The Complex Evolution of Comet Nuclei: Evidence from Deep Impact and Stardust-NExT

Spacecraft exploration of comets is revealing that far from being dirty snowballs that have changed little since their formation 4.6 billion years ago, many comet nuclei have undergone complex evolution. Observations of 9P/Tempel 1 made by Deep Impact (DI) in 2005 and Stardust-NExT (SN) in 2011 provide evidence for diverse geologic processes including the formation of layered structures, the episodic eruption of materials from the interior onto the surface, the formation of pit-like depressions and scarps by sublimation of volatiles, etc. A significant fraction of Tempel 1's activity appears to be associated with the back-wasting of scarps. Scarps on the comet display a variety of morphologies. Comparisons of DI and SN images reveal that the rate of scarp retreat varies from place to place. These observations point to differences in composition and/or texture of surface materials.