Seeing the crater would have been nice, and layering in the walls very nice, but I imagine the most important results come from the remote sensing of the ejecta composition.

In fact if there was relatively little volatile ejecta and lots of dust, as suggested so far, there may not have been a lot to see in the crater anyway. I think (admittedly on little evidence) that any difficulties may be as much caused by the comet itself as by the focus problem.

I'm an image guy so the focus thing does concern me, but if spectroscopy answers the composition questions that's probably going to be a great result. I'm not as concerned as Bruce about this. So far.

I am concerned about that cost cap decision, though.

Phil

I will be concerned about it when it passes. So many things show up in committee and versions of bills that never see the light of day that I wouldn't have any hair left if I fretted over each one. If that decision makes it into the final bill, that spells trouble.

Phil: From what I can see, deconvolving the images increases single-pixel to few-pixel noise and artifacts "several times", about what you'd expect. If the crater would have been visible in images just before shield-mode at say 5% contrast through the plume, cutting contrast several times could eat your lunch.

And unfortunately, I don't expect diddly-squat from color imaging of the nucleus now. Typically, there is very little color contrast on "small bodies" in Galileo and NEAR and Giotto images and you need to squeese out maximum signal-to-noise from the data to have anything useful. NEAR, for example, had a nasty noisy little camera and could barely see color variations in different albedo materials on Eros. Blow your signal-to-noise level on deconvolution of images of a small body, and you've lost not just single-pixel color information but lost integrating color voer enough pixels on a given feature for useful color information at all. I hope I'm, wrong, but I expect to be frustrated.

[quote=edstrick,Jul 19 2005, 09:07 AM]
Phil: From what I can see, deconvolving the images increases single-pixel to few-pixel noise and artifacts "several times", about what you'd expect. If the crater would have been visible in images just before shield-mode at say 5% contrast through the plume, cutting contrast several times could eat your lunch.

This would be a good kind of mission to repeat on a series of small bodies for comparison, with good cameras. There is no reason to think we missed a once in a lifetime opportunity.

Don

Yes, including trying to hit one of the hyperbolic orbit comets as they come in. If we build and keep a few Deep Impact probes around for such an occasion, we'll be ready.

I'm having trouble with the Quicktime plugin in Adobe Imageready (stupid Quicktime 7). Can anybody convert the two Deep Impact .movs to animated GIFs for me so I can play with them?
http://photojournal.jpl.nasa.gov/catalog/PIA02125
http://photojournal.jpl.nasa.gov/catalog/PIA02130

--Emily

I've converted the files, Emily, but they are rather large, so I'll need your email address in order to send them to you.

Ed: Yes, but I wonder what MRI got...while its resolution is poorer, it might have the S/N ration to identify broad variations.

Thanks very much for this, um3k. I took the GIFs you sent from the ITS and the MRI and deleted 5 of every 6 frames (they were redundant). I noticed that the levels varied a lot from frame to frame (I wonder why they didn't correct that? seems like it should have been easy), so I did a quick and dirty job of adjusting the levels so that the histograms of all the ITS images looked reasonably similar, which helps to smooth out the animation a bit. For the MRI animation, I rotated the frame a quarter turn so that it matched the orientation of the ITS animation a little more closely. Finally I combined the two into one movie:

Deep Impact combined animation (1.6 MB)

You can also see it on this web page:
Year in Pictures: Deep Impact

--Emily

Very nice, Emily!

I thought the development of the ejecta plume was fascinating!

Bob Shaw

Is it definitively known that the secondary plume did not have significantly more water than the conditions before the impact?


- Bob Clark

Is there the possibility that the impact destroyed materials that could not be analyzed by the flyby craft as a result?

In preparation for a probe to land on Europa and dig/melt/blast through its ice crust, perhaps a "test" mission to a comet trying out similar methods (and getting plenty of comet science in the process without having to blow up anything) should be in first order.

I am surprised that the DI mission team thought that they would have a clear view into the crater created by the impactor, rather than the cloud of debris we saw instead. Didn't they do computer modeling?

