Comet Ice May Come In Three Different Flavors

Jets spew out ice and carbon dioxide from one end of comet Hartley-2 in this EPOXI image, while water vapor gets released from the middle region. The differences suggest that the comet's core is made of at least two different ices. Ground-based measurements suggest the presence of a third ice. Credit: NASA/JPL-Caltech/UMD

A little ice, a little fizz, a few nuts and voila – we have a comet! We learned how to make a homemade comet the other week. This week NASA released a new study of Comet Hartley 2  that gives us an even better idea of a what goes into making one.

Using telescopes perched high in the mountains of Hawaii and Chile, Michael Mumma of NASA’s Goddard Space Flight Center and his team studied the comet’s coma—the envelope of gas, dust and ice particles that surrounds the core. What’s in the coma originates from the comet body itself, which is too small and shrouded in too much haze to study directly with earthbound telescopes. Astronomers deduce a comet’s composition by studying molecules buzzing around in its coma.

The location and spacing of bright lines in a spectrum act as a sort of bar code to tell us what kind of material is present.

Mumma’s team used a spectrograph to spread Hartley 2’s light into a detailed spectrum or rainbow of light. Bright and dark lines that resemble a bar code on a package of cookies stripes the rainbow from one end to the other. Each atom or molecule, through absorption and emission of light, imprints its own unique set of lines on the spectrum. By studying these ‘bar codes’, a scientist can tell you what made them, the amount present and even its temperature.

Ices in Hartley 2 are mostly made of water and carbon dioxide or dry ice. The team also discovered that the water ice contained methanol, a familiar form of alcohol that back here on Earth is mixed with gasoline or used straight to power racing cars. There were also indications of a third type of ice – ethane – a component of natural gas.

Myriads of fluffy snowballs caught up in vaporizing water and dry ice are shot into the coma by jets on the surface of 1.2-mile-long Comet Hartley 2 (right) in this photo taken by the Deep Impact probe during last November's close flyby. They range in size from pennies to basketballs. Credit: NASA/JPL-Caltech/UMD

Hartley 2’s surface is covered with small, volcano-like jets. When the comet is warmed by the sun, the jets shoot out a mix of vaporized water ice, dry ice and entrained rocky particles from the interior to create the coma. The researchers think that chunks of water ice are glued together in the comet’s core by the frozen carbon dioxide, which vaporizes before the water ice because it’s more sensitive to the sun’s heat. The carbon dioxide gas then drags along chunks of ice for the ride, which later vaporize to provide much of the water vapor in the coma.

In addition to Mumma’s studies, the EPOXI comet flyby mission last November also revealed that the carbon dioxide jets are not found at the large end of the comet, and in the middle region, water vapor is released without any carbon dioxide. And while water with methanol is released from all directions around the comet, ethane was released from just one direction. This uneven expression of ingredients across Hartley 2 may shed light on its origin. Did it condense from gas, dust and ices to form a single body or did a bunch of mini-comets of slightly different composition come together to create the comet? You can learn more about the group’s findings by clicking over to this NASA press release.