This oblique view of Saturn's moon Iapetus from high latitude shows how
the dark, heavily cratered terrain of Cassini Regio transitions to a bright,
icy terrain at high latitudes.
In this mosaic of two high resolution images taken during Cassini’s New
Year’s Eve 2004 flyby of Iapetus, the direction toward the north pole is
approximately 15 degrees below the horizontal on the right. At the equator
terrains are uniformly covered with a dark mantle of material that has a
reflectivity of about 4 percent. At latitudes toward the pole of about 40
degrees, the dark deposits become patchy and diffuse as the surface
transitions to a much brighter, icy terrain near the pole. The brightest
icy materials exhibit visual reflectivity over 60 percent.
Superimposed on the bright terrain is a subtle, ghostly pattern of crudely
parallel, north-south trending wispy streaks. The streaks, which were
discovered during this flyby of Iapetus, are typically a few kilometers wide
and sometimes tens of kilometers long. Their appearance and orientation
may be connected with the emplacement of dark materials that cover
Cassini Regio. The dark materials might represent the gradual accumulation
of dark debris falling from space, or alternatively, may represent fallout from
plume-style eruptions that may have accompanied the formation of Iapetus's
enigmatic equatorial ridge (see PIA06166).
Also seen in this mosaic are conspicuous, north-facing bright crater walls. An
example can be seen in the upper left where the bright, 4-kilometer-high (2.5
miles) walls of a 70 kilometer (44 mile) central-peak crater lies.
The bright crater walls are often higher in brightness than the corresponding
south-facing walls of the same crater. They are vaguely reminiscent of bright
north-facing crater walls that were discovered by NASA's Voyager and Galileo
spacecraft in craters near the poles of the Jovian satellites Callisto and
Ganymede. In the case of the Jovian satellites, cold-trapping of frosts on
north-facing slopes and sublimation of ices from south-facing slopes are
thought to produce the north-south asymmetries in crater wall brightness.
However, the occurrence of some young-appearing craters on Iapetus that
have bright north-facing and dark south-facing slopes, and the pattern of
streaks near the north pole of Iapetus suggests that another mechanism may
be responsible for the crater wall brightness asymmetries on Iapetus.
One possibility is that the south-facing slopes may be stained by the same
process that emplaced the low brightness coating throughout the region.
In this case, the north-pointing scarps might be bright because they face
away and are shielded from the putative falling spray of dark materials.
Bright south-facing slopes would exist primarily on young craters that have
not been exposed to the darkening agent long enough to be stained.
The image was obtained in visible light with the Cassini spacecraft narrow
angle camera on Dec. 31, 2004, at a distance of about 123,370 kilometers
(76,658 miles) from Iapetus and at a Sun-Iapetus-spacecraft, or phase,
angle of 93 degrees. Resolution achieved in the original image was 732
meters (2,401 feet) per pixel.
The Cassini-Huygens mission is a cooperative project of NASA, the
European Space Agency and the Italian Space Agency. The Jet Propulsion
Laboratory, a division of the California Institute of Technology in
Pasadena, manages the mission for NASA's Science Mission Directorate,
Washington, D.C. The Cassini orbiter and its two onboard cameras were
designed, developed and assembled at JPL. The imaging team is based at
the Space Science Institute, Boulder, Colo.
For more information about the Cassini-Huygens mission visit
http://saturn.jpl.nasa.gov. For images visit the Cassini
imaging team home page http://ciclops.org.