It's possible a small amount of material in the immediate vicinity of the impact got chemically altered due to very high temperatures, but I suspect the majority of the plume material was simply "dislodged" and ejected, followed by sublimation of volatiles underneath.


Any lander on Europa is likely to only do melting of the ice, blowing up and excavating is pretty much out of the question - you'd need to carry explosives and the ice is virtually rock at those temperatures. Also, an Europa lander that melts its way through will benefit from the surface gravity to drag it downwards, the latter is practically nonexistent on a small comet. Their respective compositions are radically different, too, comets being icy dirtballs (as opposed to dirty iceballs, which was thought until recently).

I just don't see a point in proof testing a Europa lander on a comet.


They probably assumed the comet would have a much higher percentage of ice vs dust. You can't model the behaviour of something you don't know what it's made of. That's one of the reasons this mission was flown in the first place.

I am sure I will be told that I am missing some point here, but considering that we've known for centuries about the vast amount of material constantly blown off by comets nearing the Sun (and even far away, like Hale-Bopp), why would scientists have assumed that the crater view would be relatively clear - especially since the flyby craft wouldn't be able to hang around for very long?

It's too bad the flyby craft couldn't have gone into orbit instead to wait for the debris to settle (yes, I know that is no trivial exercise and would have added great expense to the mission) or if the flyby or some future mission could go by Tempel 1 again to see what transpired after the impact.

The reason I am making a big deal out of this is that one of the big goals of the mission (stated numerous times over by NASA) was to have the flyby craft peer into the crater to learn about the comet's interior. Yes, they got measurements of materials from the blast plume, but we did not see the interior. Like I said, how did they hope to accomplish that with such a short time period?

With the deep impact mission what they could have done was release the probe much sooner so that it would have impacted the comet several hours/days ahead of the main probe coming past it.

In that way we could have seen the impact crater after the "dust cleared". Problem with this of course would be targeting the impact probe precisely to the comet if it were released that far in advance of impact. Also, some of the science of the debris plume might be have benn degrade.

And of course, that means packing a bigger battery into the impactor, and a more powerfull transmitter. Which makes it heavier and bigger and so on and so forth....

Doug

And it would have meant that the trajectories would have diverged more, so observations would have been from further away, unless you tweak the flyby craft's orbit, which requires additional fuel.....

tty

Not necessarily diverted more, the probe would have simply reduced delta V in the direction of travel. Targeting, communication and battery life would certainly have been an issue but one huge problem would have been that the probe would have to fly through the ejecta from the impact and probably not survive

And of course what they really wanted to do with the flyby craft was watch the crater develop, which they couldn't have done from a much greater distance. Basically, they had predicted a length of time it would take for the "dust to clear," and multiplied that by some number to make their guess "conservative," and then they planned their sequencing. Tempel 1 sure fooled them. Too bad they couldn't see the crater develop with the camera, but they seem to have gotten lots of interesting data from the ejecta with the spectrometer.

--Emily

You can't just reduce speed in the direction of travel without changing your trajectory sideways too. I agree about the ejecta problem.

tty

How porous/light would the comet have to have been for Deep Impact to have caused it to break apart?

Contact has been made with the Deep Impact fly-by spacecraft hibernating in solar orbit. No anomalies were reported. It is in good shape to do more science.
It will do a fly-by of Earth in about 23 months, which could be used to control its trajectory. Where it goes from there is yet to be decided.

No way that could ever have happened -- the impact was incapable even of measurably deflecting the comet's trajectory. Also keep in mind that, the softer a gravity-bound object like a comet or a "rubble pile" asteroid is, the HARDER it often is for an impact to break it apart -- the impact's energy is locally absorbed by simply throwing out some local ejecta, much of which then returns to the object through its gravity. It's like the difference between shooting at a brick and shooting at a sandbag. (Celestial objects that have actually been fragmented by a giant impact are though to often reassemble themselves this way, which indeed is what explains the existence of rubble piles -- and was once thought to be responsible for the jigsaw appearance of Miranda, although the favored theory on that has changed.)

