[X] My Assistant

[Guest_RGClark_*]

...
Finally, regarding possible mechanisms by which carbonates might form in the very early Solar System, see one paper from the 2005 LPSC which claims to have found a mechanism by which carbonates can form in pre-planetary nebulas even without the presence of liquid water, through purely gas-phase reactions (
http://www.lpi.usra.edu/meetings/lpsc2005/pdf/1894.pdf ) -- which seems to
match well with the apparent detection of carbonates in such nebulas.  I still think that the apparent absence of carbonates on the surfaces of comet nuclei, as opposed to their interiors -- and their scarcity (though not total nonexistence) in interplanetary dust particles collected out of Earth's upper atmosphere -- may be due to the fact that they're broken down pretty quickly by solar UV.

Thanks for that link to the article, Bruce. Note though that the authors had to include H2O for the carbonate to form (it did not form without H2O, whatever its phase) and they had to raise the temperature above freezing for the carbonate to form:

Experimental evidence for condensation of "astrophysical" carbonate.
"Temperatures of these ambient gases, i.e.
condensation temperatures were varied between 25°C
and ~150°C, and condensation times from ~40 to 464
min. The condensed material was collected on a
horizontal steel plate 2 to 10 cm above the target."
Lunar and Planetary Science XXXVI (2005) 1894.pdf, p. 1.
http://www.lpi.usra.edu/meetings/lpsc2005/pdf/1894.pdf

Nevertheless the authors concluded that liquid water could not be involved because of the low pressures used. However, a key fact is that water can become liquid at below its normal triple point pressure with the addition of soluble gases or solutes. That is, dissolved gases or solids can not only lower the temperature required for liquid water, it can also lower the pressure requirements. It would be interesting to find out if they observed the carbonate forming when the surrounding pressure (not just the water vapor pressure) was below that required for gas or solid water solutions.


Note also that there is now additional evidence for internal heating in comets that could have allowed liquid water to form: the finding of olivine on Tempel I.
On the space bbs uplink.space.com someone mentioned the detections of olivine in Tempel I and previously in other comets raise the possibility of more heating in comets than would be expected for objects formed far from the Sun. This would also be consistent with the theory of radiogenic heating in comets:

NASA Research Finds Green Sand Crystals Are in Comet Tempel 1.
Date Released: Thursday, September 15, 2005
http://spaceflightnow.com/news/n0007/20cometdust/

Time travel through a trail of comet dust.
NASA/GSFC NEWS RELEASE
Posted: July 20, 2000.
"We know that these dust grains change from amorphous to crystalline as they are heated, and our laboratory research revealed that the rate at which they change is extremely sensitive to temperature," Nuth added. "At the very low temperatures, where water-ice and the other volatile components of comets are frozen, the time required for amorphous silicate dust grains to change to the crystalline olivine found in comet Halley is many times longer than the age of the Universe."
http://spaceflightnow.com/news/n0007/20cometdust/

So there are at least three separate and independent indications of internal heating, likely radiogenic, in comets: carbonate/clay detected in Tempel I, olivine in Tempel I and other comets, and crystalline ice detected deep in the Kuiper belt:

Chilly Quaoar had a warmer past.
Mark Peplow
Crystalline ice suggests remote object has radioactive interior.
Published online: 8 December 2004.
http://www.nature.com/news/2004/041206/pf/041206-7_pf.html



Bob Clark

[Guest_BruceMoomaw_*]

It can't be very much liquid water, though, or the olivine would have turned into clays -- don't forget how vulnerable it is to water weathering (which, as you know, is also another reason to think that Mars has never had much surface liquid water). There do seem to be some clays in Tempel, but there's also a hell of a lot of unaltered olivine.

