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World's In Collision


FROM THE MOON TO THE OORT CLOUD
But, as with Mars, there is still a debate over whether that chance event was a particularly gigantic ancient asteroid impact that modified one whole side of the Moon (as W.C. Feldman proposed at the Conference), a predictable chance unevenness in the distribution of smaller asteroid impacts (as Yasufumi Toda said), or an asymmetry in the churning of the early Moon's molten interior rocks while it cooled (as E.M. Parmentier holds).

In any case, the fact that the side of the Moon with the thinner crust and the maria now points toward Earth may be the result of tidal tuggings after -- not before -- that thinner crust formed.

The most spectacular news about the Moon during the first half of the Conference, though, came from P.H. Schultz -- who reported both visual and spectral evidence that the fresh-looking 3 km-wide Ina caldera in

Ina is just one of at least four similar endogenic [internally-caused] features, all located around the Imbrium basin.
the Mare Felicitatus has been the site of an eruption as little as a million years ago: "In fact, our observations do not preclude the possibility that it is still in the process of formation."

While actual lunar volcanism died out completely a billion years ago as the Moon cooled, Ina may be the site of an outgassing event. "Ina is just one of at least four similar endogenic [internally-caused] features, all located around the Imbrium basin.

"Their locations suggest that volatiles were trapped near the surface and only recently escaped, or that these sites... represent conduits for gases escaping from the deep interior. In either case, they may provide a unique resource for future lunar exploration." Certainly this feature will be the subject of careful examination in the future.

The asteroids are also the subject of much examination at this conference. An entire session on Tuesday was devoted to the initial results from the NEAR spacecraft currently looping in a leisurely way around Eros -- but the contents of those papers aren't available on the Web yet. In the meantime, there were several papers on two asteroidal puzzles.

One of the biggest current asteroidal mysteries is: where did the "ordinary chondrite" meteorites come from? Most of the meteorites collected on Earth belong to this group -- but scientists have had great trouble locating any asteroids with the same composition, judging from the asteroids' infrared mineral spectra.

The obvious candidates would seem to be the silicate-rock "S" asteroids that fill the inner part of the Asteroid Belt -- but their IR spectra are distinctly "redder" in color to the sensitive eyes of the spectoscope.


An entire session on Tuesday was devoted to the initial results from the NEAR spacecraft currently looping in a leisurely way around Eros.
Solving this puzzle, by getting a close up look at the "S" asteroid Eros, was one of the primary goals of the NEAR mission. But the puzzle may already have been solved: recently it has been suggested that "space weathering" by micrometeoroid impacts and the chemical effects of billions of years of solar wind bombardment may redden the spectra of the surfaces of asteroids, while the meteorites -- being smaller chunks of rock that were broken off the larger asteroids in geologically recent times -- have been spared this effect and look "fresher".

T.H. Burbine agreed with this view at the Conference. And, in fact, we now have hopes of actually identifying the asteroids from which some of the rarer types of meteorites originally came from.

It's been recognized for some time that "basaltic achondrites" come from 4 Vesta, the only asteroid whose surface was actually covered with volcanic lava flows in its early days -- and M.J. Gaffey proposed at the meeting that "H-chondrites" can be pegged as coming from the large asteroid 6 Hebe and that "enstatite achondrites" come from the small near-Earth asteroid 3103 Eger.

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