Relationships between the distribution of Martian crater ejecta morphologies and both elevation and ridged plains thicknesses have been investigated. We observed a slight correlation between elevation and the distribution of ejecta types for the ridged plains, but not for regions of the dissected cratered plateau unit. A stronger correlation was identified between the thicknesses of Lunae and Hesperia Plana-two major occurrences of the ridged plains-and the complexity of ejecta morphologies. For a given crater size range, the complexity of the ejecta increases with decreasing thicknesses of the ridged plains. Because more of the lower unit is excavated where the upper layer is thin, we conclude that some property of the lower layer increases ejecta complexity. One possible explanation is that the lower layer may fracture to smaller ejecta sizes during excavation; thus relatively more ejecta can be entrained by the Martian atmosphere in areas where the ridged plains are thin.

Another possibility is that a greater volatile content within the lower layer may result in increased fluidization both by vapor expansion during impact, resulting in increased brecciation within the lower layer, and by the incorporation of volatiles within the ejecta. The slight relationship between elevation and the distribution of ejecta types probably results from a slight correlation between elevation and ridged plains thicknesses within Lunae Planum. This effect appears significant within our data for the ridged plains because almost half our data were obtained in this region. ¿ American Geophysical Union 1987