Comets May Not Have Solid Cores, "Impact" Shows

Brian Handwerk
for National Geographic News
September 8, 2005

NASA scientists didn't know quite what to expect when they crashed a half-ton spacecraft into the approximately four-mile-wide (six-kilometer-wide) Comet Tempel 1 on July 4.

Now initial findings from the groundbreaking experiment are challenging current theories about comets and their creation. The results could even impact theories of planetary construction and the birth of life on Earth.

One surprising revelation from the 23,000-mile-an-hour (37,000-kilometer-an-hour) collision is that Tempel 1 is a fragile ball composed of powdery fragments of ice and dust.

"We looked inside [a comet] for the first time," said Michael A'Hearn, an astronomy professor at the University of Maryland in College Park and the lead investigator for the Deep Impact mission.

"We learned that the outer several tens of meters of cometary material is unbelievably fragile, less strong than a snowbank," A'Hearn said. "It's mostly porous, mostly empty. There's no indication yet of reaching a solid layer, so if there is one, it must be down some tens of meters."

Infrared images reveal that Tempel 1's surface warms and cools quickly with changing amounts of sunlight. This rapid change suggests a porous, rather than solid, surface.

But Tempel 1's fragility is more than skindeep. Observational data also suggest that the overall density of the comet is quite low.

"I'm not convinced that there is a solid layer under there," A'Hearn said. "If you look at the icy dust and the density we've deduced for the nucleus itself, something like 75 or 80 percent of the nucleus is empty space. So that tells me that there may be no solid layer."

A'Hearn and others reported results of the Deep Impact mission in today's issue of Science Express, the online advance version of the research journal Science.

Comet Cloud

No one is quite sure just how deep the July 4 impact actually was. The collision raised a huge dust cloud that towered over the comet for more than an hour and ejected particles that hampered imaging instruments on nearby support craft.

Scientists are still processing images in hopes of positively identifying the crater, which is still obscured by the dust cloud.

Continued on Next Page >>


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