NASA's Deep Impact Adds Color To Unfolding Comet Picture

"Prior to our Deep Impact experiment, scientists had a lot ofquestions and untested ideas about the structure and composition of thenucleus, or solid body of a comet, but we had almost no realknowledge," said Deep Impact principal investigator Dr. MichaelA'Hearn, a professor of astronomy at the University of Maryland,College Park. "Our analysis of data produced by Deep Impact isrevealing a great deal, much of it rather surprising."

For example, comet Tempel 1 has a very fluffy structure that isweaker than a bank of powder snow. The fine dust of the comet is heldtogether by gravity. However, that gravity is so weak, if you couldstand on the bank and jump, you would launch yourself into space.

Another surprise for A'Hearn and his colleagues was the evidence ofwhat appears to be impact craters on the surface of the comet.Previously, two other comets had their nuclei closely observed andneither showed evidence of impact craters.

"The nucleus of Tempel 1 has distinct layers shown in topographicrelief ranging from very smooth areas to areas with features thatsatisfy all the criteria for impact craters, including varying size,"A'Hearn said. "The problem in stating with certainty that these areimpact craters is that we don't know of a mechanism by which somecomets would collide with the flotsam and jetsam in our solar system,while others would not.”

According to A'Hearn, one of the more interesting findings may bethe huge increase in carbon-containing molecules detected in spectralanalysis of the ejection plume. This finding indicates comets contain asubstantial amount of organic material, so they could have brought suchmaterial to Earth early in the planet's history when strikes byasteroids and meteors were common.

Another finding is the comet interior is well shielded from thesolar heating experienced by the surface of the comet nucleus. Missiondata indicate the nucleus of Tempel 1 is extremely porous. Its porosityallows the surface of the nucleus to heat up and cool down almostinstantly in response to sunlight. This suggests heat is not easilyconducted to the interior and the ice and other material deep insidethe nucleus may be pristine and unchanged from the early days of thesolar system, just as many scientists had suggested.

"The infrared spectrometer gave us the first temperature map of acomet, allowing us to measure the surface's thermal inertia, or abilityto conduct heat to the interior," said Dr. Olivier Groussin, theUniversity of Maryland research scientist who generated the map.

It is this diligent and time consuming analysis of spectral datathat is providing much of the "color" with which Deep Impact scientistsare painting the first ever detailed picture of a comet. For example,researchers recently saw emission bands for water vaporized by the heatof the impact, followed a few seconds later by absorption bands fromice particles ejected from below the surface and not melted orvaporized.

"In a couple of seconds the fast, hot moving plume containing watervapor left the view of the spectrometer, and we are suddenly seeing theexcavation of sub-surface ice and dust," said Deep Impactco-investigator Dr. Jessica Sunshine, with Science ApplicationsInternational Corporation, Chantilly, Va. "It is the most dramaticspectral change I've ever seen."

These findings are published in the September 9 issue of the journalScience, and were presented this week at the Division for PlanetarySciences meeting in Cambridge, England. Mission scientists are fillingin important new portions of a cometary picture that is still far fromfinished.

The University of Maryland is responsible for overall Deep Impactmission science, and project management is handled by JPL. Thespacecraft was built for NASA by Ball Aerospace & TechnologiesCorporation, Boulder, Colo. JPL is a division of the CaliforniaInstitute of Technology, Pasadena, Calif.

For more information about the Deep Impact mission on the Internet, visit: http://www.nasa.gov/deepimpact .

For information about NASA and agency programs on the Internet, visit: http://www.nasa.gov/home .