This is one of the reasons I am vexed by the official interpretatations: Sandbags do absorb, but here we had 1) A brighter-than-expected UV pulse. 2) Greater-than-expected volume of ejecta in the opposite direction from the impact.

Can anyone draw me a map that shows how an object with 90% void volume could do that?

On a side note, I was meeting with Ball Aerospace on a different project last week, and they were quite defensive about the Deep Impact results. Ok - Bruce and I diss them about the lens issue, but don't we all agree Deep Impact was a tremendous engineering and scientific success? Isn't an unexpected result better than a mission that exactly matches a prior predictions?

An new LPSC abstract on the subject by the Deep Impact science team ( http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2192.pdf ) says that the impact flash was a lot DIMMER than expected: "The overall luminous efficiency of the flash was far lower than predicted, most likely due to the high porosity and volatile content of the Tempel 1 surface." Extensive details are provided.

As for Deep Impact being "a tremendous engineering and scientific success": no quarrel about the former, but I have PLENTY of quarrels about the latter. When I look at the LPSC abstracts on the mission, I get the distinct impression that:

(1) Where science return is concerned, this thing delivered a lot less than they had hoped for, in regard to both the comet's physical structure and its chemical makeup;

(2) Most of the really useful stuff they got was from its images of the comet before the impact; and

(3) A simple CONTOUR-type mission -- perhaps with a subsurface radar sounder added -- would have produced a lot more.

From the moment this mission was picked (which apparently came as a major surprise to planetary scientists), I've always thought the selection had Captain Crazy written all over it, although admittedly I have no direct evidence of this. The resemblance to his abortive 2003 Mars Airplane just to commemorate the Wright Brothers centennial -- and to his cancellation of the Pluto mission because "Nobody gives a damn about Pluto" and astrobiology was SO much more spectacular -- is unmistakable. He was the Cecil B. DeMille of NASA, and this thing would have struck him as just the sort of Big Spectacle capable of attracting the rubes... er, voters.

DEEP NEWS

Newsletter for the Deep Impact mission

Issue #30, January/February 2005

--------------------------------------------------------------------------------

It's amazing that in the world of space exploration, in which spacecraft
sometimes take years to reach a destination, the Deep Impact mission journeyed
through funding, design, building, launch, encounter, and this month - the
release of findings from the science team. With more study still to come, Deep
Impact was proven to be a tightly scheduled mission with spectacular results.
You joined us somewhere along the way getting our monthly updates and we hope
you enjoyed the ride. This issue is the last of our monthly updates and in the
future, we will contact you with shorter news about Deep Impact's science and
technology. If for some reason you are just joining us take a look at what you
have missed on our web sites:

http://deepimpact.jpl.nasa.gov

http://deepimpact.umd.edu


PICTURE THIS - WATER, WATER EVERYWHERE!

Well, perhaps not everywhere - but results from the IR spectrometer aboard the
Deep Impact flyby spacecraft show that water ice exists on only about 0.5% of
the comet's surface. Take a look and see where the team found the water ice.

http://deepimpact.jpl.nasa.gov/gallery/Sur...eLocations.html


MISSION UPDATE: NEW RESULTS FROM THE SCIENCE TEAM

Results from the flyby spacecraft's IR spectrometer are shared in a summary from
a paper by Co-Investigator Dr. Jessica Sunshine and the science team. Also, read
Ray Brown's outline of Deep Impact findings based on a summary of telescope
observations from Co-Investigator, Dr. Karen Meech and collaborators.

