On the composition and differentiation of Ceres

The dwarf planet Ceres has a density of 2040-2250 kg m⁻³, and a dark non-icy surface with signs of hydrated minerals. As opposed to a differentiated internal structure with a nonporous rocky core and a water mantle, there are arguments for undifferentiated porous interior structure. Ceres’ mass and dimensions are uncertain and do not exclude undifferentiated interior even if hydrostatic equilibrium is attained. The rocky surface may be inconsistent with a large-scale water-rock differentiation. A differentiated structure with a thick water mantle below a rocky crust is gravitationally unstable and an overturn would have led to abundant surface salt deposits, which are not observed. A formation of hydrated surface minerals caused by internal heating implies a major density increase through devolatilization of the interior. A later accumulation of hydrated materials is inconsistent with anhydrous surfaces of many asteroids and with a low rate of the cosmic dust deposition in the inner Solar System. Ceres’ internal pressures (<140-200 MPa) are insufficient to significantly reduce porosity of chondritic materials and there is no need for abundant water phases to be present to account for the bulk density. Having the porosity of ordinary chondrites (∼10%), Ceres can consist of rocks with the grain density of pervasively hydrated CI carbonaceous chondrites. However, additional low-density phases (e.g., water ice) require to be present in the body with the grain density of CM chondrites. The likely low-density mineralogy of the interior implies Ceres’ accretion from pervasively aqueously altered carbonaceous planetesimals depleted in short-lived radionuclide ²⁶Al. Abundant water ice may not have accreted. Limited heat sources after accretion may not have caused major mineral dehydration leading to formation of water mantle. These inferences can be tested with the Dawn spacecraft in 2015.     

Photometric mapping of Asteroid (4) Vesta’s southern hemisphere with Hubble Space Telescope

We present the surface mapping of the southern hemisphere of Asteroid (4) Vesta obtained from Hubble Space Telescope (HST). From 105 images of Vesta through four filters in the wavelengths best to characterize the 1-μm pyroxene band, we constructed albedo and color-ratio maps of Vesta. These new maps cover latitudes −50° to +20°. The southern hemisphere of Vesta displays more diverse albedo and color features than the northern hemisphere, with about 15 new albedo and color features identified. The overall longitudinal albedo and color variations in the southern hemisphere are comparable with that of the northern hemisphere, with a range of about ±20% and ±10%, respectively. The eastern hemisphere is brighter and displays more diogenitic minerals than the western hemisphere. Correlations between 1-μm band depth and band width, as well as between 1-μm band depth and albedo, are present on a global scale, attributed to pyroxene composition variations. The lack of correlations between albedo and the spectral slope indicates the absence of globalized space weathering. The lack of a global correlation between 1-μm band depth and topography suggests that the surface composition of Vesta is not completely controlled by a single impact. The distribution of compositional variation on Vesta suggests a possible large impact basin. Evidence of space weathering is found in regions, including the bright rim of the south-pole crater where the steepest gravitational slope on Vesta is, and a dark area near a gravitationally flat area. We propose to divide the surface of Vesta into six geological units different from the background according to their 1-μm absorption features and spectral slopes, including two eucrite-rich units, a low-Ca eucrite unit, a diogenite-rich unit, a space weathered unit, and a freshly exposed unit. No evidence of olivine-rich area is present in these data.     

A spectroscopic comparison of HED meteorites and V-type asteroids in the inner Main Belt

