Liquid water `discovered on frozen comet`

Scientists claim to have for 1st time found concrete evidence for presence of liquid water in a comet.

Updated: Apr 06, 2011, 18:12 PM IST

Washington: Planetary scientists claim to have for the first time found concrete evidence for the presence of liquid water in a comet.
A team, led by Arizona University in the US, says 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.

The scientists made the discovery by analysing dust grains brought back to Earth from comet Wild-2 as part of the Stardust mission by NASA.

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

Comets are frequently called dirty snowballs because they consist of mostly water ice, peppered with rocky debris and frozen gases, say the scientists.

"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, much warmer than the sub-zero temperatures predicted for the interior of a comet," they 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," Berger said.