676 Encyclopedia of the Solar System
(a) (c)
(b)FIGURE 13 Saturn’s moon Phoebe as measured by theCassiniUVIS. (a) Phoebe’s FUV spectral
reflectance measured at 90◦solar phase angle. Phoebe’s spectrum is overplotted with a model
(red) that includes H 2 O frost and dark, carbonaceous material. Also shown is the lunar albedo
(blue) as measured by UVIS at the same phase angle. Note that water ice on Phoebe makes it
even darker than the Moon at short FUV wavelengths, and brighter at long FUV wavelengths.
(b) FUV wavelength image of Phoebe. Red represents 1216A where interplanetary hydrogen is ̊
bright throughout the solar system. Phoebe’s water ice makes it darker than the background IPH
on both the illuminated and the dark hemispheres so that the entire disk is visible. Blue colors
represent longer FUV wavelengths and show that the brightness varies across Phoebe’s surface
due to solar incidence, topography, and compositional variations. (c) Visible wavelength image of
Phoebe fromCassini. (Figures reproduced with permission from AAAS/Science; ISS image
courtesy of NASA.)
system, primarily near the orbit of Tethys. Similarly,Cassini
UVIS measured neutral oxygen (at 1304A), in varying ̊
amounts, with the greatest abundances near the orbit of
Enceladus. The presence of OH and O suggested that H 2 O
is produced by erosion of the inner icy satellites of Saturn by
micrometeoroid bombardment and is then broken down by
photodissociation to produce the neutral species. However,
the amounts of H 2 O necessary to produce the observed
OH and O abundances were not consistent with sputtering
rates; an additional source of H 2 O was needed—and re-
mained a mystery untilCassiniobservations of Enceladus
in 2005.
The Cassini spacecraft, through unique multi-
instrument observations, discovered active water plumes
on Enceladus. A stellar occultation byCassiniUVIS mea-
sured the presence of water vapor above the limb of the