Comet 9P/Tempel 1: Interpretation with the Deep Impact Results

According to our common understandings, the original surface of a short-period comet nucleus has been lost by sublimation processes during its close approaches to the Sun. Sublimation results in the formation of a dust mantle on the retreated surface and in chemical differentiation of ices over tens or hundreds of meters below the mantle. In the course of NASA's Deep Impact mission, optical and infrared imaging observations of the ejecta plume were conducted by several researchers, but their interpretations of the data came as a big surprise: (1) The nucleus of comet 9P/Tempel 1 is free of a dust mantle but maintains its pristine crust of submicron-sized carbonaceous grains. (2) Primordial materials are accessible already at a depth of several tens of centimeters with abundant silicate grains of submicrometer sizes. In this study, we demonstrate that a standard model of cometary nuclei explains well available observational data: (1) A dust mantle with a thickness of ~1-2 m builds up on the surface, where compact aggregates larger than tens of micrometers dominate. (2) Large fluffy aggregates are embedded in chemically differentiated layers as well as in the deepest part of the nucleus with primordial materials. We conclude that the Deep Impact results do not need any peculiar view of a comet nucleus.