Monday, July 04, 2005

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.


Anonymous JRehling said...

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?

7/04/2005 12:27:00 PM  
Anonymous Phil Stooke said...

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.


7/04/2005 01:46:00 PM  
Blogger Bruce Moomaw said...

"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."

There was quite a bit of speculation about that when the same startling features were seen on Wild 2 by Stardust. I imagine it has to do with sun angle; a lot of these things may well start as very small impact craters, but since the Sun shines on the floor of the crater for longer periods as the nucleus rotates than it shines on the walls, the ice sublimates faster from the floor -- and it sublimates faster from the near-horizontal central parts of the floor than from its sloping outer regions. One can conceive of how this could produce a feedback phenomenon that would end up simultaneously flattening the floor of the crater while steepening its walls (whose lag deposit of non-volatile dust would either fall down to the floor or actually get blown away from the wall by its sublimating water vapor and scattered evenly over the crater floor), and which would also widen the initial crater at a much faster rate than it deepened it. Voila: conversion of an initial very small bowl-shaped impact crater into a flat-bottomed but steep-sided depression whose width (but not its depth) would steadily widen with time -- until such overlapping depressions ate away most of the comet's outer surface, leaving behind only some isolated and steadily shrinking "mesas" like those on the relatively Sun-evolved Borrelly.

There's one catch, though: the Stardust imaging team reported in "Science" that the ice sublimation rate on Wild 2, since it wandered into the inner System a few decades ago, should have been only enough to remove about one meter of ice from it. However, since they also reported that Wild 2 has probably been tossed around a lot by the giant planets and that it may well have had earlier and much longer excursions into the inner System -- and since Deep Impact, whatever else it proves, as certainly proven that cometary ice can sublimate like hell, maybe at a much higher rate than the Stardust team estimated -- I don't think this is an insuperable difficulty with the crater-widening theory.

7/04/2005 06:06:00 PM  
Blogger Bruce Moomaw said...

Actually, let me modify that model. Plain old gravity may have a more important effect in turning an initial small bowl-shaped crater into a big, flat-bottomed, steep-walled, horizontally growing depression than Sun angle does.

When the ice sublimates away from the shallowly sloping floor parts of a crater, its lag deposit of dust is going to tend to just sit there without shifting (or fall back onto the same places from which the sublimating water vapor blows it) -- and in the process gradually build up a shield layer to inhibit further ice sublimation from the floor. When ice sublimates away from the more steeply sloping parts of an initial bowl-shaped impact crater's walls, on the other hand, it IS going to fall downslope -- or get blown downslope by the water vapor -- onto the crater's floor. And so the vertical slope of such walls will remain high (and they'll recede horizontally away from the center of the crater).

7/04/2005 06:15:00 PM  
Blogger Jason said...

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.

7/05/2005 11:36:00 AM  
Anonymous Phil Stooke said...

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.


7/06/2005 06:06:00 AM  
Anonymous Tayfun Oner said...

Does anybody know if there are any future plans for the flyby spacecraft, such as an asteroid flyby?

7/06/2005 09:29:00 AM  
Anonymous Phil Stooke said...

There is a possible target but no extended mission is yet funded.


7/06/2005 01:18:00 PM  
Blogger Bruce Moomaw said...

"NASA Watch" -- which reported yesterday that there was no funding for an extended mission -- now reports that NASA has changed this. :

"Late yesterday (Tuesday 5 July -- after this article was published online) the Deep Impact team was told for the first time that there would now be enough money to perform the TCM (trajectory correction maneuver) required to send the spacecraft back toward Earth for a gravity assist to whatever new target is eventually chosen. In addition, yesterday evening, the team was informed that NASA had identified funding for a very minimal flight operations team - just enough to keep the spacecraft going until a new target is chosen. Meanwhile, as noted below, no one from NASA, JPL, or University of Maryland public affairs has responded to -- or acknowledged -- my initial request several days ago."

There are apparently two possible comet targets for an extended mission. A few years ago, A'Hearn told me that the favored target ws Finlay -- but now reports that the favored target is Boethin, which would take 3.5 years to reach:

7/06/2005 04:31:00 PM  
Anonymous Jerry said...

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... 5 cents worth...

7/11/2005 10:15:00 PM  

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