Summary from the Ground Observation:

http://deepimpact.jpl.nasa.gov/mission/upd...602.html#rbrown

Summary from the IR Spectrometer:

http://deepimpact.jpl.nasa.gov/mission/upd....html#lmcfadden


TECHNICAL UPDATE FROM THE PI - DR. MIKE A'HEARN AT THE UNIVERSITY OF MARYLAND

Principal Investigator Dr. Mike A'Hearn gives his science team a technical
update on the Deep Impact flyby spacecraft after the engineering group at Jet
Propulsion Laboratory communicated with it on February 10th.

http://deepimpact.jpl.nasa.gov/mission/upd...02.html#mahearn


UP CLOSE AND PERSONAL: MEET DR. KAREN MEECH, DEEP IMPACT CO-INVESTIGATOR

Karen knew since she was very young that she would end up working in astronomy
someday and there were lots of Star Trek episodes and evenings with her father
watching the sky to help confirm that decision. Meet Karen Meech.

http://deepimpact.jpl.nasa.gov/mission/bio-kmeech.html


FOR EDUCATORS: WHAT DOES THE MOON HAVE TO DO WITH THE DEEP IMPACT MISSION?

Well, quite a lot actually. Shortly after takeoff, the science team used the
spacecraft images of the moon to check the calibrations for their instruments.
Here is a Mission Challenge based on their calculations and set to national math
standards. Have your students do real mission math!

Question: What was the distance from the Deep Impact spacecraft to the moon on
January 16, 2005?

On January 16, 2005, the Deep Impact spacecraft turned its cameras back toward
Earth and captured some beautiful images of our Moon. To aid in calibrating the
spacecraft instruments, astronomers would like to know precisely how far Deep
Impact was from the moon when this picture was taken.

http://deepimpact.jpl.nasa.gov/disczone/ch...n_Distance.html


WAY TO GO, STARDUST! - RETURN TO SENDER

Congratulations to the Stardust Mission for the safe and successful return of
their capsule containing actual particles from comet Wild 2 on January 15, 2006.
There was great excitement as their science team viewed the grid in which
squares of aerogel safely cradled the first samples of cometary and interstellar
dust to be returned to Earth for study. As a sister to Deep Impact, findings
from both missions will bring scientists closer to answering questions about the
formation of the solar system.

http://stardust.jpl.nasa.gov/news/status/060125.html


PLANETARY SOCIETY ANNOUNCES THE WINNERS OF THE CRATER CONTEST

Although the precise dimensions of the crater made by the Deep Impact mission in
Tempel 1 were hidden by ejecta from its nucleus, the Planetary Society was able
to pick 3 winners from those whose predictions fell within the range determined
by the science team. Take a look.

http://www.planetary.org/about/press/relea...ry_Society.html


DID YOU SEE OUR PAST DEEP NEWS ISSUES?

Visit http://deepimpact.jpl.nasa.gov/newsletter/archive.html to catch up on
exciting past news from the Deep Impact mission.

Deep Impact is a Discovery mission. For more information on the Discovery
Program, visit:

http://discovery.nasa.gov/

The Deep Impact mission is a partnership among the University of Maryland (UMD),
the California Institute of Technology's Jet Propulsion Laboratory (JPL) and
Ball Aerospace and Technology Corp (BATC). Deep Impact is a NASA Discovery
mission, eighth in a series of low-cost, highly focused space science
investigations. See http://deepimpact.jpl.nasa.gov or our mirror site at
http://deepimpact.umd.edu.


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site for our high web traffic time and will be returned in the future. Thanks
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Thanks, Bruce - the earliest frames clearly indicate there was significant penetration before the impact flash, although the depth of the penetration is still up-in-th-air because of the oblique angle.

There are a lot of shudda whoulda couldda's surrounding this mission. I guess I am one of those yahoo's, because I would love to do it again.

