London, Oct 9 : Dark splotches on Saturn’s moon Iapetus are found only on the sunward facing slopes of craters and mountains, new photos of the satellite by the NASA/ESA/ASI Cassini spacecraft has revealed.

The 1470-kilometre-wide Iapetus has a yin-yang appearance, with a black face and a bright back.

Astronomers believe the dark, organic-rich material on the front possibly comes from one of Saturn's other moons and is simply splattered on Iapetus's leading hemisphere.

As dark material absorbs more sunlight, some scientists have suggested that the moon's face got slightly warmer than its back.

Over time, this led to the evaporation of thin layers of ice, exposing even more dark material that was already mixed in with the ice. The net result: a feedback loop that produced the satellite's distinctive look.

Now, new pictures by the Cassini spacecraft has given boost to the theory that a runaway heating process is tainting portions of the moon.

“We got a very much better look at the relation between the black material and the ice,” said Carolyn Porco, leader of the Cassini imaging team.

Team members found that there are sharp-edged dark spots all over the surface, even on the moon's bright trailing hemisphere.

Careful analysis revealed that these isolated spots are preferentially located on the sunward facing slopes of craters.

The scientists say this suggests that as the slopes get slightly warmer, ice on the surface starts to evaporate. This exposes more dark stuff in the ice, which is then ready to retain even more solar heat.

“It's a runaway process,” said Porco.

Cassini captured the first high-resolution glimpse of the bright trailing hemisphere of Iapetus during a close flyby on September 10, 2007.

The results were presented on October 8 at a planetary sciences meeting in Orlando, Florida, US, reports New Scientist magazine. (ANI)