Adapted Modified Gaussian Model: No Detection of Olivine in Regions Predicted to Be Mantle-rich from Models of Planet-scale Collisions

Introduction: Despite many studies on the Howardite-Eucrite-Diogenite (HED) meteorite suite, the internal structure of asteroid 4-Vesta is still debated. Some studies favor the hypothesis of a global magma ocean, leading to a layered structure and a relatively thin crust [e.g. 1]. Other petrological and geochemical evidences tend to favor a more complex structure, involving intrusions, which also implies a thicker crust [e.g. 2]. Recent data from the Dawn mission now shed a new light on this question. Dawn’s global mapping have revealed that the south polar depression is actually composed of two overlapping basins, Veneneia and Rheasilvia [3]. While only one impact cannot excavate rocks from deep, the most recent numerical simulations taking into account both events show that rocks exposed in the Rheasilvia region could come from a depth of much more than 40 km [4,5]. In the case of a thin crust, a succession of two impacts would have then surely digged into the mantle, producing large outcrops of olivine-rich rocks. The Dawn spectrometer (VIR) is perfectly designed to detect mafic minerals [6]. However, no trace of olivine has been found up to now in the southern hemisphere [7,8]. It is nevertheless true that detecting olivine on the surface of Vesta is challenging [9]. An adapted version of the Modified Gaussian Model (MGM, [10]) has been recently developed to take into account the olivine-pyroxene(s) mixtures [11]. This procedure has been successfully applied to Mars