Scientists find proof that comets use water vapour, dry ice as fuel

This black-and-white image of the comet Hartley 2 provided by NASA was sent from the NASA EPOXI Mission Deep Impact spacecraft. The comet is estimated to be 2.2 kilometers long, and weigh about 280 million metric tons. Photo: AP

Images from the EPOXI mission flyby show spectacular jets of gas and particles bursting from many distinct spots on the surface of the comet.

“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 comet,” said University of Maryland astronomy professor Jessica Sunshine, who is 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 travelling to a comet, because ground based telescopes can’t detect CO2 and current space telescopes aren’t tuned to look for this gas,” Ms. Sunshine added.

The scientists have also found that water and carbon dioxide dominate the infrared spectrum of Comet Hartley 2’s environment and that organics, including methanol, are present at lower levels.

What is surprising is that there is a lot more carbon dioxide escaping this comet 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, said 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,” the researcher added.

Mission Principal Investigator and science team leader Michael A’Hearn, a University of Maryland professor of astronomy, said the mission has provided, and continues to provide, a tremendous wealth of data about Hartley 2 and the team expects to announce more science findings in the coming weeks.