玄武质小行星空间风化蚀变之谜
On the puzzle of space weathering alteration of basaltic asteroids
摘要：
大多数玄武质小行星位于主带内侧，但也有少数在主带外侧和近地空间被观测到。这些被称为V型的小行星，其表面成分类似于直径530公里的灶神星。除了成分相似外，动力学证据也将许多V型小行星与灶神星联系起来。此外，灶神星是少数几颗被确认为特定类别陨石（howardite，eucrite，diogenite achondrites）来源的小行星之一。尽管对所概述的设想达成了普遍共识，但仍有几个问题没有得到解决。特别是，目前尚不清楚灶神星、V型和HED之间观测到的光谱多样性是否是由于空间风化造成的，S型小行星被认为是这种情况。在本文中，SDSS测光是用来解决的问题，候选V型和HEDs之间的光谱差异是否可以解释空间风化。我们发现，V型的可见光谱斜率系统红相对于HEDs，在一个类似的方式，发现普通球粒陨石和S型。假设空间风化是负责的斜率不匹配，我们估计的上限约为0.5Ga的红化时间尺度。然而，所观察到的HED和V型之间的斜率不匹配对理解其起源提出了几个难题。本文还结合黎明号的灶神星任务讨论了这些发现的意义。
英文摘要：
The majority of basaltic asteroids are found in the inner main belt, although a few have also been observed in the outer main belt and near-Earth space. These asteroids -referred to as V-types- have surface compositions that resemble that of the 530km sized asteroid Vesta. Besides the compositional similarity, dynamical evidence also links many V-type asteroids to Vesta. Moreover, Vesta is one of the few asteroids to have been identified as source of specific classes of meteorites, the howardite, eucrite, diogenite achondrites (HEDs). Despite the general consensus on the outlined scenario, several questions remain unresolved. In particular, it is not clear if the observed spectral diversity among Vesta, V-types and HEDs is due to space weathering, as is thought to be the case for S-type asteroids. In this paper, SDSS photometry is used to address the question of whether the spectral diversity among candidate V-types and HEDs can be explained by space weathering. We show that visible spectral slopes of V-types are systematically redder with respect to HEDs, in a similar way to what is found for ordinary chondrite meteorites and S-types. On the assumption that space weathering is responsible for the slope mismatch, we estimated an upper limit for the reddening timescale of about 0.5Ga. Nevertheless, the observed slope mismatch between HEDs and V-types poses several puzzles to understanding its origin. The implication of our findings is also discussed in the light of Dawn mission to Vesta.
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