IMPACT CRATER SATURATION OF OUTER SOLAR SYSTEM SATELLITES: RESULTS FROM SPATIAL STATISTICS

For several decades there has been a debate over whether heavily cratered surfaces in our solar system are in “saturation equilibrium” [e.g., 1-3; a state where crater density reaches an (quasi-) equilibrium]. Saturation equilibrium is critical to understand because otherwise the crater distribution shape and/or flux may be misinterpreted. This work uses spatial statistics (quantitative measures of objects' distributions in space) to constrain whether heavily cratered surfaces of the saturnian and uranian satellites are in saturation. Using numerical simulations of crater distributions approaching and reaching saturation, I have verified that for a variety of crater size-frequency distribution (SFD) slopes the crater distribution will become more spatially uniform (more evenly spaced than expected for a random distribution) as it reaches saturation [4]. Therefore, I have computed spatial statistics for recently compiled crater distributions of several terrains on these satellites to determine whether the crater distributions are spatially random or more uniform. Then further analysis incorporating simulation results and the crater SFDs indicates if the surface is plausibly in saturation equilibrium. Preliminary results suggest that the heavily cratered terrains of Mimas, Rhea, Dione, Enceladus, and Miranda, which all have more uniform distributions, are saturated. The lightly cratered terrains of Dione and Miranda have spatially random distributions, and do not appear to be saturated. Data on additional satellite terrains will be presented and discussed. MRK acknowledges the support of NASA PGG grant NNX12AO51G. References: [1] Woronow, A. (1977) JGR 82, 2447-56. [2] Hartmann, W. K. & Gaskell, R. W. (1997) MAPS 32, 109-21. [3] Marchi, S., et al. (2012) EPSL 325-6, 27-38. [4] Kirchoff, M. R. (2013) DPS Mtg., Abst. #417.08.