Interactions between impact-induced vapor clouds and the ambient atmosphere: 2. Theoretical modeling

Interaction between impact-induced vapor clouds and the ambient atmosphere may play an important role in planetary evolution. In the companion study, Sugita and Schultz [2003] conducted a series of impact experiments detailing observation of this interaction process. The laboratory experiments using diatomic molecular spectroscopy provide several well-defined observational constraints on this process. In this study, we examine five different physical models for the interaction process against these observational constraints. Calculation results reveal that the observed high-temperature radiation could not come from either the impact-induced vapor or the shock front between impact vapor and the ambient atmosphere. Rather, the radiation is attributed to ablation vapor from the surface of small, high-speed fragments of the projectile entrained in the impact vapor cloud. Calculations with a simple ablation model indicate that the observed small ambient-pressure dependence of the initial radiation temperature requires a very large heat of vaporization. The estimated heat of vaporization is comparable to dissociation energy for formation of carbon radicals from the polymer of projectile fragments. The small dependence of radiation temperature on atmospheric composition strongly suggests that the majority of radiation comes from extremely small impact fragments whose size is comparable to the mean free path of ambient atmosphere (i.e., free-molecular flow regime). These results have a wide range of potential implications for planetary science, including augmentation of impact vapor/melt production in a thick atmosphere; production of metastable chemical compounds in ablation vapor around high-speed impact fragments; and intense downrange radiation from an oblique impact.