[SigurRosFan]

SWAS (water) observations of comet 9P/Tempel 1 and Deep Impact

http://arxiv.org/abs/astro-ph/0509850

[Guest_BruceMoomaw_*]

Jessica Sunshine has a new interview on this subject: http://www.astrobio.net/news/article1731.html

The big news is that -- while they certainly did detect a major release of organics (along with water and CO2), they're having serious trouble interpreting just what organics actually did come out: " We knew organics were going to be vaporized, and we were expecting to see each organic material have a peak at a specific wavelength, depending on what it was. But what we saw was that anything that had a carbon-hydrogen bond was vaporized, and we got a conglomerate peak. That means there were a lot of organics, and a lot of different kinds, but we couldn't tell what any of it was." Nor does she think there's any way for continued processing to tease out the different organics from DI's own spectra; the main hope lies in the higher spectral-resolution and longer duration spectra of the cloud obtained by ground-based spectrometers. "I think the ground-based data already shows clear evidence of some. They saw a tremendous change in ethane before and after; they saw a lot more ethane after. They're also seeing formaldehyde and methanol, but not large changes. But we still have a lot to do."

[ljk4-1]

Sunshine on Comets: Part I

http://www.astrobio.net/news/article1731.html

Jessica Sunshine is the Deep Impact mission scientist responsible for the
onboard infrared spectrometer. In the first half of this two-part interview, she
discusses what the comet's nucleus looked like before and after impact, and
explains why it's so difficult to piece together the spectroscopic data.

[ljk4-1]

Observations of Comet 9P/Tempel 1 made by ESA’s Rosetta spacecraft after the
Deep Impact collision suggest that comets are ‘icy dirtballs’, rather than
‘dirty snowballs’ as previously believed.

More at:

http://www.esa.int/SPECIALS/Rosetta/SEMUSK5Y3EE_0.html

[Guest_Myran_*]

That idea of 'Icy dirtballs' makes sense to me, it have been speculated that some objects we think of as asteroids might be former cometary nucleus. Most likely candidates for such former-comet asteroid nowadays are the sun grazing ones, best known example of those are Icarus.

[djellison]

I thought the move from dirty-ice-ball to icy-dirt-ball was already a popular theory before DI

Doug

[tedstryk]

I thought the move from dirty-ice-ball to icy-dirt-ball was already a popular theory before DI

Doug

Remember, this is an ESA press release. It is a totally new idea

[Guest_Myran_*]

I might be out of touch to some degree, but I thought it was the ‘dirty snowball’ theory that was mostly embraced up until the analysis of Deep impact.

[antoniseb]

I might be out of touch to some degree, but I thought it was the ‘dirty snowball’ theory that was mostly embraced up until the analysis of Deep impact.

I recall reading that description well before DI hit Tempel 1, though personally, I think that it is not realistic to assert that about all comets on the basis of this impact with one comet. I would guess that is we were to run another DI type mission at an incoming comet with a hyperbolic orbit (i.e. fresh material), that we would probably see a higher ratio of ices to silicates, and a much thinner layer of surface dust.

[Guest_BruceMoomaw_*]

There were indeed strong suspicions for years before DI that there might be more dirt than ice in comets -- there was, in fact, a two-page article on it in "Nature", which I have in my files and will track down.

[Guest_BruceMoomaw_*]

Yup -- and the article (by Mark W. Sykes) goes all the way back to the April 22, 1993 issue. It turns out that for about a decade before that, the consensus was growing among comet specialists that there was a lot more dirt than ice in the things.

The previous serious underestimates of dust mass in them had been due to visual observations, which are only sensitive to extremely fine dust a few microns or smaller -- but both Giotto's measurments during its Halley and Grigg-Skjellerup flybys, and the long-distance IR observations from the IRAS satellite in the mid-1980s, had already led to the conclusion that "short-period comets lose mass primarily in the form of refractory particles a few millimeters to centimeters in diameter", which are hard to detect visually. The average dust/gas mass ratio for short-period comets (including Halley) had already been calculated at 2 or 3 to 1, meaning that volume-wise dust and grit made up 40-50% of their nuclei. This nicely matches the rock/ice ratios for Pluto and Triton, which after all were made up an accumulation of such comets.

