Person:

Senft, Laurel E.

We present numerical simulations of crater formation under Martian conditions with a single near-surface icy layer to investigate changes in crater morphology between glacial and interglacial periods. The ice fraction, thickness, and depth to the icy layer are varied to understand the systematic effects oil observable crater features. To accurately model impact cratering into ice, a new equation of state table and strength model parameters for H2O are fitted to laboratory data. The presence of an icy layer significantly modifies the cratering mechanics. Observable features demonstrated by the modeling include variations in crater morphometry (depth and rim height) and icy infill of the crater floor during the late stages of crater formation. In addition, an icy layer modifies the velocities, angles, and volumes of ejecta, leading to deviations of ejecta blanket thickness from the predicted power law. The dramatic changes in crater excavation are a result of both the shock impedance and the strength mismatch between layers of icy and rocky materials. Our simulations suggest that many of the unusual features of Martian craters may be explained by the presence of icy layers, including shallow craters with well-preserved ejecta blankets, icy flow related features, some layered ejecta structures, and crater lakes. Therefore, the cratering record implies that near-surface icy layers are widespread on Mars.

Impact craters are potentially powerful tools for probing large-scale structure beneath planetary surfaces. However, the details of how target structure affects the impact cratering process and final crater forms remain poorly understood. Here, we present a study of cratering in layered surfaces using numerical simulations. We implement the rheologic model for geologic materials described by Collins et al. ( 2004) into the shock physics code CTH; this model includes pressure, temperature, and damage effects on strength as well the option to include acoustic fluidization. The model produces reasonable final crater shapes and damaged zones from laboratory to planetary scales. We show the effects of varying material strength parameters and discuss choosing appropriate strength parameters for laboratory and planetary situations. Results for cratering into idealized terrains with layers of differing material strength are presented. The presence of such layers in the target can significantly alter the ejecta curtain structure and the final crater morphology. Finally, we reproduce the morphologic variations that are observed in small lunar craters by modeling a weak regolith overlying competent rock.