page_title: Iapetus (moon)
section_title: Equatorial ridge
hierarchical_section_title: Iapetus (moon) / Physical characteristics / Equatorial ridge
caption_attribution_description: English: Computer simulation of the appearance of en:Saturn from its moon Iapetus. Iapetus is at the lowest point in its highly inclined orbit. Image created using the open source en:Celestia software.
context_page_description: Iapetus is the third-largest natural satellite of Saturn, eleventh-largest in the Solar System, and the largest body in the Solar System known not to be in hydrostatic equilibrium. Discoveries by the Cassini mission in 2007 revealed several unusual features, such as a massive equatorial ridge running three-quarters of the way around the moon.
context_section_description: A further mystery of Iapetus is the equatorial ridge that runs along the center of Cassini Regio, about 1,300 km (810 mi) long, 20 km (12 mi) wide, and 13 km (8.1 mi) high. It was discovered when the Cassini spacecraft imaged Iapetus on December 31, 2004. Peaks in the ridge rise more than 20 km (12 mi) 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 (120 mi) and sections with three near parallel ridges.  Within the bright regions there is no ridge, but there are a series of isolated 10 km (6.2 mi) peaks along the equator. The ridge system is heavily cratered, indicating that it is ancient. 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.
  A team of scientists associated with the Cassini mission have argued that the ridge could be a remnant of the oblate shape of the young Iapetus, when it was rotating more rapidly than it does today. The height of the ridge suggests a maximum rotational period of 17 hours. If Iapetus cooled fast enough to preserve the ridge but remained plastic long enough for the tides raised by Saturn to have slowed the rotation to its current tidally locked 79 days, Iapetus must have been heated by the radioactive decay of aluminium-26. This isotope appears to have been abundant in the solar nebula from which Saturn formed, but has since all decayed. The quantities of aluminium-26 needed to heat Iapetus to the required temperature give a tentative date to its formation relative to the rest of the Solar System: Iapetus must have come together earlier than expected, only two million years after the asteroids started to form.
  The ridge could be icy material that welled up from beneath the surface and then solidified. If it had formed away from the position of the equator at the time, this hypothesis requires that the rotational axis would have been driven to its current position by the ridge.
  Iapetus could have had a ring system during its formation due to its large Hill sphere of ~49 Iapetian radii, and that the equatorial ridge was then produced by collisional accretion of this ring.
  The ridge and the bulge are the result of ancient convective overturn. This hypothesis states that the bulge is in isostatic equilibrium typical for terrestrial mountains. It means that under the bulge there is material of low density (roots). The weight of the bulge is compensated by buoyancy forces acting on the roots. The ridge is also built of less dense matter. Its position along the equator is probably a result of the Coriolis force acting on a liquid interior of Iapetus.
caption: Computer simulation of the appearance of Saturn from Iapetus when it is at the "highest" point in its inclined orbit. Saturn's rings are clearly visible (from the other large moons they can only be seen edge-on).
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