Studying the nucleus of comet 9P/Tempel 1 using the structure of the Deep Impact ejecta cloud at the early stages of its development

Abstract

The paper presents an attempt to extract information about the comet 9P/Tempel 1 nucleus from the characteristics of the ejecta cloud produced by the impactor of the Deep Impact mission. For this purpose we use two techniques. We first study the shadow cast on the nucleus surface by the ejecta cloud and investigate how areas of different brightness are related to the varying optical thickness or albedo of the ejecta cloud. The shadow was seen during the first 2.0s after the impact (afterward it became obscured by the ejecta cloud). We have found that all brightness variations in the shadow are the result of the surface inhomogeneities, indicating that during first 2.0s the ejecta cloud was homogeneous within the MRI spatial resolution. Our second technique is to study the obscuration of the nucleus limb by the ejecta. This study covers the period 0.76–68.8s after impact and is based on comparison of the ejecta cloud brightness on the limb and just beyond the limb. At this stage we do see inhomogeneities in the ejecta cloud that relate to the albedo and optical thickness variations in the ejected dust. Specifically, we have found two distinct bands of low optical thickness and one band of a high optical thickness. Based on crater formation ideas we estimate the depth of excavation of the ejected material for the found inhomogeneities and, thus, define a potential layering structure for the comet nucleus, Our estimates suggest that the low-optical thickness material was excavated from a depth of 15–18 and 30–32 m in the case the porous nucleus material and 37–46 and 87–93m in the case of a non-porous nucleus material, and a layer of high optical thickness originated from the depth 9–11m for porous material or 20–23m for non-porous material. Based on the crater diameter estimates, we expect that the real depth of the layers is between these two cases. The rest of the ejecta do not show any signs of layering but have significant azimuthal inhomogeneity with clumps of high optical thickness and patterns of high albedo.