[X] My Assistant

[Guest_BruceMoomaw_*]

I wasn't quite sure where to put this -- maybe we need a new division of the website on "General Solar System" -- so I finally decided to put it here.

The theories as to how Earth and Mars got their substantial water supply -- despite the fact that most Solar System formation theories call for the rocky debris in the inner Sysyem to have been virtually completely dried out by heat from the accreting Sun -- is another one that continues to rage on. One long-time theory is that it was brought to them (and to Venus, Mercury and the Moon, which then lost it again) by showers of incoming comets -- but the three comets we've analyzed by remote observation (Halley, Hale-Bopp and Hyakutake) so far usually seem to contain much more deuterium in their water ice than Earth's water.

Another is that Jupiter and Saturn directed icy planetesimals inward from their own zone of formation, where the ice may originally have contained less deuterium than the ice in the Kuiper Belt (where the three comets we've analyzed thus far came from). This is one reason why we would very much like a detailed comparative analysis of the composition of Oort Cloud comets versus Kuiper Belt comets (which include all the three we've analuzed for deuterium so far) -- the Oort comets are thought likely to have also originally formed in the Jupiter-Saturn zone and then been catapulted all the way outwards into the vast regions of interstellar space by flybys of those two planets. Paradoxically, the "Jupiter-family comets" -- those with short periods lasting only a few years -- are thought to be mostly Kuiper Belt comets, directed into short-period orbits by encounters with Jupiter. This is one reason why one desirable mission is a CONTOUR-type craft that stays "on station" in near-Earth solar orbit for years, waiting for the opportunity to make an Earth flyby that can quickly redirect it to a flyby of a newly-discovered long-period comet -- or more than one.

In LPSC abstract #2347, L.P. O'Brien does a computer simulation of early Solar System conditions and concludes, interestingly, that the current orbital eccentricities and inclinations of Jupiter and Saturn -- though mild -- would be enough to keep them from diverting large numbers of nearby icy planetesimals into the inner Solar System to give the inner planets their water. However, reent models of the early Solar System suggest that the two giant planets started out with more circular and ecliptic-plane orbits than they now have -- and O'Ben's simulations show that, in those early orbits, they would have been quite efficient at shipping ice to the inner System.

There are, however, as always, dissenters -- suggesting in this cae that the inner planets may have formed in the first place out of relatively water-rich material. R. Machida suggests (#1615) that the early solar nebular disk may have been much more opaque (due to the large amount of fine dust in it) than had been thought, easily blocking enough sunlight for significant amounts of water ice to remain in the inner System and be incorporated into the inner planets (and perhaps, in other, more opaque extrasolar nebulas, causing the inner planets to be "water balls").

And R. Stimpfl says (#1395) that his lab tests show that fine rocky particles -- especially of forsterite olivine -- are very efficient at adsorbing water vapor molecules to cling to their outer surfaces, which may in itself be adequate to solve the problem.

Will we ever have enough data about conditions in the very early forming Solar System to make a reasonably probable guess as to which of these theories is correct? God knows. In http://www.cosis.net/abstracts/EGU06/10289/EGU06-J-10289.pdf , A.L. Graps and Jonathan Lunine muddy the water still further by concluding that "We find that no single [water] source satisfies all of the known [dynamical and geochemical] constraints, and indeed it is necessary to invoke multiple sources at different
times. This has important implications for the habitability of extrasolar planets, where the timing and abundance of sources of water may vary in an extreme way from that of the Earth."

[ljk4-1]

THE COMETS' TALE: MAYBE THE DIRTY SNOWBALL THEORY IS WRONG

by David L. Chandler
The Boston Globe
April 10, 2006

Three fly-by missions since 2001 have confounded almost everything astronomers thought they knew about the makeup of comets. Then, two weeks ago, University of Hawaii researchers announced the discovery of a whole new family of close-in comets -- which might help explain how the early Earth got its water.

Our lack of knowledge could have dire consequences, scientists warn, because -- unlike asteroids, whose paths can be predicted years in advance -- comets could strike Earth with little warning. The missions have proven that we don't know enough about these dazzling lumps of ice and dirt to know how to respond.

But now, one astronomer has come up with a theory that might tie some of the loose ends together. Instead of the conventional view of a comet's nucleus as a solid, several-miles-wide rubble pile or dirty snowball, Michael Belton, a lead scientist for last year's Deep Impact comet mission, suggests that the nucleus may be more like a lump of papier mache -- built up from a random assortment of irregular sheets of varying thickness.

''The presence of layers is ubiquitous" in the nuclei seen so far, Belton said, ''and may be an essential element of their internal structure." In his view, the nuclei were built up gradually as hundreds of smaller bodies smashed together over time, each flattening out and sticking to the growing body, forming one layer after another.

Astronomers were startled and confused by the dramatic and unexpected differences between the nuclei of Tempel 1 (seen by last year's Deep Impact mission), Wild 2 (as seen by the Stardust mission in 2004) and Borrelly (seen by deep Space 1 in 2001).

