Comet-like object displays unexpected crater, mysterious ice

Impressions of Asbolus based on data from the Hubble Space Telescope.   • Click each image for larger views   • Click here for QuickTime animation

By Richard Stenger
CNN.com Writer

(CNN) -- A comet-like object with a mysterious crater could offer insights into the violent past of ghostly objects hovering in the far reaches of the solar system, Hubble scientists said Thursday.

A chunk of ice and debris called Asbolus, which orbits the sun between Saturn and Uranus, surprised astronomers when they discovered it had what looked like a fresh crater.

Researchers previously thought that Asbolus and almost two dozen similar distant objects in the vicinity had simple, uniform surfaces. In a class known as centaurs, they resemble icy comet nuclei and reside far enough away to elude close observation from Earth.

Even the powerful Hubble Space Telescope could not directly see the crater, but the satellite observatory, studying the spectral light reflections from Asbolus, offered some startling data.

An analysis suggested a surprisingly bright impact crater less than 10 million years old. Moreover, the underlying ice looked like nothing seen before.

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"The ice does have some strong similarities to water ice, but in places it really doesn't match," said lead investigator Susan Kern of the Massachusetts Institute of Technology in a statement. "This could be a mixture of things we've seen before, but not in this combination."

Centaurs, which orbit the sun between Neptune and Jupiter, are thought to have broken free from a reservoir of comets in the far reaches of the solar system known as the Kuiper Belt, which extends at least 10 times the distance of Pluto from the sun.

"We suspect the Kuiper Belt extends much farther than this distance. Because such objects are so distant and faint, astronomers do not understand much about their physical and chemical properties. They may be made of materials dating to the formation of the solar system," Kern said.

High energy particles from the solar wind have darkened objects in the distant belt. Impacts like the one that hit Asbolus, however, can dislodge the pristine material underneath, said Kern and her colleague Donald McCarthy.

McCarthy, a University of Arizona researcher, suggested that the collision creating the impact crater dislodged Asbolus and sent it on its way to its closer orbit around the sun.

Kern recently earned her bachelor's degree from the University of Arizona. A report from Kern and fellow researchers will be published soon in Astrophysical Journal Letters.

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The Space Science Telescope Institute

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