Off-Topic: Deep Impact
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.
posted by Jason Perry at 9:58 AM
7 Comments:
Tidbit: Does a comet have a north? The rotation potentially changes as new jets open, including the one we artificially created last night. A coordinate system for the nucleus should be imposed without (fixed) reference to its rotation.
The flash should be due to the components of the comet reaching white heat when all the kinetic energy was lost. Certainly ices won't stay white hot long, but for a moment.
I agree that we are seeing pits that are left behind as outgassing takes place. What we need is a model of the dynamics of how an initial "leak" causes erosion to peel back the sides and leave more of a shallow cylindrical pit rather than the partial sphere shape typical of small impact craters.
I suppose the most interesting question to me is: What is the nonice stuff, and what determines its placement on the ambient surface of the comet? Is it simply that ice grains are unstable, so the surface becomes, ipso facto, whatever is nonice? Or does the nonice from jets end up falling back in? Or perhaps other space dust does that?
I am a member of the IAU working group on cartographic coordinates etc., and in our most recent report, to be published soon, we have anticipated this concern. Halley, Toutatis and Hyperion all seem to share the slow long axis mode of rotation with very wide precession angles, and comets might change rotation over time as you suggest. (Hyperion is not really chaotic in rotation as used to be claimed...) In these cases north is defined by using the right-hand rule about the rotation axis - the long axis for these odd rotators, the short axis for most other objects. This is used regardless of the actual orientation of the axis including any variations in orientation. If I understood Mike A'Hearn correctly at today's press conference, north on this nucleus would be the end of the short axis, facing roughly towards the sun at the time of encounter. The top of the image is rotating towards us in the best views.
Phil
Phil, absolultely, there is no basis for me saying north, south, or otherwise. I'm just one of those people who prefers cardinal directions than left and right directions, but that will of course cause problems when you don't know which way north is.
In terms of the model for outgassing, I agree with Bruce here. I just think we are seeing the mesas here in the "southern" (there I go again ;D) portion of what we can see and in the north, we do appear to be seeing pits, presumably altered impact craters. Maybe the mesas represent areas where there are relatively few volatiles (there are a few small isolated mesas in this region "west" of the main scarp) and these take longer to sublime than other regions that have more volatiles and lead to further scarp retreat.
Jason, what you did was perfectly legitimate under the circumstances. I was just replying to jRehling's question 'does a comet have a north?'. The whole question has a complex history and would make a good study. I look forward to getting good rotation and shape data from this mission so a decent map can be compiled.
Phil
Does anybody know if there are any future plans for the flyby spacecraft, such as an asteroid flyby?
There is a possible target but no extended mission is yet funded.
Phil
NASA is spread a little thin these days, and rather preoccupied with internal politics, so responses will be understandably slow.
I urge a note of caution here - The interpretations being expressed are based upon the premise comets are primarily ice/dirt with icy cores, and I don't think the data from Deep Impact to date have provided any evidence to support that hypothesis.
The Dust raised by the impact, if I am reading correctly, is dampening the hydrogen spectra, not enhancing it. Without prior expectations, the best interpretation may be that this comet is mostly dirt and rock.
I am very anxious to see the spectrographic results of the initial impact - NASA said the spectra was difficult to interpret, and it should be, because of the velocity of the ejecta, but it would also be difficult to decipher if it contained heavier elements...iron, nickel...
...my 5 cents worth...
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