V-type asteroids in the inner Main Belt (a < 2.5 AU) and the HED meteorites are thought to be genetically related to one another as collisional fragments from the surface of the large basaltic Asteroid 4 Vesta. We investigate this relationship by comparing the near-infrared (0.7-2.5 μm) spectra of 39 V-type asteroids to laboratory spectra of HED meteorites. The central wavelengths and areas spanned by the 1 and 2 μm pyroxene-olivine absorption bands that are characteristic of planetary basalts are measured for both the asteroidal and meteoritic data. The band centers are shown to be well correlated, however the ratio of areas spanned by the 1 and 2 μm absorption bands are much larger for the asteroids than for the meteorites. We argue that this offset in band area ratio is consistent with our currently limited understanding of the effects of space weathering, however we cannot rule out the possibility that this offset is due to compositional differences. Several other possible causes of this offset are discussed.Amongst these inner Main Belt asteroids we do not find evidence for non-Vestoid mineralogies. Instead, these asteroids seem to represent a continuum of compositions, consistent with an origin from a single differentiated parent body. In addition, our analysis shows that V-type asteroids with low inclinations (i < 6°) tend to have band centers slightly shifted towards long wavelengths. This may imply that more than one collision on Vesta’s surface was responsible for producing the observed population of inner belt V-type asteroids. Finally, we offer several predictions that can be tested when the Dawn spacecraft enters into orbit around Vesta in the summer of 2011.     

Boulders and ponds on the Asteroid 433 Eros

There are ∼300 features on the Asteroid 433 Eros that morphologically resemble ponds (flat-floored and sharply embaying the bounding depression in which they sit). Because boulders on Eros are apparently eroding in place and because ponds with associated boulders tend to be larger than ponds without blocks, we propose that ponds form from thermally disaggregated and seismically flattened boulder material, under the assumption that repeated day/night cycling causes material fatigue that leads to erosion of the boulders. Results from a simple boulder emplacement/thermal erosion model with boulders emplaced in a few discrete events (i.e., large impacts) match well the observed size distribution. Under this scenario, the subtle color differences of ponds (somewhat bluer than the rest of the surface) might be due to some combination of less space-weathered material and density stratification of silicate-rich chondrules and more metal-rich matrix from a disaggregated boulder. Volume estimates of ponds derived from NEAR Laser Rangefinder profiles are consistent with what can be supplied by boulders. Ponds are also observed to be concentrated in regions of low slope and high elevation, which suggests the presence of a less mobile regolith and thus a contrast in the resistance to seismic shaking between the pond material and the material that makes up the bounding depression. Future tests include shake-table experiments and temperature cycling (fatigue) of ordinary chondrites to test the thermal erosion mechanism.     

Compositional heterogeneity of Asteroid 4 Vesta’s southern hemisphere: Implications for the Dawn mission

High signal-to-noise, rotationally-resolved spectra of Asteroid 4 Vesta’s southern hemisphere from the 2007 opposition were used to constrain its compositional and mineralogical variations. The spectra were rotationally-phased using closely timed HST observations of Vesta by Li et al. (Li, J.-Y., McFadden, L.A., Thomas, P.C., Mutchler, M.J., Parker, J.Wm., Young, E.F., Russell, C.T., Sykes, M.V., Schmidt, B.E. [2010]. Icarus 208, 238–251). The average surface of Vesta’s southern hemisphere is analogous to a howardite or polymict eucrite assemblage similar to the northern hemisphere, although the band parameters are distinctly shifted towards the diogenite zone on the Band–Band plot. A few distinct compositional units were detected and they might be related to albedo features detected by Hubble Space Telescope (Li et al., 2010). We have identified two compositionally distinct regions overlaying the background surface. The first unit is a polymict eucrite and/or low-Ca eucrite compositional unit at 143° longitude that border the eucrite zone on the Band–Band plot and the second is a diogenite unit at 159°. While we did not detect any distinct olivine units as suggested by Gaffey (Gaffey, M.J. [1997]. Icarus 127, 130–157), we cannot rule out the possibility of smaller olivine-rich units that are below the detection limit of the instrumentation we used. Based on the analysis and the limitations of the data, we do not suggest that Vesta’s surface is olivine-free. Mean pyroxene chemistry estimates for both hemispheres broadly agree with one another (to within one-sigma) with the northern hemisphere ferrosilite (Fs) and wollastonite (Wo) values being slightly higher than southern hemisphere.     

