Anton I. Ermakov^a, Maria T. Zuber^a, David E. Smith^a,b, Carol A. Raymond^c, Georges Balmino^d, R.R. Fu^a and B.A. Ivanov^e

^aDepartment of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA
^bNASA Goddard Space Flight Center, Greenbelt, MD, USA
^cJet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA
^dCentre national d’études spatiales (CNES), Toulouse, France
^eInstitute for Dynamics of Geospheres, Russian Academy of Sciences, Moscow, Russian Federation

Vesta is a differentiated asteroid as confirmed by gravity and spectroscopy measurements from the Dawn mission. We use the shape and gravity field of Vesta determined from observations of the Dawn spacecraft to develop models of the asteroid’s interior structure. We compute a three-layer interior structure model by minimizing the power of the residual gravity anomaly. The densities of the mantle and crust are based on constraints derived from the Howardite-Eucrite-Diogenite (HED) meteorites.
Vesta’s present-day shape is not in hydrostatic equilibrium. The Rheasilvia and Veneneia impact basins have a large effect on Vesta’s shape and are the main source of deviation from hydrostatic shape. Constraining a pre-giant-impact rotation rate and orientation of the spin axis from an ellipsoidal fit to the parts of Vesta unaffected by the giant impacts, and using the theory of figure, we can constrain the shape of the core.
Our solution for Vesta’s crust-mantle interface reveals a belt of thick crust around Rheasilvia and Veneneia. The thinnest crust is in the floor of the two basins and in the Vestalia Terra region. Our solution does not reveal an uplift of the crust-mantle boundary to the surface in the largest basins. This, together with the lack of olivine detected by the Visible and Infrared Spectrometer (VIR) data in Rheasilvia and Veneneia, indicates that Vesta’s presumed olivine mantle was either not brought to the surface by these large impacts or was covered by ejecta from subsequent impacts.

Reference
Ermakov AI, Zuber MT, Smith DE, Raymond CA, Balmino G, Fu RR and Ivanov BA (in press) Constraints on Vesta’s Interior Structure Using Gravity and Shape Models from the Dawn Mission. Icarus
[doi:10.1016/j.icarus.2014.05.015]
Copyright Elsevier

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Jiao He and Gianfranco Vidali

When dust grains have a higher temperature than they would have in dense clouds, and when H, H2, and O2 have a negligible residence time on grains, the formation of water should still be possible via the hydrogenation of OH and Eley–Rideal-type reactions. We determined that the OH desorption energy from an amorphous silicate surface is at least 143 meV (1656 K). This is 400 K higher than the value previously used in chemical models of the interstellar medium and is possibly as high as 410 meV (4760 K). This extends the temperature range for the efficient formation of water on grains from about 30 K to at least 50 K, and possibly over 100 K. We do not find evidence that water molecules leave the surface upon formation. Instead, through a thermal programmed desorption experiment, we find that water formed on the surface of an amorphous silicate desorbs at around 160 K. We also measured the cross-sections for the reaction of H and D with an O₃ layer on an amorphous silicate surface at 50 K. The values of the cross-sections, σ_H = 1.6 ± 0.27 Ų and σ_D = 0.94 ± 0.09 Ų, respectively, are smaller than the size of an O₃ molecule, suggesting the reaction mechanism is more likely Eley–Rideal than hot-atom. Information obtained through these experiments should help theorists evaluate the relative contribution of water formation on warm grains versus in the gas phase.

Reference
He J and Vidali G (2014) Experiments of water formation on warm silicates. The Astrophysical Journal 788:50.
[doi:10.1088/0004-637X/788/1/50]

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