The Nucleus of Comet 9P/Tempel 1: Shape & Geology From Two Flybys
By Peter Thomas
The flyby of comet Tempel 1 by Stardust-NExT on February 14, 2011 revealed new parts of the surface hidden from view of the Deep Impact spacecraft in July 2005. The new data allow mapping of nearly ¾ of the surface and show the shape of this object is somewhat pyramidal, with a mean radius of 2.8 km. From the mass inferred from Deep Impact observations, the surface gravity is only 1/30000 that on the earth (200 pound Earthling would weigh only about 1/10th ounce on the surface). The shape, viewed from six directions, and colored by the relative heights, is shown in the top of the illustration.
One of the striking features of this low-gravity object is that there are regions of different types of “geology” and that these occur at different relative heights. Areas that are relatively high are rough and have many pits, tens to hundreds of m across. Areas that are low have much smoother topography and even display deposits that have many characteristics of having been emplaced as flows along the surface filling in the low areas. These likely flow deposits are up to tens of meters thick, and some are over 3 km in length. The three main low regions and outlines of likely flows are shown in the lower part of the illustration.
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These findings emphasize some of the conclusions emerging from close-up exploration of comet nuclei: they have complicated histories and are not just evaporating snowballs, and that even very low gravity can control geological processes.
Shape and surface of Tempel 1. Top: views of the shape from six orthogonal directions. Mean radius is 3 km. Colors show heights relative to a reference surface: red is high, blue low. Bottom: Three images projected to views down on areas with smooth topography indicating flow deposits. The lowest row of color views shows the outlines of the flows and the topography, emphasizing the smoother materials have been deposited in the low regions.