Followup: Deep Impact crater on Tempel 1

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Feb. 16 2011 7:00 AM

Followup: Deep Impact crater on Tempel 1

At the Stardust press conference yesterday, they displayed an image that purported to show the crater where the Deep Impact impactor slammed into the Tempel 1 comet nucleus back in 2005. When I wrote up my previous post that picture was not available, but it's been a few hours and it's now online. So here you go:

On the left is the Deep Impact image taken by the impactor not long before the end. On the right is the Stardust image; note the resolution isn't as good. That's what you get when you're 178 km away instead of a dozen or so! Still, the arrows mark the outline of the impact crater rim. On the left, I marked a dark mound of material that existed in 2005... but is gone in the new image. Not too surprising! The impact was like setting off nearly 5 tons of TNT, so that mound is now most likely vaporized water orbiting the Sun on its own.

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death from the Skies!  

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I'll note this image is much better than what impact expert Pete Schultz had available at the press conference (they were scrambling to get the images together right up until the conference started). I couldn't see the crater in the image he showed then, but in this one the rim is actually fairly clear. You can also see the faint central mound inside the crater; that's probably material that was lifted up by the explosion and then fell back in. I wonder though: In big impacts on rocky bodies (like the Moon) you get a central peak, and that's due to molten material flowing back from the rim and splashing up (the process is called isostatic rebound, a phrase I love). I don't know if that would apply here or not, but it'll be interesting to hear what the scientists say as they have more chance to study the images.

Also in the Deep Impact image on the left is a heart-shaped hole or depression. In the Stardust image it appears to have rounded out a bit. Hard to say with the lower resolution, but things do look different.

In fact, take a look at this image:

These show a smooth area, elevated a bit above the rest of the surface. The top image is from 2005, the bottom from 2011. There are several changes; the yellow lines mark a couple of craters that have apparently merged into a trough of some kind in the intervening years.

Also at the top right of the 2005 image, the Sun is brilliantly illuminating a steep cliff. It's hard to say if the cliff is gone in the bottom image because the lighting is different -- it looks like it's disappeared, but that may simply be due to the Sun not shining on it as strongly. Still, the shape is different. Scientists think the cliffs may have receded by as much as 30 meters in the past few years. In the bottom image there are a series of mounds running horizontally across the top of the plain; these are not apparent in the older image. Again I'm not sure if that's an illumination issue, but I imagine that, since scientists have multiple Stardust images from different angles, they can construct at least a crude 3D map. That will help them figure out what's changed.

Overall, these images show that scientists have their work cut out for them, but that the data they need are there! I've known Pete Schultz for a few years -- he was one of our experts on "Bad Universe" -- and I expect it'll be about a week before his feet even touch the ground.

My congratulations to the Stardust folks for threading the needle on this one! Amazing work.