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NASA NTRS2017

Core Formation on Asteroid 4 Vesta: Iron Rain in a Silicate Magma Ocean

Geochemical observations of the eucrite and diogenite meteorites, together with observations made by NASA's Dawn spacecraft, suggest that Vesta resembles H chondrites in bulk chemical composition, possibly with about 25% of a CM-chondrite like composition added in. For this model, the core is 15% by mass (or 8 volume %) of the asteroid. The abundances of moderately siderophile elements (Ni, Co, Mo, W, and P) in eucrites require that essentially all of the metallic phase in Vesta segregated to form a core prior to eucrite solidification. Melting in the Fe-Ni-S system begins at a cotectic temperature of ~940 deg. C. Only about 40% of the total metal phase, or 3-4 volume % of Vesta, melts prior to the onset of silicate melting. Liquid iron in solid silicate initially forms isolated pockets of melt; connected melt channels, which are necessary if the metal is to segregate from the silicate, are only possible when the metal phase exceeds about 5 volume %. Thus, metal segregation to form a core does not occur prior to the onset of silicate melting.

Kiefer, Walter S.↗
NASA NTRS1980

Radar detection of Vesta

Asteroid 4 Vesta was detected on November 6, 1979 with the Arecibo Observatory's S-band (12.6-cm-wavelength) radar. The echo power spectrum, received in the circular polarization opposite to that transmitted, yields a radar cross section of (0.2 + or - 0.1)pi a-squared, for a = 272 km. The data are too noisy to permit derivation of Vesta's rotation period.

Ostro, S. J.↗
NASA NTRS2012

Overview of Vesta Mineralogy Diversity

4 Vesta is known to have a surface of basaltic material through visible/near-infrared reflectance spectroscopy (1). Vesta s spectrum has strong absorption features centered near 0.9 and 1.9 m, indicative of Fe-bearing pyroxenes. The spectra of HED (howardite, eucrite and diogenite) meteorites have similar features (1). This led to the hypothesis that Vesta was the parent body of the HED clan (2,3) and the discovery of a dynamical Vesta family of asteroids (Vestoids) provides a further link between Vesta and HEDs (4). Data from the Dawn VIR (Visible InfraRed mapping Spectrometer) (5) characterize and map the mineral distribution on Vesta, strengthen the Vesta - HED linkage and provide new insights into Vesta s formation and evolution.

DeSanctis, M. C.↗
NASA NTRS2017

Volatile Concentrations and H-Isotope Composition of Unequilibrated Eucrites

Eucrites are among the oldest and best studied asteroidal basalts (1). They represent magmatism that occurred on their parent asteroid, likely 4-Vesta, starting at ~ 4563 Ma and continuing for approx. 30 Myr. Two hypotheses are debated for the genesis of eucrites, a magma ocean model (2), and a mantle partial melting model. In general, volatiles (H, C, F, Cl) have been ignored for eucrites and 4-Vesta, but solubility of wt% levels of H2O are possible at Vestan interior PT conditions. Targeted measurements on samples could aid our understanding considerably. Recent studies have found evidence of volatile elements in eucrites, but quantifying the abundance of volatiles remains problematic (6). Volatile elements have a disproportionately large effect on melt properties and phase stability, relative to their low abundance. The source of volatile elements can be elucidated by examining the hydrogen isotope ratio (D/H), as different H reservoirs have drastically different H isotope compositions. Recent studies of apatite in eucrites have shown that the D/H of 4-Vesta matches that of Earth and carbonaceous chondrites, however, the D/H of apatites may not represent the D/H of a primitive 4-Vesta melt due to the possibility of degassing prior to the crystallization of apatite. Therefore, the D/H of early crystallizing phases must be measured to determine if the D/H of 4-Vesta is equal to that of the Earth and carbonaceous chondrites.

