The appearance of the two-toned Iapetus has always been a mystery since the Saturn moon was first discovered in the late seventeenth century.

Now, new high-resolution images from Cassini have uncovered details on the strange bright and dark patterns. The moon's bright trailing hemisphere, especially in the equatorial regions, are coated in dark material - which is collected in the floors of crators and other low regions.

It suggests that the waning of the Sun in removing bright ice from the sunward-facing surfaces then leaves behind the native dark material that is normally mixed with the ice. Tilmann Denk, imaging team associate at the Free University in Berlin, explains that "the equator-facing slopes get more sunlight, and the bright ice there evaporates, leaving behind the darker stuff."

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