We analyzed Deep Impact High Resolution Instrument (HRI) images acquired within the first seconds after collision of the Deep Impact impactor with the nucleus of comet 9P/Tempel 1. These images reveal an optically thick ejecta plume that casts a shadow on the surface of the nucleus. Using the 3D radiative transfer code HYPERION we simulated light scattering by the ejecta plume, taking into account multiple scattering of light from the ejecta, the surrounding nuclear surface and the actual observational geometry (including an updated plume orientation geometry that accounts for the latest 9P/Tempel 1 shape model). Our primary dust model parameters were the number density of particles, their size distribution and composition. We defined the composition through the density of an individual particle and the ratio of its material constituents, which we considered to be refractories, ice and voids. The results of our modeling indicate a dust/ice mass ratio for the ejecta particles of at least 1. To further constrain the parameters of the model, we checked for consistency between the ejecta mass resulting from our modeling with the ejecta mass estimated by the crater formation modeling. Constraining the particle size distribution by results of other studies of the Deep Impact ejecta, we find the number density of ejecta particles equal to ~10^4 particles/cm^3 at the base of the plume.
A Quarter-Century of Observations of Comet 10P/Tempel 2 at Lowell Observatory: Continued Spin-Down, Coma Morphology, Production Rates, and Numerical Modeling
A Look at Eight Outbursts of Comet 7P/Pons-Winnecke
University of Maryland, College Park（马里兰大学帕克分校）；Space Telescope Science Institute（太空望远镜科学研究所）；Boston University（波士顿大学）；Las Cumbres Observatory（拉斯坎伯斯天文台）；Auburn University（奥本大学）；Appalachian State University（阿巴拉契亚州立大学）；Queen’s University Belfast（贝尔法斯特女王大学）；Open University（开放大学）；Cardiff University（卡迪夫大学）
Erosion of volatiles by micro-meteoroid bombardment on Ceres, and comparison to the Moon and Mercury
(1)谷神星是主带中最大的水库，黎明号宇宙飞船最近访问了它，发现了几个带有H PDMATHAMM O冰特征的区域。独立的望远镜观测显示了一个目前未知来源的水散逸层。我们探讨流星体的影响谷神星考虑到从黎明任务获得的地形，使用广泛使用的微流星体模型和射线跟踪技术。考虑直径为$0.01-2$ mm的流星体群。我们分析了谷神星在当前轨道上的短期影响以及整个岁差周期的长期影响。我们发现整个表面都受到流星体的轰击，没有留下任何区域处于流星体流入的永久阴影中。谷神星的赤道部分产生的$80\%$喷出物比极地地区多，这是因为长周期彗星的撞击速度很大。质量通量，能量通量，和喷出物生产季节性变化的$3-7$的因素，由于倾斜的偏心轨道。与水星和月球相比，谷神星经历的微流星体撞击的影响要小得多，总质量流量为$4.5\pm1.2\times10^{-17}$ kg m $^{-2}$ s $^{-1}$。平均而言，水星受到更大的$50\times$质量通量，并产生比谷神星更多的喷出物$700\times$，而月球的质量通量是$10\times$更大，喷出物的产生是$30\times$大于谷神星。由于这些原因，我们发现流星体的影响是一个不太可能的候选人生产的水散逸层或显着挖掘的表面特征。所考虑的微流星体群的表面更新率估计为谷神星上的$1.25$百万年。
