Where does the 19 to 25 kilometer value come from?
How certain are these estimates of Europas ice shell thickness? There is some uncertainty in the exact thickness using these techniques. This is due mostly to uncertainties in the details of impact cratering mechanics, which are very difficult to duplicate in the laboratory. The uncertainties are probably only between 10 and 20%, however, so we can be reasonably sure that Europa’s ice shell is not a few kilometers thick. Could the ice shell have been thinner in the past? There is evidence in the crater topography that the thickness of ice on Ganymede has changed over time, and the same might be true for Europa. The estimate for ice shell thickness of 19 to 25 kilometers is relevant to the icy surface we now see on Europa. This surface has been estimated to be 30 to 50 million years or so. Most surface materials older than this have been destroyed by tectonism and resurfacing. This older icy crust could have been thinner than todays crust, but we presently have no way of knowing. Could
