Scientists find strong evidence for presence of liquid water in comet

University of Arizona scientists have found convincing evidence for the presence of liquid water in a comet.

The finding has shattered the current paradigm that comets never get warm enough to melt the ice that makes up the bulk of their material.

UA graduate student Eve Berger, who led the study, and her colleagues from Johnson Space Center and the Naval Research Laboratory made the discovery analyzing dust grains brought back to Earth from comet Wild-2 as part of the NASA's Stardust mission.

"In our samples, we found minerals that formed in the presence of liquid water. At some point in its history, the comet must have harbored pockets of water," said Berger.

"When the ice melted on Wild-2, the resulting warm water dissolved minerals that were present at the time and precipitated the iron and copper sulfide minerals we observed in our study.

"The sulfide minerals formed between 50 and 200 degrees Celsius (122 and 392 degrees Fahrenheit), much warmer than the sub-zero temperatures predicted for the interior of a comet," she said.

In addition to providing evidence of liquid water, the discovered ingredients put an upper limit to the temperatures Wild-2 encountered during its origin and history.

"The mineral we found - cubanite - is very rare in sample collections from space. It comes in two forms - the one we found only exists below 210 degrees Celsius (99 degrees Fahrenheit). This is exciting because it tells us those grains have not seen temperatures higher than that," said Berger.

"Wherever the cubanite formed, it stayed cool. If this mineral formed on the comet, it has implications for heat sources on comets in general," she added.

According to Berger, two ways to generate heat sources on comets are minor collisions with other objects and radioactive decay of elements present in the comet's mixture.

Heat generated at the site of minor impacts might generate pockets of water in which the sulfides could form very quickly, within about a year (as opposed to millions of years). This could happen at any point in the comet's history.

Radioactive decay on the other hand, would point to a very early formation of the minerals since the radioactive nuclides would decay over time and cause the heat source to flicker out.

According to Lauretta, the findings show that comets experienced processes such as heating and chemical reactions in liquid water that changed the minerals they inherited from the time when the solar system was still a protoplanetary disk, a swirling mix of hot gases and dust, before it cooled down enough for planets to form.

The discovery is to be published in an upcoming online edition of the journalGeochimica et Cosmochimica Acta.