Impact crater relaxation on Dione and Tethys and relation to past heat flow

Science images
Science images

Relating relaxation of impact crater topography to past heat flow through the crusts of icy satellites is a technique that has been applied to satellites around Jupiter and Saturn. This study led by Oliver White, a SETI researcher based at Ames, was published in Icarus on 15 May 2017 and uses global digital elevation models of the surfaces of Dione and Tethys generated from Cassini data to obtain crater depth/diameter (d/D) data.

Comparison with measurements obtained in a similar 2013 study (also led by White) that focused on relaxation on the other saturnian satellites Rhea and Iapetus, reveals that relaxation is found to affect craters down to smaller diameters on Dione and Tethys than on Rhea (Iapetian craters display negligible relaxation). Relaxation simulations performed in the present study allow quantification of the heat flow necessary to relax craters on Dione and Tethys to their present morphologies.

Heat flows exceeding 60 mW m-2 are required to relax several craters on both satellites, and relaxation appears to be subject to geographical controls. On Dione, a ‘relaxation dichotomy’ is defined that separates the more relaxed craters in sparsely cratered plains from the less relaxed craters in heavily cratered terrain. This is demonstrated in (a) in the accompanying figure: colored diamonds represent craters with measured d/D ratios, with violet indicating minimally relaxed, through increasing relaxation fractions of blue, green, yellow, orange and red, while relaxation simulations were performed for the highlighted craters. The configuration of this dichotomy resembles that of the structural-geological dichotomy on Enceladus, implying that a similar resonance-induced tidal heating mechanism concentrated in the southern hemisphere may have affected both satellites. Strong contrasts in crater relaxation across short lateral scales are also observed on Tethys, although defining geographical distribution of relaxation here is hindered by the presence of the young Odysseus impact basin and its associated ejecta (see (b) in the accompanying figure). The relaxation simulations raise the possibility that Odysseus is partially relaxed despite its markedly young age, implying that high heat flow conditions may have affected Tethys comparatively recently.



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