In the case of Iapetus it is a ridge around the moon's equator, clearly visible in the photograph.

ECONOMIST: Those space probes in full

Observations of the material coating the dark face of Iapetus indicate it has a similar composition to the surface material on Phoebe.

BBC: New ring detected around Saturn

The discovery would appear to resolve a longstanding mystery in planetary science: why the walnut-shaped Iapetus has a two-tone complexion, with one side of the moon significantly darker than the other.

BBC: New ring detected around Saturn