Article on Deep Impact in the latest The Planetary Society The Planetary Report:

Deep Impact: Understanding Comet Tempel 1

On July 4, 2005, the Deep Impact spacecraft sent a 370-kilogram (820-pound) copper ball on a collision course with comet Tempel 1 to give us our first look inside a comet. Within minutes of the impact, the spacecraft returned to Earth spectacular images of the explosive event. Exactly what these images and other data revealed, however, took much longer to analyze. Now, 6 months later, Deep Impact coinvestigator Lucy McFadden and coauthor Ray Brown detail what scientists are discovering about comet Tempel 1 and what Deep Impact has taught us about the oldest components of our solar system.

http://www.planetary.org/programs/planetary_report.html

The printed article is a little disappointing - There is an artist's rendition of the nucleus that makes it look like it has a heavy frosting of ice, and the caption calls it a 'dirty snow ball' 0.04% surface moisture is not consistent with either the rendition or description.

Astrophysics, abstract
astro-ph/0603306

From: Damien Hutsemekers [view email]

Date: Mon, 13 Mar 2006 13:46:17 GMT (26kb)

Deep Impact : High Resolution Optical Spectroscopy with the ESO VLT and the Keck 1 telescope

Authors: E. Jehin, J. Manfroid, D. Hutsemekers, A.L. Cochran, C. Arpigny, W. M. Jackson, H. Rauer, R. Schulz, J.-M. Zucconi

Comments: Accepted for publication in ApJ Letters

We report on observations of comet 9P/Tempel 1 carried out before, during, and after the NASA DEEP IMPACT event (UT July 4), with the optical spectrometers UVES and HIRES mounted on the telescopes Kueyen of the ESO VLT (Chile) and Keck 1 on Mauna Kea (Hawaii), respectively. A total observing time of about 60 hours, distributed over 15 nights around the impact date, allowed us (i) to find a periodic variation of 1.709 +/- 0.009 day in the CN and NH flux, explained by the presence of two major active regions; (ii) to derive a lifetime > ~ 5 x 10^4 s for the parent of the CN radical from a simple modeling of the CN light curve after the impact; (iii) to follow the gas and dust spatial profiles evolution during the 4 hours following the impact and derive the projected velocities (400 m/s and 150 m/s respectively); (iv) to show that the material released by the impact has the same carbon and nitrogen isotopic composition as the surface material (12C/13C = 95 +/- 15 and 14N/15N = 145 +/- 20).

http://fr.arxiv.org/abs/astro-ph/0603306

So is Temple 1 highly homogenius, or did the probe truly penetrate deep into the comet? I think this data, combined with 1) The lack of a temperature gradient in the dust 2) low moisture content in the ejecta. suggest that all we did was rattle a bunch of dust off a resilient surface. The water lies beneath.

So experimental results indicate the science behind the assumption that the probe penetrated a weak surface is far from conclusive - especially since the error bars on the gravity/density determinations are so wide.

Except that the reason Housen thinks this ( http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1068.pdf ) is precisely that the plume particles shot away from the nucleus at several times the speed they would have travelled had they just been dry, loose dust particles ejected by the force of the impact -- which means that "comet-like acceleration mechanisms [by which he means a powerful blast of released gas from thawed-out volatiles inside the comet] are required to move the ejected mass to 100 km ranges in a few-hour time frame. But if there were such mechanisms, then up to all of the ejected mass could be transported to large ranges over an hour or two and observed. Then, to within the large uncertainties in particle sizes, albedo, ejecta scaling and so forth, any of the above strength cases, from 0 to 12 kPa, could furnish the amounts of total mass estimated. So, either gravity or strength craters can be consistent with the observations. Thus, the observations to date do not discern between the relative importance of strength and gravity in the DI event." That is, not only did the impact lead to a massive outburst of thawed-out ices from inside the comet -- that outburst was so powerful that it has ruined our ability to estimate the actual hardness of the outer dirt layer on the comet, except that it must be somewhere below 12 kilopascals (which is about 120 grams per square cm, or 1.8 lb. per square inch).

