The European Space Agency says its Rosetta spacecraft circling Comet 67P has detected more than 100 patches of what may be water ice exposed on the comet's surface.

Analysis of gases being emitted by comets as they approach the sun and are warmed has shown they are rich in ice.

Researchers have used images from the high-resolution science camera on Rosetta to conduct an analysis of bright patches of exposed ice on the surface of Comet 67P/Churyumov-Gerasimenko.

In images captured by Rosetta's OSIRIS camera, scientists have been able to identify 120 different spots on the comet's surface as much as 10 times brighter than the average surface brightness, they report in the journal Astronomy & Astrophysics.

The bright areas are all seen in areas with shadows, such as below cliffs, where any patches of surface ice could be protected from sunlight, they say.

While they acknowledge they cannot definitively say the areas are water ice, all the evidence points to that as the most likely explanation, researchers say.

"At the time of our observations, the comet was far enough from the Sun such that the rate at which water ice would sublimate (turn to gas) would have been less than one mm per hour of incident solar energy," study lead author Antoine Pommerol explains.

"By contrast, if carbon dioxide or carbon monoxide ice had been exposed, it would have rapidly sublimated when illuminated by the same amount of sunlight. Thus we would not expect to see that type of ice stable on the surface at this time."

Some of the bright patches are grouped in clusters, the scientists say, while others are more isolated. At visible wavelengths, the patches are bluer in color compared with the comet's generally reddish background, consistent with their having an icy component, the researchers say.

As the comet continues on its voyage toward perihelion — its closest approach to the sun — further confirmation of the patches and what they're made of could be forthcoming, they say.

"As the comet continues to approach perihelion, the increase in solar illumination onto the bright patches that were once in shadow should cause changes in their appearance, and we may expect to see new and even larger regions of exposed ice," says Rosetta project scientist Matt Taylor.

Rosetta's science mission was scheduled to terminate in December of this year, but the ESA has announced it is extending it through September 2016.