Iapetus: Major discoveries from the Cassini imaging experiment
2008
Over the course of more than three years orbiting
Saturn, the Imaging Subsystem (ISS) [1] of the
Cassini spacecraft has acquired high-resolution
images of the Saturnian moon Iapetus during a
number of flybys. The most recent and only targeted
Iapetus flyby occured on 10 September 2007, and
allowed a >50x closer look at the surface than any
previous observation. The surface of Iapetus is heavily
cratered down to the resolution limit of ~10 meters per
pixel. The crater size-frequency distribution shows no
measurable difference between the leading and the
trailing hemisphere, arguing for planetocentric
projectiles as the main impactor source. The equatorial
ridge can now be clearly tracked along half of
Iapetus's circumference, from ~50°W to ~245°W; it is
mainly absent on the other hemisphere. However, we
argue that it presumably spanned the full globe shortly
after formation. Very small bright-ray and bright-rim
craters have been detected deep within the dark
hemisphere, suggestive for a dark blanket with a
thickness in the order of decimeters to meters only. On
the trailing side at low and mid-latitudes, very dark
terrain is located immediately adjacent to bright
terrain, with almost no gray shading in between. In
many cases, crater walls facing towards the equator
are dark, while poleward-facing walls and slopes are
bright. This effect vanishes at both north and south
high latitudes. We interpret these observations to
indicate that thermal segregation of water ice is
responsible for these complex small-scale dark-bright
patterns. On the trailing side, a bright polar cap has
been observed at high latitudes on both hemispheres
(north and south). A global color dichotomy has been
detected in addition to the long-known global
brightness dichotomy, with the leading side showing a
significantly redder color than the trailing side. Unlike
the more ellipsoidal-shaped brightness dichotomy, the
color dichotomy is quite well separated into two
different hemispheres, with the sub-Saturn (~0°W)
and anti-Saturn (~180°W) meridians as the approximate
boundaries [2]. This global pattern indicates an
exogenic origin. Earlier hypotheses for the origin of
the brightness dichotomy, like the infall of dust from
retrograde outer moons, might actually offer a better
explanation for the color dichotomy than for the
brightness dichotomy. We propose that this so far unknown
process forming the color dichotomy has also
reddened and somewhat darkened Hyperion, another
moon of Saturn. The color dichotomy also provides a
key element to the explanation of the brightness
dichotomy in the model of Spencer et al. [3].
References
[1] Porco, C.C. et al. (2004) Space Sci. Rev.115, 363.
[2] Denk, T. et al. (2006) EGU06-A-08352.
[3] Spencer, J.R. et al. (2005) 37th DPS, abstract, 39.08.
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