Sarafian, Adam R.↗
NASA NTRS1974

Radiometric diameters and albedos of 40 asteroids

The radiometric method of measuring the albedos and sizes of small airless objects was applied to forty asteroids. The largest asteroids were revealed to be 1 Ceres, 2 Pallas, 4 Vesta, 10 Hygiea, and 511 Davida. The highest albedo belonged to 4 Vesta. The darkest asteroids were 19 Fortuna, 324 Bamberga, and 747 Winchester. A comparison of the derived diameters and albedos with photometrically measured colors reveals three distinct classes of objects. The largest group is those with red color and albedos in the lunar range. The second group, which includes most of the largest objects, is characterized by more neutral color and lower albedo. The third class has one member, 4 Vesta, an object of high albedo and intermediate color. Objects in the first group have surfaces composed presumably of silicate minerals. The second and third groups may be similar in composition to the carbonaceous chondrites and the basaltic achondrite meteorites, respectively.

Morrison, D.↗
NASA NTRS2009

Spitzer IRS (8-30 micron) Spectra of Basaltic Asteroids 1459 Magnya and 956 Elisa: Mineralogy and Thermal Properties

We report preliminary results from Spitzer IRS (Infrared Spectrograph) spectroscopy of 956 Elisa, 1459 Magnya, and other small basaltic asteroids with the Spitzer IRS. Program targets include members of the dynamical family of the unique large differentiated asteroid 4 Vesta ("Vestoids"), several outer-main-belt basaltic asteroids whose orbits exclude them from originating on 4 Vesta, and the basaltic near-Earth asteroid 4055 Magellan. The preliminary thermal model (STM) fit to the 5--35 micron spectrum of 956 Elisa gives a radius of 5.4 +/- 0.3 km and a subsolar- point temperature of 282.2 +/- 0.5 K. This temperature corresponds to eta approximately equals 1.06 +/- 0.02, which is substantially higher than the eta approximately equals 0.756 characteristic of large main-belt asteroids. Unlike 4 Vesta and other large asteroids, therefore, 956 Elisa has significant thermal inertia in its surface layer. The wavelength of the Christiansen feature (emissivity maximum near 9 micron), the positions and shapes of the narrow maxima (10 micron, 11 micron) within the broad 9--14 micron silicate band, and the 19--20 micron minimum are consistent with features found in the laboratory spectra of diogenites and of low-Ca pyroxenes of similar composition (Wo<5, En50-En75).

Lim, Lucy F.↗
NASA NTRS2019

Elemental Variation and Petrogenesis of Pyroxenes in HED Meteorites by Multivariable Discriminant Analysis Method

HED (Howardite, Eucrite and Diogenite) are meteorites with mafic and ultramafic igneous composition. Previous studies suggested HED came from asteroid (4) Vesta and they were generated by magmatic melting followed by differentiation crystallization, metamorphic, and impact. Uniform oxygen isotopic composition of HED samples favors global magma ocean for (4) Vesta. However, the petrological diversity of HED may indicate more complex magma processes in the meteorites. In general, most of the geochemical studies are using traditional methods (e.g. element to element or ratio to ratio plots) to classify the different rock type and groups. With the traditional methods, only limited elements can be shown in a figure. As such, some meteorites may have been identified as HED by a traditional method but found different either in isotopic composition or other elemental characteristic. These anomalous HED meteorites may or may not come from asteroid (4) Vesta. In fact, magma is a unit system where any elemental change should affect to all other elements as a whole, it would be reasonable to consider all elements together to look for the systematical changes. Using multivariable discriminant analysis (MDA) method is one of such testing for their geochemical variation. The method may be able to help us to better understand the petrologic processes and the relationship among elements. This study is to test the MDA method by focus mainly on pyroxene composition of eucrite and diogenite.

