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.

But they do know that the impact was very big.

"I think from the amount of material that came out, we probably did excavate a crater about 100 meters [330 feet] across," A'Hearn said.

NASA's Spitzer Space Telescope captured infrared spectrograph images, which are being used to analyze the material in the dust cloud. So far scientists have been able to determine that the cloud contained some 5 million kilograms (11 million pounds) of water and even more dust.

Many of the organic materials currently identified from Tempel 1, including hydrogen cyanide and methyl cyanide, have previously been seen in other comets.

Some scientists theorize that comets might have sparked life on Earth by serving as delivery mechanisms for organic material, and the abundance of organics in the Tempel 1 debris may give this theory a boost.

"I'd argue that [the theory of comets spawning life on Earth] is more likely because we saw a big enhancement of organic material coming out" of Tempel 1, A'Hearn said.

Scientists have identified other known comet components in Tempel 1, such as silicates or sand. But the researchers also found clay and chemicals known as carbonates, which are found in seashells. Finding these materials was a surprise, because scientists have long believed that the compounds could be formed only in the presence of liquid water.

"How did clay and carbonates form in frozen comets?" asked Carey Lisse of Johns Hopkins University's Applied Physics Laboratory in Laurel, Maryland.

"We don't know. But their presence may imply that the primordial solar system was thoroughly mixed together, allowing material formed near the sun, where water is liquid, and frozen material from out by Uranus and Neptune to be included in the same body."

The dust may contain other materials that are unchanged since the earliest days of the solar system some 4.5 billion years ago, which could shed light on processes of planetary formation.

Unprecedented Observation

Scientists watching the Deep Impact collision operated more than 70 telescopes around. Participants say it was the largest group astronomical effort of its kind.

The unusually intensive effort allowed sky-watchers to spot comet activities that may be common but have not been frequently observed, like natural outbursts from the comet itself.

Karen Meech, of the Institute for Astronomy at the University of Hawaii at Manoa, notes that the natural eruptions shared parallels with the Deep Impact explosion.

"They looked similar from the point of view of length of time, duration, how fast the dust was flowing out, and how it spread away and floated down within a few days," Meech said.

The European Space Agency's Rosetta spacecraft, currently en route to visit another comet, observed the Tempel 1 impact from the relatively close distance of under 50 million miles (80 million kilometers).

Rosetta's mission to land on a comet in 2014 may have to be reevaluated in light of Deep Impact's findings about the comet's density.

"It could mean that you can't really land on this surface, that you'd just sink in," said Horst Uwe Keller of the Max-Planck Institute for Solar System Research in Katlenburg-Lindau, Germany.

But researchers are unsure how typical Comet Tempel 1 may be, and data suggest that comets are not as uniform as previously believed.

Deep Impact also could have another important benefit—learning how to defend Earth against rogue comets.

"All along we predicted that any change [the collision] made in the [Tempel 1's] orbit would be so small that it would be undetectable," said the University of Maryland's A'Hearn.

"What we have learned is important to designing a diversion technology. Knowing that it's a highly porous, fragile aggregation of tiny dust and ice particles—that's important in how you design a diversion maneuver," he said.

"But to actually cause a measurable change [in orbit], you'd need an experiment a hundred to a thousand times larger than Deep Impact."

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