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Physical Chemistry

Hartley 2 Reveals More Surprises

The comet’s tail is composed largely of CO2, but its nucleus is surrounded by “dandelion puffs” of water ice

by Elizabeth K. Wilson
November 29, 2010 | A version of this story appeared in Volume 88, Issue 48

COMET CLOSE-UP
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Credit: NASA/JPL-Caltech/UMD
The peanut-shaped comet Hartley 2, as seen from the EPOXI spacecraft.
Credit: NASA/JPL-Caltech/UMD
The peanut-shaped comet Hartley 2, as seen from the EPOXI spacecraft.

Images of comet Hartley 2 captured by the spacecraft EPOXI show not only that the jets of gas driving the comet’s tail are composed largely of CO2 (dry ice) but also that the comet’s nucleus is surrounded by “dandelion puffs” of water ice, a team announced on Nov. 18. NASA’s EPOXI was known as Deep Impact when it blasted a crater in the comet Tempel 1 five years ago. The craft was redirected to the peanut-shaped Hartley 2 on a second mission. Hartley 2’s CO2-based tail has given researchers a surprise, because the ice in comets that sublimes when warmed by the sun, spewing vapor and dust outward, is usually water. But fluffy puffs of water ice as big as golf balls and even basketballs also surround the nucleus, like a swarm of ghostly bees. “We’re seeing snowballs, not ice cubes,” said Jessica M. Sunshine, a scientist for the EPOXI mission. The team was also surprised to find a tremendous amount of water vapor coming from the “waist” of the comet’s body, implying that subsurface ice from that section is evaporating.

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