Belton's new theory, which he outlined at a conference in Houston last month, identifies all the varied and unexplained features seen on these comets -- including supposed craters on Wild 2, mesa-like plateaus on Borrelly, and distinctly different, overlapping surface textures on Tempel 1 -- as different aspects of the layered model he nicknamed Talps (for ''splat" spelled backwards).

Clark Chapman, a specialist in asteroids and comets at the Southwest Research Institute in Boulder, Colo., agrees with Belton that ''it looks like comets have layers in them," but he said the theory is still untested. ''It's a first step toward trying to understand comets differently."

The new model would have significant implications for the life cycle of comets and for how we might attack a comet headed for Earth. Pushing aside a solid ball with a huge rocket or nuclear blast might make sense, but using the same approach against a ball of many layers might cause the comet to splinter and could magnify the damage rather than avert it, Belton suggests.

The find of a new type of comet -- the third known -- adds a lot of new questions to comet research and possibly helps answer a longstanding mystery: How the Earth has so much water when models suggest it shouldn't.

As the solar system's inner planets coalesced from the cloud of gas and dust swirling around the sun, the sun's heat caused water to evaporate. The new discovery suggests that Earth's water supply might have been replenished by some comets or asteroids that initially formed just a bit farther out and so might have retained their ice as they hurtled around the sun and eventually smashed into our planet.

Astronomers Henry Hsieh and David Jewitt of the University of Hawaii announced late last month that they have found comets with asteroid-like orbits -- circling the sun as planets do, between Mars and Jupiter, instead of the very elongated orbits characteristic of all previously known comets. Finding comets like these suggests that there could be icy asteroids or comets that formed much closer to the sun than previously thought. They would have replenished Earth's water supply when they crashed into its surface.

''I think it's very significant," Jewitt said, to find such a fundamentally different group of comets, which must have formed separately from all the others. But it will take more study to figure out how this new population will compare to the others and what kind of structure they might have. Being born in a hotter region of the growing solar system, for example, might have produced a different kind of layering, if any.

Belton, president of Belton Space Exploration Initiatives in Tucson, said he'd like to have a chance to prove his model by getting a closer look at some of these comets, particularly with a radar analysis -- which past missions couldn't perform -- that could clearly show whether the orb is layered deep down.

It may be a while before he gets that wish, but the European Space Agency's Rosetta mission will provide close-up views in 2014 of another comet nucleus and will use microwaves to probe its inner structure. Other comet missions have been proposed. ''The reconnaissance is over," Belton said. ''It's time to get into the detailed exploration phase."

[Guest_BruceMoomaw_*]

This is the theory described (with a nice drawing) in http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1232.pdf . (One might also call it the "Dirty Snowball Fight" theory.)

[The Messenger]

This is the theory described (with a nice drawing) in http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1232.pdf . (One might also call it the "Dirty Snowball Fight" theory.)

An onion rather than a potato? Layers would seem to require layers of stuff floating out there, and aside from the explosion of the crystal planet Krypton, I don't see it. At the moment, a big boat and a bunch of pairs of animals seems to be as good of an explanation as any

It there really that much difference between the comets we have observed 'closely', or are the differences primarily an expression/interpretation of different resolutions?

I hope Rosetta lives up to its name...

[ljk4-1]

What happens when a fragmented comet hits the surface of a planet? It makes a chain of craters. Researchers are looking for evidence of these crater chains here on Earth--and finding them in some unexpected places.

FULL STORY at

http://science.nasa.gov/headlines/y2006/12....htm?list161084

[Guest_BruceMoomaw_*]

An onion rather than a potato? Layers would seem to require layers of stuff floating out there,...

Wrong-o. As Belton carefully explains, all it requires is an object made out of material with very low cohesiveness hitting another, larger object (whose surface has already been somewhat compacted by earlier impacts) and going "splat!" Thus his grim determination to name his theory "Talps" ("splat" spelled backwards).

[The Messenger]

Wrong-o. As Belton carefully explains, all it requires is an object made out of material with very low cohesiveness hitting another, larger object (whose surface has already been somewhat compacted by earlier impacts) and going "splat!" Thus his grim determination to name his theory "Talps" ("splat" spelled backwards).

Interesting.

I have to agree loosely consolidated layering is emerging as candidate. Again, I get to wonder out-loud: We have witnessed the breakup of two comets and there are (apparently) recent pecker-tracks of many others -that seems to me to be a high attrition rate. So I get to wonder if this stuff is really originating in the Oort Clouds/Kuiper Belt, or is there loose supernova debris all over the place?

[Guest_BruceMoomaw_*]

Since there are supposed to be several trillion comets over 1 km in size in the Oort Cloud, and about a billion in the Kuiper Belt, we don't need to go that far afield to look for all the garbage that keeps coming back into the inner Solar System.