This almost surreal view of Iapetus was acquired by Cassini about 10
minutes after the spacecraft's closest approach to the icy moon during
a close flyby on New Year's Eve 2004.
The image shows Iapetus' surface illuminated by reflected light from Saturn
(not by the Sun) and is the highest resolution view acquired to date of this
part of Iapetus' surface.
Compared to the approximately one second exposure times used for
imaging Iapetus' sunlit side, this view required a very long exposure time
of 82 seconds. Cassini was designed to pivot while moving in order to
keep its cameras and other remote sensing instruments pointed 'on
target' with great precision. Consequently, despite the large relative speed
between Iapetus and the spacecraft during this long exposure -- about 2
kilometers per second or almost 4,500 miles per hour at closest
approach -- the image of the moon’s surface is un-smeared (although
the background stars are smeared).
This image reveals a heavily cratered surface and shows the boundary
between Iapetus' bright trailing hemisphere and Cassini Regio -- a large,
dark region that covers the leading hemisphere of the moon’s surface.
Some of the dark material appears to have collected inside the rim of a
large impact structure about 250 kilometers across (155 miles) that lies
just beyond the edge of the dark region (seen here near the right of the
image). NASA's Voyager images (see PIA02268) this feature appeared as a dark 'moat'
and had been hypothesized to be an impact structure. The recent images
from Cassini confirm an impact origin for this feature.
In contrast, the origin of the dark material is currently unknown and the
recent images have sparked exciting debates among Cassini imaging
scientists. Some characteristics of the dark region revealed in this and
other images taken during the encounter suggest that dark material
from elsewhere in the Saturnian system -- perhaps the result of an impact
on another nearby moon -- has coated this side of Iapetus with a relatively
thin layer. However, an internal origin for the material has not been ruled
out and, if correct, may be related to the long equatorial ridge discovered
in Cassini images to span Cassini Regio.
Regardless of its origins, the dark material appears to lie on top of other
geologic features seen on Iapetus thus far, implying that the event which
formed the dark coating occurred later in Iapetus' history. A closer
encounter with this moon later in the Cassini mission (September 2007)
may reveal more detail and help answer the question of the origin and
age of the dark material on Iapetus.
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
(76,659 miles) from Iapetus. The image scale is 730 meters (2,395 feet)
per pixel. The image has been contrast enhanced to aid visibility of
surface features.
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.
