^1,2D. Turrini, ³V. Svetsov, ⁴G. Consolmagno, ⁵S. Sirono, ⁶S. Pirani
Icarus (in Press) Link to Article [doi:10.1016/j.icarus.2016.07.009]
¹Istituto di Astrofisica e Planetologia Spaziali INAF-IAPS, Via Fosso del Cavaliere 100, 00133 Rome, Italy
²Departamento de Fisica, Universidad de Atacama, Copayapu 485, Copiapó, Chile
³Institute for Dynamics of Geospheres, Leninskiy Prospekt 38-1, Moscow 119334, Russia
⁴Specola Vaticana, V-00120, Vatican City State
⁵Graduate School of Earth and Environmental Sciences, Nagoya University, Tikusa-ku, Nagoya 464-8601, Japan
⁶Lund Observatory, Department of Astronomy and Theoretical Physics, Lund University, Box 43, SE-221 00 Lund, Sweden
Copyright Elsevier

The survival of asteroid Vesta during the violent early history of the Solar System is a pivotal constraint on theories of planetary formation. Particularly important from this perspective is the amount of olivine excavated from the vestan mantle by impacts, as this constrains both the interior structure of Vesta and the number of major impacts the asteroid suffered during its life. The NASA Dawn mission revealed that olivine is present on Vesta’s surface in limited quantities, concentrated in small patches at a handful of sites not associated with the two large impact basins Rheasilvia and Veneneia. The first detections were interpreted as the result of the excavation of endogenous olivine, even if the depth at which the detected olivine originated was a matter of debate. Later works raised instead the possibility that the olivine had an exogenous origin, based on the geologic and spectral features of the deposits. In this work we quantitatively explore the proposed scenario of a exogenous origin for the detected vestan olivine to investigate whether its presence on Vesta can be explained as a natural outcome of the collisional history of the asteroid over the last one or more billion years. To perform this study we took advantage of the impact contamination model previously developed to study the origin and amount of dark and hydrated materials observed by Dawn on Vesta, a model we updated by performing dedicated hydrocode impact simulations. We show that the exogenous delivery of olivine by the same impacts that shaped the vestan surface can offer a viable explanation for the currently identified olivine-rich sites without violating the constraint posed by the lack of global olivine signatures on Vesta. Our results indicate that no mantle excavation is in principle required to explain the observations of the Dawn mission and support the idea that the vestan crust could be thicker than indicated by simple geochemical models based on the Howardite-Eucrite-Diogenite family of meteorites.

^1,2Christopher Hamann, ^1,3Dieter Stöffler, ^1,3Wolf Uwe Reimold
Geochimica et Cosmochmica Acta (in Press) Link to Article [doi:10.1016/j.gca.2016.07.018]
¹Museum für Naturkunde, Leibniz-Institut für Evolutions- und Biodiversitätsforschung, Invalidenstraße 43, 10115 Berlin, Germany
²Institut für Geologische Wissenschaften, Freie Universität Berlin, Malteserstraße 74–100, 12249 Berlin, Germany
³Humboldt-Universität zu Berlin, Unter den Linden 6, 10099 Berlin, Germany
Copyright Elsevier

We analyzed the interaction of spherical, 6.36-mm-diameter, Cu-bearing aluminum projectiles with quartz sand targets in hypervelocity impact experiments performed at NASA Ames Vertical Gun Range. Impact velocities and inferred peak shock pressures varied between 5.9–6.5 km/s and ∼41–48 GPa, respectively. Shocked particles (“impact melt particles”) coated with thin crusts of molten projectile material were recovered from the floors of the ca. 33-cm-diameter craters and the respective ejecta blankets. Through petrographic and chemical analyses (optical microscopy, FE-EMPA, SEM-EDX, and XRF analysis) we show that these particles have a layered structure manifested in distinct layers of decreasing shock metamorphism. These can be characterized by the following physical and chemical reactions and alteration products: (i) complete melting and subsequent recrystallization of the projectile, forming a distinct crystallization texture in the fused metal crust; (ii) projectile–target mixing, involving a redox reaction between Cu-bearing Al alloy und SiO2, leading to formation of khatyrkite (CuAl2), Al2O3 melt, euhedral silicon crystals, and spherical droplets of silicon; (iii) melting of quartz to lechatelierite and formation of planar deformation features in relic quartz grains; and (iv) shock lithification of quartz grains with fracturing of grains, grain-boundary melting, planar deformation features, and complete loss of porosity. To our knowledge, this is the first report of khatyrkite formed experimentally in hypervelocity impact experiments. These results have implications for the understanding of a similar redox reaction between Al–Cu metal and siliceous impact melt recently postulated for the Khatyrka CV3 carbonaceous chondrite. Moreover, these results bear on the processes that lead to layers of regolith on the surfaces of planetary bodies without atmospheres, such as asteroids in the main belt (e.g., 4 Vesta), and on the Moon. Specifically, impacts of mm-sized projectiles at velocities between 4–6 km/s into regolith-covered, asteroidal surfaces in the main belt should yield similar impact melt particles that feature a continuum of shock effects, i.e., partially to completely molten projectile remnants adhering to impact-melted regolith agglomerates, as well as projectile-contaminated impact melts and local shock melting along grain boundaries.