Compositional Gradients Across Mare and Highlands Contacts: The Importance and Geological Implication of Lateral Mixing

L. Li, J. F. Mustard (Brown University)

Variation in mare abundance across mare-highland contact depends on the relative importance of vertical and lateral mass transport. Nonlinear spectral mixing analysis of Clementine VIS-NIR data of the Grimaldi basin indicates that the abundance of mare and highland are approximately equal to 50% at the geologic contact and the amount of mare materials transported to the highlands is approximately equal to the amount of highland materials to the mare. Thus, vertical mixing is relatively unimportant except near the contact. There are apparently two mixing zones, a steep mixing gradient near the mare and highland contact and a more diffuse zone that extends some distance from the contact. The net transfer of mare across the geologic contact can be approximated by a diffusion process since the transport is a random process driven by impact cratering. We have developed a diffusion model and the results indicate that a typical profile of mare abundance requires two superimposed diffusion curves to match this compositional gradients. A third curve is found to be statistically invalid. The two diffusion parameters can be related to the geologic processes of regolith reworking and development that created the variations. This suggests that there are at least two superimposed processes at work, the one is the reworking of the surface ejecta debris having a larger diffusion coefficient, the other is the development of new regolith from subsurface layers having smaller diffusion coefficient.