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I was looking at pictures of the moons of Saturn and noticed that the craters on Hyperion have a strange shape, somewhat resembling sinkholes. They look a lot deeper than the impact craters on other moons (including ours). Is it an optical illusion (perhaps caused by the presence of dark material at the bottom of the craters?) that somehow makes them look deeper? If not, what process caused these holes to appear?

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According to Thomas et al. (2007), these craters are not unusually deep:

It is unlikely that unusual crater depths significantly enhance the sponge-like appearance. Crater depth-to-diameter ratios for the 13 examples that can be measured reliably using shadow lengths average 0.21 ± 0.05 (s.d.). These ratios are similar to values for fresh lunar craters, and are slightly greater than for some small rocky objects. They are slightly larger than that for craters on large icy satellites, 0.14.

What is unusual and gives a "sponge-like" appearance to Hyperion is the high density of craters and lack of intercrater plains:

For craters with diameters between 2 km and 11 km, Hyperion has a cumulative number of craters per unit area twice that on Phoebe; at similar resolution these objects are strikingly different.

To explain this, they proposed that ballistic ejecta from impact events, which usually cover older craters, are lost to space instead.

A more recent study (Howard et al., 2012) suggested that mass wasting processes (landslides) and CO$_2$ sublimation also account for this special morphology, especially for the dark-floored craters. This has been confirmed by Dalton et al. (2012), who called these craters "suncups":

The irregular shapes of the sublimation pits (suncups) support the hypothesis of extensive mass wasting and sublimation degradation (Moore et al., 1996, Howard et al., 2011, Howard et al., 2012) largely associated with the loss of volatiles (including water ice). When the ices evaporate, entrained low-albedo grains accumulate in lag deposits, which are subject to solar heating. Warm grains accelerate both the local evaporation of ices and their own spatial concentration.

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