic activity on Ceres today—is it from decay of radionuclides (Bhatia & Sahijpal, 2017), or are there additional considerations? These fundamental questions remain drivers for understanding the basic nature of the inner solar system bodies and planets and make a compelling case for further study of Vesta and Ceres.
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