Distinctive space weathering on Vesta from regolith mixing processes

摘要

The surface of the asteroid Vesta has prominent near-infrared absorption bands characteristic of a range of pyroxenes, confirming a direct link to the basaltic howardite-eucrite-diogenite class of meteorites. Processes active in the space environment produce 'space weathering' products that substantially weaken or mask such diagnostic absorption on airless bodies observed elsewhere, and it has long been a mystery why Vesta's absorption bands are so strong. Analyses of soil samples from both the Moon and the asteroid Itokawa determined that nanophase metallic particles (commonly nanophase iron) accumulate on the rims of regolith grains with time, accounting for an observed optical degradation. These nanophase particles, believed to be related to solar wind and micrometeoroid bombardment processes, leave unique spectroscopic signatures that can be measured remotely but require sufficient spatial resolution to discern the geologic context and history of the surface, which has not been achieved for Vesta until now. Here we report that Vesta shows its own form of space weathering, which is quite different from that of other airless bodies visited. No evidence is detected on Vesta for accumulation of lunar-like nanophase iron on regolith particles, even though distinct material exposed at several fresh craters becomes gradually masked and fades into the background as the craters age. Instead, spectroscopic data reveal that on Vesta a locally homogenized upper regolith is generated with time through small-scale mixing of diverse surface components.%在2011年7月16日和2012年9月5日之间，美rn国国家航空航天局的空间探测器Dawrl在绕rnVesta运行，后者被认为是自太阳系形成过程rn早期阶段以来基本上没有什么变化的一个原始rn行星。在本期Nature上，两个小组报告了这段rn时间Dawn对Vesta的探测情况。Carle Pietersrn及其同事发现，Vesta上的太空风化过程与在rn月球和Itokawa(在一次返回地球任务中被取样rn研究过的小行星)上观测到的不同。在Vestarn上，风化涉及小颗粒风化土的混合，该过程消rn除了最近碰