Other
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p41f..07p&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P41F-07
Other
[5418] Planetary Sciences: Solid Surface Planets / Heat Flow, [5420] Planetary Sciences: Solid Surface Planets / Impact Phenomena, Cratering, [6280] Planetary Sciences: Solar System Objects / Saturnian Satellites
Scientific paper
Over billions of years, crater depths relax at a rate that is dependent on the internal properties of the target body. Thus, measuring the depth of craters can give insight into the thermal history and subsurface structure of terrestrial bodies and icy satellites (Dombard and McKinnon, 2006). The extensive surface imaging coverage provided by Cassini (and more modest coverage from Galileo), in combination with the development of new automated stereo imaging programs, now allows for detailed measurements of crater depths on the moons of Saturn and Jupiter, and thus more accurate estimates of crater relaxation. We utilize these resources to create digital elevation models (DEMs) of large craters (D>70km) on icy satellites, beginning with Rhea and Dione. We extract crater profiles from our DEMs to determine current crater depths. An estimate of initial crater depth requires extrapolations from a crater assumed to be unrelaxed, either scaled up in size if on the same body, or scaled by gravity if on another satellite; initial and current crater depths are combined to yield a measured relaxation percentage for different crater diameter size bins. Our topographic measurements are compared with the results of a coupled thermal evolution-viscoelastic relaxation code, allowing us to investigate the thermal history of each satellite. Our model predicts the expected degree of crater relaxation for craters of different sizes and ages based on assumptions about the initial thermal state of the satellite and its subsurface structure. So far, in the case of Rhea, our numerical simulations under-predict the amount of crater relaxation we observe, suggesting that Rhea is warmer than we initially modeled; in fact, it appears that internal temperatures must approach the melting point of ice in order to achieve the amount of relaxation we observe. Our numerical model has been benchmarked against standard analytical solutions (Robuchon et al., 2011), and thus we believe that the code itself is not in error and that Rhea experienced more heating early in its history than previously thought. We also find that for 100 km diameter craters on Rhea, other factors in addition to viscous relaxation are important in determining their final depth. We have completed our measurements for all large craters on Rhea that are captured, to date, in Cassini ISS stereo pairs, and are currently working to produce topographic profiles of all available large craters on Dione. We will present results from our Dione crater profiles and numerical modeling of Dione's thermal history, and will compare our results for degree of crater relaxation and subsurface thermal profile with those previously determined for Rhea. Since Rhea and Dione have similar compositions and surface gravities, craters of equivalent diameters on their surfaces likely had similar initial depths. Thus, our comparisons of final crater depths on these two satellites will help us understand the details of any similarities or differences in their relaxation histories. Dombard, A. J., and McKinnon W. B. (2006), JGR, 111 E01001; Robuchon, G., et al. (2011), Icarus 214, 82-90
Beyer Ross A.
Hammond N. P.
Nimmo Francis
Phillips Cynthia B.
Roberts James Hirsch
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