[meteorite-list] The Comets' Tale - Maybe the Dirty Snowball Theory Is Wrong

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue Apr 11 11:26:41 2006
Message-ID: <200604102255.k3AMtus18987_at_zagami.jpl.nasa.gov>

http://www.boston.com/news/globe/health_science/articles/2006/04/10/the_comets_tale/

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."
Received on Mon 10 Apr 2006 06:55:56 PM PDT