Iapetus is the 3rd largest moon around Saturn and like many of Saturn's moons; it is locked in synchronous rotation where the same hemisphere faces Saturn all the time. A striking two-tone colouration exists over the leading and trailing hemispheres of Iapetus. The leading hemisphere and sides are dark (albedo 0.03 - 0.05), while most of the trailing hemisphere and poles are bright (albedo 0.5 - 0.6). This two-tone colouration has a pattern analogous to a spherical yin-yang symbol.
Figure 1: A mosaic showing an entire hemisphere of Iapetus. Credit: NASA/JPL/Space Science Institute.
Iapetus has a remarkable equatorial ridge system extending over 110 degrees in longitude. Parts of the ridge have peaks that rise up to 20 km above the surrounding landscape, making these mountains amongst the highest in the Solar System. The prominent equatorial ridge system gives Iapetus an overall walnut-like appearance. A number of endogenic (i.e. processes such as tectonism or volcanism arising from the interior of Iapetus) and exogenic (i.e. processes such as debris in-fall that originate from beyond the surface of Iapetus) mechanisms have been proposed to explain the origin of the equatorial ridge system.
A recent study by Lopez Garcia et al. (2014) suggests an exogenic formation mechanism for the equatorial ridge on Iapetus. Using a total of 506 topographic profiles of the ridge system obtained by NASA’s Cassini spacecraft, a topographic analysis was performed where 6 types of ridge morphologies were identified - triangular (33% of profiles), trapezoidal (21% of profiles), crowned (8% of profiles), twinned (14% of profiles), dissimilar (7% of profiles) and saddle (17% of profiles). The triangular peaks form the most common morphology and have the steepest slopes, with some slopes reaching ~40 degrees. Also, the triangular peaks are probably the least impact-modified ridge morphology on Iapetus.
Figure 2: Representative examples of the six ridge morphological types observed in the topographic profiles. Vertical exaggeration ~10 times. Source: Lopez Garcia et al. (2014).
As proposed by Ip (2006), an exogenic origin for the ridge system would most probably occur via debris in-fall, whereby the ridge is the remains of an early ring system that collapsed onto the equator of Iapetus. Depending on the material, the resulting ridge would have slopes with angles close to the angle of repose. For rounded icy grains, the angle of repose is ~25 degrees; and for snow mixed with particles of hail, the angle of repose is ~45 degrees. Indeed, as shown in this study, the presence of slope angles close to the angle of repose favours an exogenic origin for the equatorial ridge on Iapetus.
- Lopez Garcia et al. (2014), “Topographic Constraints on the Origin of the Equatorial Ridge on Iapetus”, arXiv:1404.2337 [astro-ph.EP]
- Ip (2006), “On a ring origin of the equatorial ridge of Iapetus”, Geophysical Research Letters 33, L16203.