In other words, the very fact that D.I.'s measurement of the comet's surface hardness is so inexact (according to Housen) is because the impact DID punch through the outer layer of dry crust to a large reservoir of ices beneath -- just as the D.I. team has insisted, and The Messenger has denied. Jessica Sunshine noted that not only was a fair amount of finely powdered ice and water vapor released by the impact (as detected by the craft's mapping near-IR spectrometer), but that jet of water wasn't spread out in a very wide cone of ejecta like the dust splattered by the impact -- it was "very collimated"; that is, it shot up in a relatively narrow jet from the bottom of the crater, proving that it was material that had been vaporized underneath the crater and had broken up through the thinnest part of the crater's dirt-layer floor. She also notes that the three patches of water ice located before the impact on the nucleus' surface by the near-IR spectrometer were all very dilute -- only about 3-6% of the material in them was ice, with the rest being dust -- and therefore sublimation from them couldn't even account for the natural jets of water vapor seen coming from the comet before the impact; those jets must have come from ice being vaporized underneath and breaking open ventholes in the outer dry layer. ( http://www.planetary.org/blog/article/00000499/ ; http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1890.pdf )

And the impact also released much lower-temperature ices than mere water ice. The team noted from the start that the jet of gas coming out from the impact contained a much higher ratio of CO2 to water vapor than the pre-impact natural jets coming from the comet ( http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1978.pdf . According to A'Hearn's "Science" article, the CO2-to-water ratio increased fivefold.) D.I. also detected a large amount of organic gases squirting out of the impact site, but didn't have the spectral resolution to identify any of them -- but Michael Mumma, using the Keck Telescope, WAS able to measure the pre- to post-impact ratios of three of them. Sure enough, ethane -- which has a very low freezing point -- shot up dramatically after the impact relative to the amount of water vapor released by the comet; while HCN and methanol -- which, unlike ethane and CO2, have much higher freezing points comparable to water -- did not increase in their released amounts relative to water vapor (although they did increase to about the same degree as the water vapor did after the impact). See his article in the Oct. 14 "Science".

So The Messenger is simply totally wrong on that particular point of his: the impact unquestionably did break all the way through the comet's dry outer crust to release a very violent jet of material from the ices underneath -- and it also broke through the layer of higher-temperature ices just beneath that dry layer (including water, HCN and methanol) to release some of the still lower-temperature frozen materials underneath that (including CO2 and ethane).

As for Housen's statement that the D.I. impact did not, after all, prove that the comet's surface was made of totally loose powdery dust but that it may have been caked instead: it meshes well both with the tendency of gas jets from comets to suddenly break through the outer layer and then erupt violently, and with the fact (indicated by some Earth-based spectral observations, and apparently confirmed by the sudden burst of particles Stardust ran into when it was very far from Wild 2's nucleus) that a lot of the debris blown off comets acts like "fireworks" -- that is, relatively large bits of caked dust and ice get blown off the comet, and then later explode into a burst of finer dust particles when the ice inside them is in turn vaporized by the Sun and produces gas pressure inside them.

Bruce:

And comets, historically, have broken apart and formed new tails and so forth, often when warmed - which tends to suggest a 'pressure cooker' that's burst rather than a fluffy pile of dust.

However, I think it's also true that the trend over recent years *has* been away from the primeval rocky iceberg model, and towards a class of objects with rather more of a history - and a constitution closer to many asteroids.

Bob Shaw

Yeah; one would think, if the dust layer was entirely loose and fluffy, gases produced underneath would tend to diffuse upwards between the grains fairly gently and steadily -- and that this would be pretty evenly distributed all over the nucleus' surface. (Also, it's kind of hard to conceive how, underneath such circumstances, the quite sharp cliffs that we've seen all over the surface of all three of the nuclei we've gotten a really good look at could be sustained, even in such extremely weak gravity. I'll be having more to say about those shortly.)

Anyone interested, or indeed perturbed, by references on UMSF to the Elder Gods (who may or may not like having innocent comets whacked by the engines of an upstart mankind) may care to visit the following website:

http://www.cthulhulives.org/cocmovie/index.html

Aiiii!

