Ice Exists on Surface of Comet, But Most Lies Deeper (Forwarded)

Office of University Communications
University of Maryland

Contacts:
Lee Tune, 301-405-4679

For Immediate Release: February 2, 2006

Ice Exists on Surface of Comet, But Most Lies Deeper

COLLEGE PARK, Md. -- Scientists for Deep Impact, the University of
Maryland-led NASA mission that made history when it smashed into a comet
this past July 4th, have added another first to their growing list: the
first finding of water ice on the surface of a comet.

By analyzing data and images taken prior to impact, Deep Impact
scientists have detected water ice in three small areas on the surface
of comet Tempel 1. This is the first time ice has been detected on the
nucleus, or solid body, of a comet. The findings are published today in
the online version of the journal Science.

"These results show that there is ice on the surface, but not very much
and definitely not enough to account for the water we see in the
out-gassed material that is in the coma [the cloud of gas and dust that
surrounds the comet]," said lead author Jessica Sunshine of Science
Applications International Corporation.

"These new findings are significant because they show that our technique
is effective in finding ice when it is on the surface and that we can
therefore firmly conclude that most of the water vapor that escapes from
comets is contained in ice particles found below the surface," said Deep
Impact Principal Investigator Michael A'Hearn of the University of Maryland.

Where's the Ice?

Through observations of ice grains and water vapor in the coma of
comets, scientists have long known that "dirty snowballs," as comets are
sometimes described, must indeed contain substantial amounts of water
ice. However, prior to Deep Impact they didn't have any knowledge about
how such ice was distributed between the surface, subsurface and inner
core of a comet's nucleus.

In the Science article, the authors say that prior to Deep Impact there
existed few observations of nuclei not obscured by the coma. Among
previous cometary missions, the most notable of such observations was
the Deep Space-1 mission to comet Borrelly, which searched
unsuccessfully for evidence of water ice and other volatiles on that
comet's surface. Limited ground based observations of possibly bare
cometary nuclei have also failed to find clear evidence of surface ice.

The fact that the Deep Impact team found water on the surface, but only
in a few scattered places, all but eliminates the possibility that there
is a lot of undetectable surface ice "just hiding in the surface
darkness," explained Sunshine.

The surface ice that the team detected was not located where the impact
later occurred. This means, Sunshine explained, that the water ice and
water vapor the team already had found in analyses of material ejected
by the July 4 impact must have come from ice located close to, but not
on, the surface of the comet.

Bright Spots in a Dark Landscape

As a comet approaches the Sun, it releases gas and dust forming a cloud
(coma) that obscures the nucleus from view unless spacecraft can get
very close. Deep Impact did just that. The Deep Impact science team used
four types of data in their search for ice on the mostly coal black
surface of Tempel 1.

First, images from Deep Impacts high resolution and medium resolution
instruments (the HRI and MRI) showed three small regions that were about
30 percent brighter than surrounding areas. After scaling the images to
the average brightness value of the nucleus, these three discrete areas
on the nucleus where found to be brighter in the ultraviolet and darker
in the near-infrared, a combination that is consistent with water ice.
In addition, Sunshine's analysis of the spectra of light emitted and
absorbed in those regions showed the distinctive spectral signature of
water ice. The combination of the relative colors and the spectral
signature make a powerful case that there is water ice at these specific
locations on Tempel 1.

Using visual images and spectral mapping of the impact side of the
surface of Tempel 1, the team found that the patches of surface ice
represented only 0.5 percent of the total observed surface.

Team member Olivier Groussin, a University of Maryland research
scientist, made a temperature map and combined it with the color map to
show that two of the three ice patches regions were in colder regions of
the nucleus. Stereo images show the largest area of ice to be a
depression 80 meters below surrounding areas.

"Water Ice on the Surface of Comet Tempel 1," J. M. Sunshine, Science
Applications International Corporation (SAIC); M. F. A'Hearn, University
of Maryland; O. Groussin, University of Maryland; J.-Y. Li, University
of Maryland; M. J. S. Belton, Belton Space Exploration Initiatives; W.
A. Delamere, Delamere Support Services; J. Kissel, Max-Planck-Institute
for Solar System Research; K. P. Klaasen, Jet Propulsion Laboratory; L.
A. McFadden, University of Maryland; K. J. Meech, University of Hawaii;
H. J. Melosh, University of Arizona; P. H. Schultz, Brown University; P.
C. Thomas, Cornell University; J. Veverka, Cornell University; D. K.
Yeomans, Jet Propulsion Laboratory; I. C. Busko, Space Telescope Science
Institute; M. Desnoyer, Cornell University; T. L. Farnham, University of
Maryland; L. M. Feaga, University of Maryland; D. L. Hampton, Ball
Aerospace & Technology Corporation; D. J. Lindler, Sigma Scientific; C.
M. Lisse, Applied Physics Laboratory, Johns Hopkins University; D. D.
Wellnitz, University of Maryland. Published in Science Express, February
2, 2006.

Deep Impact Slide Show,
http://www.newsdesk.umd.edu/images/D...ISlideShow.cfm