Peng, Z. X.↗
NASA NTRS1977

Vesta - The first pyroxene band from new spectroscopic measurements

Spectral reflectance measurements of asteroid 4 Vesta were obtained using a silicon vidicon spectrometer with a resolution of 2 to 4 thousandths of a micron. A flow chart illustrating the data reduction procedure is given, and the location, absorptivity, and bandwidth of an absorption band detected in the near infrared region are described. The absorption band has been interpreted as due to electronic absorptions in ferrous iron in sixfold coordination in the pyroxene, pigeonite. The heightened spectral resolution obtained from the silicon vidicon imaging system permits determination of the relative calcium content of the pyroxene (10 to 12%). Furthermore, the symmetry of the absorption band implies an absence of olivine, which is commonly associated with pyroxene in terrestrial and meteoritic mineral samples. Support for these conclusions is found in spectral analyses of pyroxene, as well as in spectral reflectance measurements of 4 Vesta obtained by other observers.

Mcfadden, L. A.↗
NASA NTRS2008

Spitzer IRS Spectra of Basaltic Asteroids: Preliminary Results

We present preliminary results of a Spitzer program to observe the 5.2--38 micron spectra of small basaltic asteroids using the Spitzer IRS (Infrared Spectrograph). Our targets include members of the dynamical family of the unique large differentiated asteroid 4 Vesta ("Vestoids"), four outer-main-belt basaltic asteroids whose orbits exclude them from originating on 4 Vesta, and the basaltic near-Earth asteroid (NEA) 4055 Magellan. We will compare the compositions and thermophysical properties of the non-Vestoid objects with those of the dynamical vestoids to provide insight on the extent of metal-silicate differentiation on planetsimals during the epoch of planet formation in the early Solar System. As of this writing, spectra of asteroids 10537 (1991 RY16) and 2763 Jeans have been returned. Analysis of these data are ongolng. Observations of 956 Elisa, 2653 Principia, 4215 Kamo, 7472 Kumakiri, and 1459 Magnya have been scheduled and are expected to be available by the time of the DPS meeting. NIR spectra and lightcurves o f the target asteroids are also being observed in support of this program.

Lim, Lucy F.↗
NASA NTRS2009

Diogenite-like Features in the Spitzer IRS (5-35 micrometers) Spectrum of 956 ELISA

We report preliminary results from the Spitzer Infrared Spectrograph (IRS) observations of the V-type asteroid 956 Elisa. Elisa was observed as part of a campaign to measure the 5.2-38 micron spectra of small basaltic asteroids with the Spitzer IRS. Targets include members of the dynamical family of the unique large differentiated asteroid 4 Vesta ("Vesroids"), several outer-main-belt basaltic asteroids whose orbits exclude them from originating on 4 Vesta, and the basaltic near-Earth asteroid 4055 Magellan.

Lim, Lucy F.↗
NASA NTRS2012

The Vestal Cataclysm

The currently operating Dawn mission shows asteroid 4 Vesta to be an extensively cratered body, with craters in a variety of morphologies and preservation states The crater size-frequency distribution for Vesta, modeled using the lunar chronology and scaled to impact frequencies modeled for Vesta, shows that both the north and south pole areas are ancient in age [1]. We have in our meteorite collection products from 4 Vesta in the form of the HED (howardite, eucrite, diogenite) meteorites. The HED parent body globally differentiated and fully crystallized by approx.4.56 Ga; subsequently, the eucrites were brecciated and heated by large impacts into the parent body surface, reflected in their disturbance ages [2, 3]. Dawn images have also shown that Vesta is covered with a well-developed regolith that is spectrally similar to howardite meteorites [4, 5]. Howardites are polymict regolith breccias made up mostly of clasts of eucrites and diogenites, but which also contain clasts formed by impact into the regolith. Impact-melt clast ages from howardites extend our knowledge of the impact history of Vesta, expanding on eucrite disturbance ages and helping give absolute age context to the observed crater-counts on Vesta.

