[meteorite-list] Comet's Crater Hidden, But Plume Tells Story

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue Jul 5 13:22:29 2005
Message-ID: <200507051721.j65HLfJ12052_at_zagami.jpl.nasa.gov>

http://www.newscientistspace.com/article.ns?id=dn7629

Comet's crater hidden, but plume tells story
David L Chandler
New Scientist
July 5, 2005

Seeing the crater produced by the Deep Impact mission's violent
encounter with Comet Tempel 1 on Monday - one of the mission's key goals
- could now be impossible.

The plume of gas and dust kicked up by the impact was much bigger,
brighter and less transparent than expected. As a result, the crater
itself, hidden behind the plume, will be very difficult to detect in the
images taken by the flyby spacecraft.

But the science team has already figured out some indirect ways of
determining the crater's dimensions, if the optical images cannot
provide enough information. In any case, any problem with getting data
on the crater challenge is far outweighed by the wealth of information
returned from the first-ever deliberate comet impact.

And the show may go on for a while yet. Measurements by the Hubble space
telescope and other observatories show the comet continued to brighten -
and its new plume of ejected material continued to expand - for at least
several hours after the impact.

And if the impact exposed a lot of fresh, volatile material at the
bottom of the crater, the growing plume "could go on for weeks",
according to the mission's chief scientist, Mike A'Hearn, at mission
control at NASA's Jet Propulsion Laboratory in Pasedena, California, US

NASA has produced a series of videos showing:

o The flash when the comet ran over the probe, as seen by the
high-resolution camera <http://www.nasa.gov/mov/121520main_HRI-Movie.mov>.

o The flash, as seen by the medium-resolution camera
<http://www.nasa.gov/mov/121527main_MRI_impact.mov>.

o The approach of the probe to the comet
<http://www.nasa.gov/mov/121530main_its_approach_x4.mov>, right up to
seconds before impact.

Clearly, the science team has plenty to work on. Analysis of infrared
images should soon reveal the precise temperatures of every part of the
comet's surface. This, in turn, will reveal how solid or porous the
surface is, by showing how quickly it heats in response to changes in
the amount of sunlight it receives.

As for the crater, it should be cooler than the surface, meaning it
could be revealed by the infrared images.

And because the ejected plume casts a very clear shadow on the comet's
nucleus in optical images, it should be possible to determine the size
of the plume's base - and that should correspond exactly to the size of
the crater.

Impact depth

Analysis of the exact shape of the plume as it developed could also show
just how deeply the impactor penetrated into the nucleus, and even the
consistency of the materials, and whether it has a layered structure.

The plume will be crucial in revealing the composition of material in
the comet's hidden interior, via spectroscopic analysis which is still
being processed.

There are also the spectacularly detailed images of the comet's
mountain-sized nucleus, taken before impact, which reveal a highly
varied surface. The science team will try to interpret crater-like
rings, dark linear scarp-like features, flat areas, and scattered bright
patches.

Until that analysis is completed the science team is sticking to food
analogies. A'Hearn said the nucleus "does not look like a pickle or a
cucumber, it's closer to a loaf of bread or a muffin". And Jay Melosh,
another Deep Impact team member, said that its internal porosity may
make it "weaker than the weakest souffle".
Received on Tue 05 Jul 2005 01:21:41 PM PDT