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Dry ice fuels comet jets
Posted: 11 November 2010

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Less than week after NASA's flyby of Comet Hartley 2 and scientific discoveries are already being announced, including the latest finding that dry ice is the fuel for the comet's jets seen bursting from its surface.

This image montage shows comet Hartley 2 as NASA's EPOXI mission approached and flew under the comet on 4 November. The images progress in time clockwise, starting at the top left. The Sun is to the right. Image: NASA/JPL-Caltech/UMD.

During the flyby, which saw the recycled Deep Impact spacecraft pass within 700 kilometres of Hartley's 2 by 0.4 kilometre nucleus, spectacular jets were seen erupting from many spots on the surface. Spectral analysis finds that the primary constituent of these jets is carbon dioxide gas, accompanied by particles of dust and ice. Smaller amounts of organic materials are also present.

“Previously it was thought that water vapour from water ice was the propulsive force behind jets of material coming off of the body, or nucleus, of the comet,” say University of Maryland astronomy professor Jessica Sunshine, deputy principal investigator for the EPOXI mission. “We now have unambiguous evidence that solar heating of subsurface frozen carbon dioxide (dry ice), directly to a gas, a process known as sublimation, is powering the many jets of material coming from the comet. This is a finding that only could have been made by traveling to a comet, because ground based telescopes can’t detect CO2 and current space telescopes aren’t tuned to look for this gas.”

Distribution maps of water, carbon dioxide, and dust around 103P/Hartley. Image: NASA/JPL-Caltech/UMD.

Although the presence of carbon dioxide is no surprise, the volume of gas escaping from the comet was much more than expected.

“The distribution of carbon dioxide and dust around the nucleus is much different than the water distribution, and that tells us that the carbon dioxide rather than water takes dust grains with it into the coma as it leaves the nucleus,” says Assistant Research Scientist Lori Feaga. “The dry ice that is producing the CO2 jets on this comet has probably been frozen inside it since the formation of the Solar System.”

Previously, the Deep Impact spacecraft had visited Comet Tempel 1, and only five comets in total have been toured by spacecraft. With data from the EXPOXI mission being returned at a rate of some 2,000 images a day, the project team say they will have many more science findings to report in the coming weeks.