A Deep Moho in “Small Planet” Vesta and Implication Regarding the Chondritic Nature of Protoplanets

Abstract:

Asteroid 4-Vesta is the only differentiated protoplanet [1] which has been extensively explored by a space probe, and is represented by a large collection of basaltic and ultramafic samples, the Howardite-Eucrite-Diogenite (HED) meteorites. High-resolution global mapping by the Dawn probe revealed that the south polar depression is composed of two overlapping impact basins, Veneneia and Rheasilvia [2]. Numerical simulations taking into account both sequential events [3] show that surface material in the northern hemisphere of Vesta came from a depth of about 20 km, whereas the exposed southern material comes from a depth of 60 to 100 km. Recent magma-ocean crystallization models [4], which consider a chondritic initial composition, predict a crustal thickness of 30-40 km. Thus a succession of two impacts would have dug well into the olivine-rich mantle. Here we use the results from 3D smooth particle hydrodynamics impact simulations to show that, not only the floor of the basins should expose very deep lithologies, but that a significant proportion of the rocks which have escaped from Vesta during the second impact originated from depth much greater than 40 km. We also mapped locally Vesta’s mineralogy with images from the Dawn VIR spectrometer. The lack of olivine-rich lithologies in the Veneneia/Rheasilvia region, and the simultaneous lack of mantle samples among the vestoids and HEDs together provide evidence that the crust-mantle boundary (or Moho) was not reached during the two impacts, and entails that Vesta’s Moho is deeper than 80 km [5]. With a deep Moho, it appears clear that the mantle is much thinner than expected, leading to the conclusion that Vesta contains far less olivine than predicted by magma-ocean crystallization models. This could be an evidence that its bulk chemical composition deviates substantially from a chondritic composition for major elements, alike as for sodium and other volatiles.

[1] Russell et al, Science 336, 684-686, 2012
[2] Schenk et al, Science 336, 694-697, 2012
[3] Jutzi et al, Nature 494, 207-210, 2013
[4] Mandler and Elkins-Tanton, Meteorit Planet Sci 48, 2333-2349, 2013
[5] Clenet et al, Nature 511, 303-306, 2014