Bob Shaw

Ahem. Getting back to Deep Impact, I continue to be struck by how apparently little we actually learned from running into that comet as opposed to just flying past it a la CONTOUR. Spectral studies of the ejecta cloud have apparently told us very little about the composition of comets that we didn't already know (or wouldn't have learned from CONTOUR and Stardust); we never did see the crater; and if Kevin Housen is right, the impact didn't even give us much useful information about the hardness of the surface layer. The impact was a spectacular light show, but not very cost-effective scientifically -- which further bolsters my suspicions that Captain Crazy was behind its original selection as a Discovery mission.

Actually, to my mind, the most interesting DI-connected LPSC papers do indeed concern the craft's photos of the comet nucleus before impact. Joe Veverka has a very nice one on the peculiar structures seen on Tempel's surface ( http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1364.pdf ) -- Emily, alas, apparently missed his talk during all her frantic running around to try to drop in on separate LPSC sessions simultaneously. Once again, we are seeing the layered structures, scarps and mesas that we saw earlier on Borrelly and Wild 2 (Giotto's photos of Halley were too fuzzy to be useful here). I've always believed that what we are seeing is evidence that comet nuclei literally "peel from sunburn". That is, when you get an quite small impact crater or vent pit in an initial nucleus surface that has a lot of ice in it, solar warmth sublimates the ice away and leaves a lag deposit of dry dirt. On a flat surface like an impact crater's floor, this lag deposit simply grows to a certain thickness and then serves as insulation to keep the ices underneath from boiling away further. But on a crater's or pit's side slope, provided it's steeper than the angle of repose of dry dust on that comet, the loosened rock dirt slides down the slope, exposing more fresh ice-dust mixture to be eroded away by the Sun -- so that such craters don't increase much in depth, but instead grow sideways into wider and wider flat-bottomed depressions (like those on Wild 2) that can ultimately wrap clean around the comet and leave only isolated mesas like those seen on Borrelly. Then, when new small impact or venting pits appear in that newly exposed layer that are deep enough to expose more ice, the whole process starts all over again -- and it's even possible then for ice to sublimate out from underneath the layer of dry caked dirt on top, producing actual overhangs such as have apparently been seen in a few places on Wild 2.

Nice theory, and Veverka seems to agree with it -- but Mike Belton proposes a startling alternative ( http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1232.pdf ). Namely: we may be seeing evidence that comets (or at least Jupiter-family comets, which is all we've gotten a good look at yet), may have been originally formed not by smaller blobs of ice/dirt that bumped into each other and stuck together lightly while retaining most of their original shape (making the accumulated comet nucleus a "lumpy" conglomerate), but by such blobs hitting the accumulating nucleus hard enough to smear themselves flatly over its surface like a snowball hitting a wall. In that case, the nucleus is made up of an onion-like patchwork of multiple flattened layers from different cometesimals, almost none of which stretches all the way around the nucleus -- and "Mesas are formed, following the suggestion by Britt et al., as a result of erosional sublimation at the boundaries of the outermost layers during passages near the sun. Cometary splitting and tidal disruption is seen as the result of detachment of entire layers or, possibly, disassembly of essentially the entire, presumably weakly bonded, layer structure." Belton calls this the "talps" model (it took me a while to realize that this is "splat" spelled backwards). I'm inclined to agree with Veverka that, while Belton's theory is interesting, as yet we simply do not have the data yet to decide which of these two models is true (or whether both of them are). As Belton says, this makes the data from the CONSERT radar-sounding experiment on Rosetta even more important.

There are, however, two other very interesting kinds of surface structures seen on Tempel by DI and mentioned by Veverka. First: "Two areas of extensive smooth terrain are evident. They are completely uncratered, suggesting a young relative age, and very smooth at meter scales. Both occur in gravitational lows." The bigger one shows marks suggesting that it flowed downhill into this gravitational low, and "A potential source area (only a few hundred meters wide) can be identified. The extremely smooth surface of this feature suggests that this putative flow consisted of materials which are extremely fine and uniform in texture." We are, I imagine, looking at an analogy of the "ponds" of fine regolith seen in the gravitational lows of Eros and Itokawa -- some of the dust squirted off the comet's surface by gas jets and impacts does not escape completely, but tends to sift and slide back to form a deep, loose "pool" filling low spots on the nucleus (and later hardening a bit so that the erosive solar processes I mentioned earlier can nibble at its edge to produce a steep edge scarp, such as has also been seen for this big smooth area on Tempel).

