Equatorial ridge on Iapetus

Lore & Background

The equatorial ridge on Iapetus was discovered when the Cassini spacecraft imaged the moon on 31 December 2004. Peaks in the ridge rise more than 20 km above the surrounding plains, making them some of the tallest mountains in the Solar System. The ridge forms a complex system including isolated peaks, segments of more than 200 km, and sections with three near parallel ridges. It is split into three mountain ranges: Carcassone Montes, Toledo Montes, and Tortelosa Montes. There are bright areas on the sides of the equatorial ridge near Iapetus' bright trailing hemisphere, which were visible in Voyager 2 and were nicknamed the 'Voyager Mountains'; they were eventually formally named Carcassone Montes.

The ridge system is heavily cratered, indicating that it is ancient. Within the bright regions there is no ridge, but there are a series of isolated 10 km peaks along the equator. The prominent equatorial bulge gives Iapetus a walnut-like appearance. It is not clear how the ridge formed. One difficulty is to explain why it follows the equator almost perfectly. There are at least four current hypotheses, but none of them explains why the ridge is confined to Cassini Regio.

One hypothesis suggests the ridge could be a remnant of the oblate shape of the young Iapetus when it was rotating more rapidly, with a maximum rotational period of 17 hours. Another proposes the ridge could be icy material that welled up from beneath the surface and then solidified. A third suggests Iapetus may have had a ring system during its formation, and the ridge could have been produced by collisional accretion of this ring. A fourth hypothesis states that Iapetus could have been impacted by a body that created both a ring system and a subsatellite, which then forced the ring system down onto Iapetus before escaping. Additionally, the ridge and bulge could be the result of ancient convective overturn, with the bulge in isostatic equilibrium.

Reader's Guide

The equatorial ridge on Iapetus is significant as the third tallest mountain structure in the Solar System, discovered by the Cassini probe in 2004. Its origin remains unknown, with at least four current hypotheses but none explaining why the ridge is confined to Cassini Regio. The ridge's height suggests a maximum rotational period of 17 hours for a young Iapetus, and if it cooled fast enough to preserve the ridge while tides slowed its rotation to the current 79 days, Iapetus must have been heated by radioactive decay of aluminium-26. This gives a tentative formation date of only two million years after the asteroids started to form. The ridge's heavily cratered nature indicates it is ancient. The ridge is split into three named mountain ranges, and the bright areas near the trailing hemisphere were nicknamed 'Voyager Mountains' before being formally named Carcassone Montes. The ridge's legacy lies in its unresolved origin and its role in constraining Iapetus's early history and rotation.

Did You Know?

• The ridge was discovered by the Cassini spacecraft on 31 December 2004.
• The bright areas on the sides of the ridge near Iapetus' trailing hemisphere were nicknamed the 'Voyager Mountains' and later formally named Carcassone Montes.