Scientists have always maintained that the celestial rocks never get warm enough to melt their frozen bodies.
But a team of U.S. scientists now say they have convincing evidence of the presence of water in one comet sample.
Fire and ice: This artist s impression shows the irregular surface of comet Wild-2 and jets spouting into space at speeds of several hundred miles per hour
Researchers at the University of Arizona and the Johnson Space Center analysed grains of dust scooped from a comet's surface by the Stardust spacecraft in 2004 and returned to Earth in a capsule two years later.
'In our samples, we found minerals that formed in the presence of liquid water,' researcher Eve Berger said.
'At some point in its history, the comet must have harbored pockets of water.
The university's Professor of Cosmochemistry, Dante Lauretta, added: '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.
'This study shows the high science value of sample return missions. These grains would never been detected by remote sensing or by flying a spacecraft past the comet to make observations without collecting a sample.'