[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."

[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.)