For humans, a bulging waistline may be a signal to exercise more, but for Saturn's moon Iapetus it may be a hint of past calamity.

The bulge, a long ridge visibly running across the moon's midsection, is apparent in this image taken by NASA's Cassini spacecraft during a recent flyby past the Saturnian satellite.

Astronomers are unclear what actually caused the ridge, but they do have their theories. The ridge may be the result of a simple mountain belt, the eruption of Iapetus' interior through a massive, lengthy surface crack or even the accumulation of dark material cast out by impacts on Saturn's other moons, NASA officials said.

The geologic feature, which is unlike any other found on moons throughout the solar system, is a long narrow ridge that lies almost exactly on Iapetus' equator and runs across its entire dark hemisphere. At its highest point the ridge reaches 12 miles (19 kilometers) up as it stretches across more than 808 miles (1,300 kilometers) along the moon's midsection.

Although Cassini took this image during its Dec. 31 pass by Iapetus, it was released on Jan. 7.

-- SPACE.com Staff

Credit: NASA/JPL