MODELING OF SUBLIMATION-DRIVEN EROSION AND ICE PINNACLE FORMATION ON CALLISTO

Most of the areas observed at high resolution on the Galilean satellite Callisto have a morphology that implies sublimation-driven landform modification and mass wasting is at work [Moore et al., 1999]. These areas comprise rolling dark plains with interspersed bright pinnacles. Howard and Moore [2008], using the MARSSIM landform evolution model, simulated evolution of this landscape as a combination of bedrock volatile sublimation, mass wasting of the dark, non-coherent residue, and redeposition of ice at high-elevation cold traps sheltered from thermal re-radiation to form the pinnacles.

Howard and Moore [2008] was essentially a feasibility study that showed that formation of the pinnacles through sublimation and redeposition of bedrock volatiles was a viable process. The goal of our study is to further investigate the details of pinnacle formation by refining this model, and by constraining values for the variable environmental parameters within the model such that they are consistent with the current understanding of Callisto’s surface environment. We present the results of the updated model and our experimentation with varying key parameters.

Our refinement of the model has caused us to revise the result of Howard and Moore [2008] that the pinnacles represent an ice cover of several tens to hundreds of meters. Instead, our results indicate an ice coverage reaching several meters at most, a figure that is consistent with the prediction of Moore et al. [2004]. We have also modified the model such that ice contained within the pinnacles is now subject to sublimation itself.

Using Fick’s Law to solve for the diffusive transport rate between a volatile table and an atmosphere [Moore et al., 1996], we have determined that the loss rate of H₂O ice from the volatile-refractory bedrock through sublimation is too slow (~10^-20 kg m^-2 s^-1) to account for the formation of the ice pinnacles, and that a volatile mixture that contains H₂O ice is necessary to facilitate its loss. We find that CO₂ hydrate fulfills this role well: loss rates of CO₂∙6H₂O (~10^-10 kg m^-2 s^-1) are sufficient to produce deposited ice thicknesses reaching several meters, with the volatile CO₂ resulting from dissociation remaining in the tenuous atmosphere [Carlson, 1999; Liang et al., 2005].