Physics Institute, Space Research and Planetary Sciences, Center for Space and Habitability, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland.
Nature. 2013 Feb 14;494(7436):207-10. doi: 10.1038/nature11892.
Asteroid 4 Vesta seems to be a major intact protoplanet, with a surface composition similar to that of the HED (howardite-eucrite-diogenite) meteorites. The southern hemisphere is dominated by a giant impact scar, but previous impact models have failed to reproduce the observed topography. The recent discovery that Vesta's southern hemisphere is dominated by two overlapping basins provides an opportunity to model Vesta's topography more accurately. Here we report three-dimensional simulations of Vesta's global evolution under two overlapping planet-scale collisions. We closely reproduce its observed shape, and provide maps of impact excavation and ejecta deposition. Spiral patterns observed in the younger basin Rheasilvia, about one billion years old, are attributed to Coriolis forces during crater collapse. Surface materials exposed in the north come from a depth of about 20 kilometres, according to our models, whereas materials exposed inside the southern double-excavation come from depths of about 60-100 kilometres. If Vesta began as a layered, completely differentiated protoplanet, then our model predicts large areas of pure diogenites and olivine-rich rocks. These are not seen, possibly implying that the outer 100 kilometres or so of Vesta is composed mainly of a basaltic crust (eucrites) with ultramafic intrusions (diogenites).
小行星 4 灶神星似乎是一颗主要的完整原行星，其表面成分与 HED（辉长-无球粒陨石）陨石相似。南半球主要由一个巨大的撞击疤痕主导，但以前的撞击模型未能再现观测到的地形。最近发现灶神星的南半球主要由两个重叠的盆地主导，这为更准确地模拟灶神星的地形提供了机会。在这里，我们报告了在两次重叠的行星尺度碰撞下灶神星全球演化的三维模拟。我们紧密再现了它的观测形状，并提供了撞击挖掘和喷射物沉积的地图。在大约 10 亿年历史的较年轻的赖希斯维亚盆地中观察到的螺旋图案归因于撞击坑塌缩过程中的科里奥利力。根据我们的模型，暴露在北部的表面材料来自大约 20 公里的深度，而暴露在南部双挖掘内部的材料来自大约 60-100 公里的深度。如果灶神星最初是一个分层的、完全分化的原行星，那么我们的模型预测会有大片的纯 diogenites 和富含橄榄石的岩石。这些都没有被观察到，这可能意味着灶神星的外 100 公里左右主要由玄武质地壳（eucrites）和超镁铁质侵入体（diogenites）组成。