The far-ultraviolet albedo of Šteins measured with Rosetta-ALICE

During Rosetta’s flyby of the asteroid (2867) Šteins in 2008, we used the ALICE instrument to measure the first far-ultraviolet (FUV) reflectivity spectrum of an asteroid (850-2000 Å). It is very dark in the FUV, ∼4%, compared to its very high reflectivity (40%) at optical wavelengths. The FUV albedo does not exhibit a systematic color trend across the spectral range, but there is a broad absorption feature, not yet identified with a specific mineral, with maximum depth near 1650 Å. The shape of this feature implies a very low abundance of Fe²⁺ ions in the surface minerals. The FUV brightness exhibits a significant opposition surge at phases below 10°. The visible/FUV color gets much redder with increasing phase angle inside the opposition surge and gets gradually redder at larger phase angles. We also conducted a deep search for an exosphere of atoms sputtered from the surface and set upper limits on any column densities of oxygen and hydrogen atoms at the time of our observations. The upper limit for H is comparable to that predicted by the only theoretical prediction of which we are aware, while that for O is higher than predicted by about an order of magnitude.     

The cratering history of asteroid (2867) Steins

The cratering history of main belt asteroid (2867) Steins has been investigated using OSIRIS imagery acquired during the Rosetta flyby that took place on the 5th of September 2008. For this purpose, we applied current models describing the formation and evolution of main belt asteroids, that provide the rate and velocity distributions of impactors. These models coupled with appropriate crater scaling laws, allow the cratering history to be estimated. Hence, we derive Steins’ cratering retention age, namely the time lapsed since its formation or global surface reset. We also investigate the influence of various factors—like bulk structure and crater erasing—on the estimated age, which spans from a few hundred Myrs to more than 1 Gyr, depending on the adopted scaling law and asteroid physical parameters. Moreover, a marked lack of craters smaller than about 0.6 km has been found and interpreted as a result of a peculiar evolution of Steins cratering record, possibly related either to the formation of the 2.1 km wide impact crater near the south pole or to YORP reshaping.     

Asteroid impact in the Black Sea. Death by drowning or asphyxiation?

Normally the consequences of an impact of an asteroid in a water body are discussed in terms of the beach run up of impact-generated waves. If even a small asteroid would hit the Black Sea, however, the potential emissions of toxic and flammable gases could be more disastrous to the region (comprised of Romania, Ukraine, Russia, Georgia, Turkey and Bulgaria) than the tsunami caused by the impact.     

Spectral study of the Eunomia asteroid family : I. Eunomia

We present color ratio curves of the S-Asteroid 15 Eunomia, which have been extracted from high-precision photometric lightcurves obtained in three different VNIR wavelength bands at the Bochum Telescope, La Silla. The measured color ratio curves and near infrared spectra were used to derive a detailed surface composition model whose shape has been computed by V-lightcurve inversions. According to this analysis, the asteroid shows on one hemisphere a higher concentration of pyroxene, which causes an increased 440/700 nm and a reduced 940/700 nm reflectance ratio as well as a pronounced 2-μm absorption band. The remaining surface shows a higher concentration of olivine, leading to a reduced 440/700 nm and slightly increased 940/700 nm color ratio. In addition, we found that the maximum of the 440/700 nm color ratio curve coincide with the minimum of the 940/700 nm color ratio curve and vice versa. We demonstrate on the basis of USGS laboratory spectra that this anti-cyclical behavior can be explained by choosing Fe-rich olivine and a pyroxene with moderate Fe content as varying mineral phases. Furthermore, our observations confirm that 15 Eunomia is an irregular elongated and at least partially differentiated body. Previous spectral investigations of several smaller fragments of the Eunomia asteroid family revealed that the amount of fragments showing an increased pyroxene content exceeds the amount of pyroxene-poor fragments (Nathues, 2000, DLR Forschungsbericht, ISSN 1434-8454). This finding together with the observation that the major fraction of Eunomia's surface is enriched in olivine let us claim that a large fraction of the original pyroxene-enriched crust layer has been lost due to a major collision that created the Eunomia asteroid family. Significant spectral evidences, consistent with high concentrations of metals have been found neither in the rotational resolved spectra of 15 Eunomia nor in its fragments. This led to the conclusion that either no core consisting mainly of metals exists or that an eventual one has not been unearthed by the impact.