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.
References:
- 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.