Second, there are other spots on Tempel that "appear to preserve evidence of past cratering, suggesting that the nucleus spent significant amounts of time in environments in which sublimation erosion rates were very low compared to cratering rates. At least 60 craters ranging in diameter from 50 to 2500 meters can be identified. The resulting population has a slope of about -2 on a cumulative plot, consistent with a highly eroded crater population. The crater density is about one tenth that found on asteroid Gaspra. This value, low by asteroid standards, nevertheless suggests a remarkably old age for portions of the surface of 9P/Tempel 1." I had assumed that these craters -- one of which the Impactor just missed -- were steadily growing sublimation pits of the sort I've mentioned, like those seen by Stardust on Wild 2; but the DI team says that they have the same gradual bowl-shaped cross-sections seen on standard impact craters, unlike the Wild 2 "cookie-cutter" pits with their very flat floors suddenly bordering onto very deep and steep side slopes. It may still be that these really are also sublimation pits, which take on a different shape on Tempel due to a somewhat different surface consistency -- or it may be that these are patches of the nucleus which, very early in the comet's history, became hard enough (from the removal of their volatiles) and thick enough that no more gas vents can break through them from underneath, leaving them free to be gradually covered by regular impact craters like any standard non-active asteroid surface. Again, viewing the subsurface layering of comets seems to be crucial -- and doing this with radar sounding seems to be much better than trying to do it with expensive and localized artificial impacts.

In that connection, one more brief note: P.H. Schultz ( http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2294.pdf ) confirms that the nature of the ejecta plume rising off the surface of Tempel after the impact suddenly changed in nature about 50 seconds after impact, suggesting again that at that point a jet of sublimating ice underneath started blowing more dust and ice particles into space (after the initial dust ejecta had simply been tossed out by the impact shock).

Thanks, Bruce, the CO2 and ethane increases make the case for a deeper penetration much more compelling...I don't mind putting the timpanic model on the shelf, and you/they have presented enough persuasive data to says it should be. It is still difficult to figure out how/why an impact would release so much fine dust, and yet the comet appears to be able to build-and-hold internal pressure - at least enought to make the jets periodic.

Other Questions that remain:

1) Is each jet discrete - like a bursting soap bubble, or are they like bubble gum, where the surface vent replugs and then builds pressure again?

2) It there enough heat energy absorbed to account for all the energy in the jets?

3&4) What are the surface features that resemble craters, and if they are craters, what could have impacted the comet without blasting such a fragile clinker to bits?

5) How much of the dust we see represents the primal make-up of the nucleus, and how much has been accumulated in the eons since the nucleus formed?

6) Will Rosetta answer these questions, and more?

Thanks for the review. One piece of information does have potential bombshell like importance if confirmed. That is the observations of Lisse et.al. of large amounts of carbonates and clays on Tempel I. Clays especially would suggest the long term presence of liquid water.
Their results have been submitted to Science for publication. I'm looking forward to reading it.


Bob Clark

It will certainly be very interesting to see if these are confirmed in the Stardust samples.

Considering that the Tagish Lake meteor seems to have been composed mainly of clays, and considering that the Tagish Lake meteor behaved like a volatile-rich body when it entered Earth's atmosphere (acquiring some odd vectors as pockets of volatiles were released, and finally exploding well above the surface), we have some fairly decent proof that cometary bodies *can* contain clays. And that such bodies also have a good admixture of volatiles.

It's more a matter, I think, of trying to come up with a set of mechanisms that accounts for such differentiation and clay formation in cometary bodies...

-the other Doug

http://www.spaceref.com/news/viewnews.nl.html?id=1103



I hope they fund it - it would settle for once and all the depth issue.