Although their insides are primordial, the outsides of comets whose orbital paths take them into the inner solar system have seen a lot of change. Cometary activity happens when comets get close enough to the Sun for some of their icy material to vaporize into gas; the expanding gas flies off the comet, sometimes in collimated jets, much of it never to return to the surface. Some of the dust goes with the gas in cometary jets, but other dust falls back to the comet surface, and some is left behind as a lag deposit. The process changes the surface of the comet in some way, but how exactly?

Before Philae, we wondered: What is the surface like? Is it made of a dust-particle snow with a very poofy texture like powder snow? Is it sticky and compressible, like fresh snow? Does it have some strength but is compressible, like styrofoam? Or is it strong and dense like ice or rock? Understanding the nature of the cometary surface would help scientists develop models for how and why comets have jets, how activity begins and ends, how long comets are active before becoming dormant, and would even help engineers figure out how to safely land a bigger spacecraft on a comet.