What an exciting 24 hours. I know this doesn't have anything to do with outer planetary satellites, but I still found this fascinating enough to post here. You can comment if you like here. All Deep Impact-related discussion by myself or you, the reader, will be limited to this post.
For the most, comets have not interested me. Sure, I looked for Hale-Bopp, but that was pretty much the limit of my interest. Stardust and Deep Space 1 didn't really help much. The images they returned were lackluster at best, at least the ones released to the public. Being more of a geologist at heart, such images hold little interest to me. Enter Deep Impact.
The images this mission has returned have been spectacular. Even small thumbnails circulating on the web are spectacular, showing the impact event as well as the impact region before the event. The latter has been holding my interest the most (sure I like that "flash" image from HRI, but that only holds my attention for so long). The pre-impact impactor images are fascinating, showing wonderful detail of the comet's surface at up to 7 m/pixel. The above image shows the region a minute or so before impact. You can see circular features as well as a number of "north"-ward facing scarps. I should point out that all directional phrases I use assume north is up, which has no basis in reality, I'm just using it till I hear otherwise. Anyways, the circular features obviously look like craters, at least that would be the initial interpretation, but look closely. Sunlight is coming from lower right. The circular features are surrounded by dark "stuff" that has a sharp inner contact and a diffuse outer one. This shading, I posit, is more suggestive of an outward facing scarp rather an inward facing crater rim.
So what do we make of this morphology? Combine this with some earlier impactor images and the bright markings you see in the above image, and you start to see the full picture. I suspect we are looking at cliffs that are being eroded away by cometary outgassing. The jets remove surface layer material (water ice, which volatilizes, and "other" material) and eject it into space. Over time, you go from the circular pits seen by Stardust at Wild 2, to the terraced formations we see here at an older, more evolved comet like Tempel 1. The lack of craters on the surface (the artificial one notwithstanding) can explained by saying that the rate of resurfacing (or the removal of surface) is faster than the rate of impacts.
Now to the impact itself. We saw quite a bright flash :o So what explains that? First, the HRI images appear to show low angle ejecta with radial structure. Perhaps we are seeing the formation of rays? Surprising considering the size of the nucleus. The impact kicked up a considerable amount of material that didn't return. Maybe the impact created a new jet as it exposed fresh water ice?
I should note that this is just my own speculation, so take it with a grain of salt, but I thought I'd get my speculation out there for comments. Though, given the impending news conference at 11am, I could be completely wrong in only 30 minutes.