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Thickness constraints on the icy shells of the galilean satellites from a comparison of crater shapes

Abstract

A thin outer ice shell on Jupiter's large moon Europa would imply easy exchange between the surface and any organic or biotic material in its putative subsurface ocean1,2,3,4. The thickness of the outer ice shell is poorly constrained, however, with model-dependent estimates ranging from a few kilometres5,6 to ten or more kilometres7. Here I present measurements of depths of impact craters on Europa, Ganymede and Callisto that reveal two anomalous transitions in crater shape with diameter. The first transition is probably related to temperature-dependent ductility of the crust at shallow depths (7–8 km on Europa). The second transition is attributed to the influence of subsurface oceans on all three satellites3,8,9, which constrains Europa's icy shell to be at least 19 km thick. The icy lithospheres of Ganymede and Callisto are equally ice-rich, but Europa's icy shell has a thermal structure about 0.25–0.5 times the thicknesses of Ganymede's or Callisto's shells, depending on epoch. The appearances of the craters on Europa are inconsistent with thin-ice-shell models1 and indicate that exchange of oceanic and surface material could be difficult.

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Figure 1: Impact crater landforms on the icy galilean satellites, representing the major morphologic types.
Figure 2: Depth/diameter measurements for fresh impact craters on the icy galilean satellites.

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Acknowledgements

This work was supported by NASA Planetary Geology and Geophysics.

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Schenk, P. Thickness constraints on the icy shells of the galilean satellites from a comparison of crater shapes. Nature 417, 419–421 (2002). https://doi.org/10.1038/417419a

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