通过碳质球粒陨石撞击向灶神星输送暗物质
Delivery of Dark Material to Vesta via Carbonaceous Chondritic Impacts
摘要：
美国宇航局的黎明号航天器对小行星（4）灶神星的观测显示，它的表面是我们迄今为止观测到的任何小行星中最高的亮度和颜色变化。使用黎明分幅相机（FC）0.75微米滤光片图像，在几种地质环境中发现了富含低散射暗物质的地形：与撞击坑有关（在喷出物覆盖层材料中和/或撞击坑壁和边缘上）;通常与地形高点有关的流动状沉积物或射线;以及撞击坑附近的暗点（可能是二次撞击）。这DM可能是一个遗迹的古代火山活动或外源的起源。我们报告说，DM的光谱的大部分是类似的碳质球粒陨石混合的材料土著灶神星。使用高分辨率的七色图像，我们比较了DM的颜色特性（色度，带深）与实验室测量的可能的模拟材料。DM的条带深度和宽度与富碳球粒陨石捕虏体的Howardite Mt.普拉特（PRA）04401。Murchison CM 2碳质球粒陨石和玄武质钙铝榴石Millbillillie的实验室混合物也显示了带深度和对DM的亲合力。DM（1-6体积%）中碳质球粒陨石丰度的建模与豪威陨石一致。我们没有发现大规模的火山活动（暴露的岩脉/火山碎屑瀑布）作为DM的来源的证据。我们使用撞击坑比例律和喷出物再增生的数值模型进行的建模工作表明，在低速（<2公里/秒）碳质撞击物形成约400公里Veneneia盆地期间，该DM在灶神星上的输送和就位。这一发现很重要，因为它强化了长期以来的观点，即原始天体是早期太阳系中碳和可能挥发物的来源。
英文摘要：
NASA's Dawn spacecraft observations of asteroid (4) Vesta reveal a surface with the highest albedo and color variation of any asteroid we have observed so far. Terrains rich in low albedo dark material (DM) have been identified using Dawn Framing Camera (FC) 0.75 μm filter images in several geologic settings: associated with impact craters (in the ejecta blanket material and/or on the crater walls and rims); as flow-like deposits or rays commonly associated with topographic highs; and as dark spots (likely secondary impacts) nearby impact craters. This DM could be a relic of ancient volcanic activity or exogenic in origin. We report that the majority of the spectra of DM are similar to carbonaceous chondrite meteorites mixed with materials indigenous to Vesta. Using high-resolution seven color images we compared DM color properties (albedo, band depth) with laboratory measurements of possible analog materials. Band depth and albedo of DM are identical to those of carbonaceous chondrite xenolith-rich howardite Mt. Pratt (PRA) 04401. Laboratory mixtures of Murchison CM2 carbonaceous chondrite and basaltic eucrite Millbillillie also show band depth and albedo affinity to DM. Modeling of carbonaceous chondrite abundance in DM (1-6 vol%) is consistent with howardite meteorites. We find no evidence for large-scale volcanism (exposed dikes/pyroclastic falls) as the source of DM. Our modeling efforts using impact crater scaling laws and numerical models of ejecta reaccretion suggest the delivery and emplacement of this DM on Vesta during the formation of the ~400 km Veneneia basin by a low-velocity (<2 km/sec) carbonaceous impactor. This discovery is important because it strengthens the long-held idea that primitive bodies are the source of carbon and probably volatiles in the early Solar System.
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