Sun light caused mysterious blemishes on Iapetus

From the NewScientist online magazine

Blame the Sun for the mysterious dark blemishes on Saturn's moon Iapetus. New photos from the Cassini spacecraft reveal the splotches are mainly found on the sunward-facing slopes of craters and mountains, suggesting a runaway heating process is tainting portions of the moon. Cassini's imaging 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. That suggests that as the slopes get slightly warmer, ice there starts to evaporate. That exposes more dark stuff in the ice, which is then ready to retain even more solar heat.

thanks to Shaun Saunders for the update