[ljk4-1]

"Moreover, the expanses of interplanetary space are not the forbidding barrier they once seemed. Over the past 20 years scientists have determined that more than 30 meteorites found on Earth originally came from the Martian crust, based on the composition of gases trapped within some of the rocks. Meanwhile biologists have discovered organisms durable enough to survive at least a short journey inside such meteorites.

Although no one is suggesting that these particular organisms actually made the trip, they serve as a proof of principle. It is not implausible that life could have arisen on Mars and then come to Earth, or the reverse. Researchers are now intently studying the transport of biological materials between planets to get a better sense of whether it ever occurred. This effort may shed light on some of modern science's most compelling questions: Where and how did life originate? Are radically different forms of life possible? And how common is life in the universe?"

http://www.sciam.com/article.cfm?chanID=sa...4FF83414B7F0000

[Guest_RGClark_*]

Further evidence for comets around other star systems.

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

This was released today:

NASA's Spitzer Finds Possible Comet Dust Around Dead Star
For Release: January 11, 2006
http://www.spitzer.caltech.edu/Media/relea...4/release.shtml

Then this may support the idea that the carbonate seen in planetary nebulae indeed stems from comets in those those systems. So rather than the extrasolar detections providing an alternative origin for carbonate that does not require liquid water, they may support the idea the carbonate in comet Tempel I may indeed be due to liquid water.


- Bob Clark

Thanks for that link to the article, Bruce. Note though that the authors had to include H2O for the carbonate to form (it did not form without H2O, whatever its phase) and they had to raise the temperature above freezing for the carbonate to form:

Experimental evidence for condensation of "astrophysical" carbonate.
"Temperatures of these ambient gases, i.e.
condensation temperatures were varied between 25°C
and ~150°C, and condensation times from ~40 to 464
min. The condensed material was collected on a
horizontal steel plate 2 to 10 cm above the target."
Lunar and Planetary Science XXXVI (2005)    1894.pdf, p. 1.
http://www.lpi.usra.edu/meetings/lpsc2005/pdf/1894.pdf

Nevertheless the authors concluded that liquid water could not be involved because of the low pressures used. However, a key fact is that water can become liquid at below its normal triple point pressure with the addition of soluble gases or solutes. That is, dissolved gases or solids can not only lower the temperature required for liquid water, it can also lower the pressure requirements. It would be interesting to find out if they observed the carbonate forming when the surrounding pressure (not just the water vapor pressure) was below that required for gas or solid water solutions.
Note also that there is now additional evidence for internal heating in comets that could have allowed liquid water to form: the finding of olivine on Tempel I.
On the space bbs uplink.space.com someone mentioned the detections of olivine in Tempel I and previously in other comets raise the possibility of more heating in comets than would be expected for objects formed far from the Sun. This would also be consistent with the theory of radiogenic heating in comets:

NASA Research Finds Green Sand Crystals Are in Comet Tempel 1.
Date Released: Thursday, September 15, 2005
http://spaceflightnow.com/news/n0007/20cometdust/

Time travel through a trail of comet dust.
NASA/GSFC NEWS RELEASE
Posted: July 20, 2000.
"We know that these dust grains change from amorphous to crystalline as they are heated, and our laboratory research revealed that the rate at which they change is extremely sensitive to temperature," Nuth added. "At the very low temperatures, where water-ice and the other volatile components of comets are frozen, the time required for amorphous silicate dust grains to change to the crystalline olivine found in comet Halley is many times longer than the age of the Universe."
http://spaceflightnow.com/news/n0007/20cometdust/

So there are at least three separate and independent indications of internal heating, likely radiogenic, in comets: carbonate/clay detected in Tempel I, olivine in Tempel I and other comets, and crystalline ice detected deep in the Kuiper belt:

Chilly Quaoar had a warmer past.
Mark Peplow
Crystalline ice suggests remote object has radioactive interior.
Published online: 8 December 2004.
http://www.nature.com/news/2004/041206/pf/041206-7_pf.html
  Bob Clark