The image on the left was taken on Nov. 1, 1980, by NASA's Voyager
spacecraft from a distance of 5.3 million kilometers (3.3 million miles).
It shows a very strong narrow shadow cast on the equatorial region of
Saturn's atmosphere by the rings. During the Voyager encounters, the Sun
was close to the plane of the rings so that the ring shadow was very deep
and localized to low latitudes.
Radio signals detected by Voyager were interpreted as lightning coming
from a persistent, extended storm system at low latitudes. It is possible
that the ring shadow was partly responsible for generating this storm by
promoting strong convection at the boundary of the colder shadowed
atmosphere and the adjoining sunlit atmosphere. This image was previously
released on June 19, 1999. For original caption see PIA00335.
The image on the right was acquired by the Cassini spacecraft on May 10,
2004, from a distance of 27.2 million kilometers (16.9 million miles) and
shows the complex set of ring shadows cast over a large region of Saturn's
northern hemisphere. This shadow pattern is due to the Sun being well
below the ring plane during Cassini's approach to Saturn. This image was
previously release on May 25, 2004. For original caption see PIA05394.
Unlike the situation when NASA's Voyager spacecraft flew by Saturn, these
ring shadows are not as deep and are not localized at a very narrow range
of latitudes. Should these shadows drive convection in Saturn's
atmosphere, the location would likely be very much different than the
near-equatorial shadow observed by the Voyagers in the early 1980s. It
is possible that this very different ring shadow geometry is one reason
for different morphologies of thunderstorms observed by Cassini and
Voyager. Voyager observed lightning apparently from one persistent,
low-latitude storm system, whereas Cassini observes lightning from storms
which seem to come and go on time scales of a day or so, and perhaps from
more than one storm system at a time.
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 Cassini-Huygens mission for NASA's Science Mission Directorate,
Washington, D.C. The Cassini orbiter was designed, developed and assembled
at JPL. The radio and plasma wave science team is based at the University
of Iowa, Iowa City.
For more information about the Cassini-Huygens mission, visit
http://saturn.jpl.nasa.gov and the instrument team's home page,
http://www-pw.physics.uiowa.edu/plasma-wave/cassini/home.html.