Cohen, Barbara A.↗
NASA NTRS2014

Bunburra Rockhole: Exploring the Geology of a New Differentiated Basaltic Asteroid

Bunburra Rockhole (BR) is the first recovered meteorite of the Desert Fireball Network. It was initially classified as a basaltic eucrite, based on texture, mineralogy, and mineral chemistry but subsequent O isotopic analyses showed that BR's composition lies significantly far away from the HED group of meteorites. This suggested that BR was not a piece of the HED parent body (4 Vesta), but other explanations could also account for the observed oxygen signatures. Possible scenarios include contamination by components from other bodies (chondrites or other achondrites) or that 4 Vesta may not be as equilibrated as hypothesized. After examining multiple pieces with different instruments (CT scans and x-ray maps), no obvious evidence of contamination was found. If BR is not from Vesta, a conundrum exists as no unusual features were found in mineral and bulk trace element chemistry as exist for other anomalous basaltic achondrites such as Ibitira or Asuka 881394. These meteorites have distinct petrological and geochemical characteristics, in addition to their anomalous O isotope compositions, that set them apart from eucrites. Thus, early results provided a somewhat ambiguous picture of BR's petrogenesis and parentage. To clarify the nature of the relationship, if any, between BR and eucrites, we have performed a correlated stable isotope and bulk chemical study of several lithologic fragments.

Benedix, G.K.↗
NASA NTRS1979

Albedo and color contrasts on asteroid surfaces

Asteroids in general display only small or negligible variations in spectrum or albedo during a rotational cycle. Color variations with rotation are described in the literature but are usually comparable to the noise in the measurements. Twenty-four asteroids have been systematically monitored for such color changes. Only 3 Juno, 4 Vesta, 6 Hebe, 71 Niobe, 349 Dembowska, and 944 Hidalgo display color variations larger than 0.03 mag. In each of these cases the asteroid appears redder near maximum brightness. Of seven asteroids monitored polarimetrically, only 4 Vesta shows a convincing variation, attributed to an albedo change with rotation. The lightcurve can be explained by albedo differences alone; Vesta apparently has a nearly spheroidal shape. Nothwithstanding the above results, the degree of uniformity of most asteroid surfaces is remarkable. If asteroids exist with large discrete domains of ferrosilicate, metallic, and/or carbonaceous material together on their surfaces, they have not yet been identified.

Degewij, J.↗
NASA NTRS2009

Chemistry of Diogenites and Evolution of their Parent Asteroid

Diogenites are orthopyroxenite meteorites [1]. Most are breccias, but remnant textures indicate they were originally coarse-grained rocks, with grain sizes of order of cm. Their petrography, and major and trace element chemistry support an origin as crustal cumulates from a differentiated asteroid. Diogenites are genetically related to the basaltic and cumulate-gabbro eucrites, and the polymict breccias known as howardites, collectively, the HED suite. Spectroscopic observations, orbit data and dynamical arguments strongly support the hypothesis that asteroid 4 Vesta is the parent object for HED meteorites [2]. Here we discuss our new trace element data for a suite of diogenites and integrate these into the body of literature data. We use the combined data set to discuss the petrologic evolution of diogenites and 4 Vesta.

Mittlefehldt, D.W.↗
NASA NTRS2012

Siderophile Element Constraints on the Conditions of Core Formation in Mars

Siderophile element concentrations in planetary basalts and mantle samples have been used to estimate conditions of core formation for many years and have included applications to Earth, Moon, Mars and asteroid 4 Vesta [1]. For Earth, we have samples of mantle and a diverse collection of mantle melts which have provided a mature understanding of the how to reconstruct the concentration of siderophile elements in mantle materials, from only concentrations in surficial basalt (e.g., [2]). This approach has led to the consensus views that Earth underwent an early magma ocean stage to pressures of 40-50 GPa (e.g., [3,4]), Moon melted extensively and formed a small (approx. 2 mass %) metallic core [5], and 4 Vesta contains a metallic core that is approximately 18 mass % [6,7]. Based on new data from newly found meteorites, robotic spacecraft, and experimental partitioning studies, [8] showed that eight siderophile elements (Ni, Co, Mo, W, Ga, P, V and Cr) are consistent with equilibration of a 20 mass% S-rich metallic core with the mantle at pressures of 14 +/- 3 GPa. We aim to test this rather simple scenario with additional analyses of meteorites for a wide range of siderophile elements, and application of new experimental data for the volatile siderophile and highly siderophile elements.

Righter, K.↗