Click on image for larger annotated version
Cassini made a close flyby of Saturn's moon Iapetus on Sept. 10, 2007, and
the visual and infrared mapping spectrometer obtained these images showing
surface composition and particle size.
The visual and infrared mapping spectrometer is like a digital camera, but
instead of using three colors, it makes images in 352 colors, or
wavelengths, from the ultraviolet to the near-infrared. The many
wavelengths produce a continuous spectrum in each pixel, and these
spectra measure how light is absorbed by different materials. By analyzing
the absorptions expressed in each pixel, a map of the composition at each
location on the moon can be constructed.
The left image in the figure shows the amount of reflected light at a
wavelength of 1.75 microns in the infrared (green light seen by our eyes
is 0.53 microns). The color image on the right shows the results of
mapping for three components of Iapetus' surface: carbon dioxide that is
trapped or adsorbed in the surface (red), water in the form of ice
(green), and a newly-discovered effect due to trace amount of dark
zsa~particles in the ice creating what scientists call Rayleigh scattering
(blue). The Rayleigh scattering effect is the main reason why the Earth's
sky appears blue.
The Rayleigh scattering effect on Iapetus provides evidence that tiny
grains, less than the wavelength of visible light (less than 0.5 microns)
have been embedded in the surface of Iapetus. The tiny grains must be
well-separated for the Rayleigh effect to become prominent, so the
abundance of particles must be less than about 2 percent. The Rayleigh
scattering effect shows in all areas, although weakly in dark regions (the
red carbon dioxide dominates the colorimage), and it appears stronger away
from the equator. Investigating the trend from dark to bright areas, the
Rayleigh effect changes with the amount of dark material in the ice, and
becomes weaker as more dark material is added. This points to cleaner ice
as one moves north or south from the equator and away from the dark
leading side of the moon (toward the right in the image).
This provides additional evidence for an external source for the dark
material coating Iapetus, and for ice transport away from the warm dark
regions and equator to the cooler poles. The ice transport away from the
equator increases the concentration of dark material there and reduces the
Rayleigh effect. With the volatile transport from the dark warm regions,
the strong carbon dioxide signature is a surprise because frozen carbon
dioxide is more volatile than water ice. Therefore, the carbon dioxide
must be trapped, making its presence stable in the warm equatorial region.
The trapping mechanism is currently under study.
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 was designed, developed and assembled at JPL. The Visual
and Infrared Mapping Spectrometer team is based at the University of
Arizona.
For more information about the Cassini-Huygens mission visit
http://saturn.jpl.nasa.gov/home/index.cfm. The visual and infrared mapping
spectrometer team homepage is at http://wwwvims.lpl.arizona.edu.