The Scars of Impact

Impacts by projectiles that have crashed into Europa have not had very much effect on the surface we see today. However, they do provide some critical information about what has gone on there. The numbers of impact features indicate how long the surface has been in place. Specifically, the paucity of craters suggests a young surface. The scars of impacts also depend on what lies below. On Europa, the character of impacts sites may reflect the distance down to liquid water.

In order to translate the number of European impact craters into information about the surface age, we would need to know the bombardment rate. If the rate had been low, even a very old surface might have few craters. The good news is that the numbers of craters are so very small that, even with only rough estimates of bombardment rates, we can be confident that the surface is extremely young compared with the age of the solar system. The other good news is that astronomers have made great progress in understanding the bombarding populations(especially of small asteroids and comets), which will improve the accuracy of surface-age estimates.

Like the measurements of age, measuring ice thickness using impact features requires information about a variety of phenomena. We need to understand what types of scars would be produced by impacts into ice of various thickness. Studies of the Earth, Moon, and other terrestrial planets have taught us a great deal about crater formation on solid bodies, but in order to interpret the record on Europa we need to know what happens when an ice crust, floating over liquid water, is bombarded. What sort of surface feature will result, and how does it depend on the thickness of the ice and the size and speed of the projectile? Based on careful experimentation and computer modeling, planetary scientists can now make meaningful predictions of what sort of impact feature to expect under various circumstances. Even so, comparison with observed craters will be somewhat subjective. Moreover, whatever is learned about the thickness of ice will only apply to a specific place and time of impact, and impact sites are few and far between on Europa.