[X] My Assistant

[ljk4-1]

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

[The Messenger]

Astrophysics, abstract
astro-ph/0603306

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

...(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.

[The Messenger]

But Housen argued that laboratory experiments have shown that gravity-controlled growth is not required for the plume to remain attached to the surface, and he showed some experiments that got a murmur from the audience. "The strength of the surface could be as much as 10 kilopascal for the plume to still remain attached to the surface," Housen said; that's more than a thousand times the current estimate for Tempel 1.

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.

[Guest_BruceMoomaw_*]

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.

[Bob Shaw]

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

[Guest_BruceMoomaw_*]

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.)

[Bob Shaw]

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

[Guest_BruceMoomaw_*]

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).

[The Messenger]

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.)

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?

[Guest_RGClark_*]

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.

...

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

[Guest_BruceMoomaw_*]

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

[dvandorn]

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

[The Messenger]

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

In a dramatic bid to maximize the utilization of existing, low- cost planetary spacecraft,
researchers want to divert the NASA/Lockheed Martin Stardust comet-sample-return mothership
to intercept and image a second comet, blasted open last July 4 by the Deep Impact mission.
Stardust is about 20 million mi. behind the Earth in a solar orbit that earlier enabled it to collect
samples from the comet Wild 2....

The new intercept, to take place about 2010, would be managed by the Jet Propulsion Laboratory in Pasadena, Calif. Developed for JPL by Lockheed Martin, Stardust and its 1999 launch on a Boeing Delta II cost only $210 million.

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