Comets: Powdery Puffballs in Space?

By Elise Kleeman
Oct 24, 2005 5:00 AMNov 12, 2019 5:16 AM


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Soon after NASA crashed an 820-pound copper bullet into comet Tempel 1 on July 4, astronomers began to crunch reams of data from more than 60 observatories on Earth and in space. Although researchers have examined only a small fraction of the material from the Deep Impact mission, they are beginning to reach some preliminary conclusions.

Surface Comets probably do not have hard, rocky exteriors. The impact caused Tempel 1 to glow five times brighter than normal, apparently the result of flying debris illuminated by the sun. That much dust—tens of thousands of tons—could not have been created by the blow alone, says Mike A’Hearn, the project’s lead scientist and an astronomer at the University of Maryland: “You cannot pulverize that much material in an impact.” He concludes that the surface layer must have “negligible strength”—meaning it is soft and fine like talcum powder.

Interior Astronomers hoped the probe would confirm that comets fit the popular model of a “dirty snowball”: an icy core made up of a solid mix of water ice, dirt, and frozen gases like carbon dioxide and carbon monoxide, coated with a thin crust of debris. To their surprise, the amount of water they predicted did not appear. So far, data show the impact ejected only slightly more water vapor than had been trailing the comet beforehand, suggesting the probe did not reach a frozen core.

However, A’Hearn’s team reported an increase in carbon dioxide after the collision. Because the gas would have remained frozen within the comet’s interior unless exposed by the impact, that suggests the probe did penetrate some sort of icy layer—but perhaps one much different from what astronomers originally predicted.

Scientists have many other questions they want to answer. A’Hearn says he hopes eventually to understand the comet’s geologic structure and even estimate its gravity by watching how the debris falls back to its surface. “We have a lot of work to do.”

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