Testing Alternative Hypotheses to Olivine on Vesta with Multiple Data from the Dawn Mission: Implications on its Internal Structure, Origin and Evolution

This investigation aims at clarifying the presence and distribution of olivine at the surface of asteroid 4 Vesta and proposing alternative interpretations. The Dawn mission at Vesta reported outcrops of olivine-rich lithologies, mostly in the northern region encompassing craters Bellicia Arruntia and Pomponia, using reflectance data from the Visible and Infrared mapping spectrometer (VIR). Those materials exhibit a broad and complex absorption band at 1 μm. An exposure of spectrally- and spatially- resolvable olivine suggests a mantle origin, and implies extensive mineral differentiation similar to that of a planet. However, this interpretation is challenged in several ways:

1) Reported olivine detections on Vesta are not associated with the largest impact craters, where it would be expected if it had been excavated from the mantle.

2) Elemental composition analyses from gamma-ray and neutron spectroscopy of Bellicia Arruntia and Pomponia indicate similarities with a type of diogenite meteorite (Yamato, of type B) that is not olivine-rich.

3) Preliminary investigations revealed that other minerals such as low-calcium, high-iron pyroxenes may explain the broad and complex absorption band at 1 μm without requiring olivine, and that a regional correlation with the molecular group hydroxyl (OH) may indicate an exogenous origin.

4) The responsivity function of VIR was profoundly revised after the Dawn mission of Vesta in order to match telescopic observations, and few published results have accounted for the new calibration for a revised interpretation of the composition of Vesta’s surface.

5) Performing again a comparison with meteorites, especially the type-B, is needed to better account for the spatial resolution of GRaND.

In Task 1 we will calibrate VIR and GRAND data with the most up-to-date corrections. For VIR, those include a revised responsivity function, a temperature-dependence correction of the visible detector data and a photometric correction.

Task 2 will be dedicated to spectral measurements in the laboratory of mineral samples (mostly olivine and pyroxene with various Mg, Fe and Ca numbers) at surface temperatures relevant to Vesta under various phase angles and for different grain sizes in order to calculate their optical constants in the visible and near-infrared.

Task 3 will focus on the analysis of VIR data and mapping of the surface composition of Vesta. With VIR, we will emphasize radiative transfer modeling for absolute abundance evaluations of minerals on areas where olivine was initially reported.

In Task 4, we will make comparisons with forward-modeling of elemental compositions with GRaND data based on VIR -derived mineralogical models.

In Task 5 we will determine the implication of the observed petrology on the geological history and internal structure of Vesta, and investigate alternative starting compositions for a Vesta Magma Ocean, by means of gravity-constrained modeling, to explain these observations.

The presence or absence of olivine exposed at the surface could help further determine the differentiation processes within the crust and clarify the possible exogenous origin of the outcrops of interest, especially where the surface is hydroxylated in the region Bellicia Arruntia and Pomponia. The possible absence of olivine would imply 1) no surface exposure of mantellic lithologies, consistent with no olivine detected in the deepest part of Vesta excavated by the impact that formed Rheasiliva, 2) inclusions of plutonic, non-mantellic lithologies in the upper crust, exposed by impacts craters, and 3) a possible exogenous origin for the hydroxylated materials in the region encompassing the craters Bellicia, Arruntia and Pomponia. The expected outcome of the proposed investigation is to converge to fewer hypotheses than in the current literature about the petrogenesis of Vesta and the role of foreign materials.