Spokes, those ghostly radial markings on Saturn's B ring, appear bright
compared to the rings in this image taken a little more than a month after
the planet's August 2009 equinox.
Spokes appear bright when they are viewed at phase, or Sun-Saturn
spacecraft, angles higher than about 45 degrees. The phase angle in this
image is 64 degrees. Also, the contrast is even greater in this image
since the surrounding rings are darkened during the equinox period at
To learn more about spokes, see PIA11144.
Saturn's northern latitudes appear dark in this image because of the
camera filter used. This view uses a spectral filter sensitive to
absorption of certain wavelengths of light by methane in Saturn's
atmosphere. In the north, the light at these wavelengths reaches slightly
greater depth -- compared to the equatorial regions -- before being
reflected off the cloud tops, and therefore passes through more light
absorbing methane along the way out.
The novel illumination geometry that accompanies equinox lowers the sun's
angle to the ringplane, significantly darkens the rings, and causes
out-of-plane structures to look anomalously bright and cast shadows across
the rings. These scenes are possible only during the few months before and
after Saturn's equinox, which occurs only once in about 15 Earth years.
Before and after equinox, Cassini's cameras have spotted not only the
predictable shadows of some of Saturn's moons (see PIA11657), but also the
shadows of newly revealed vertical structures in the rings themselves (see
The moon Janus (179 kilometers, or 111 miles across) is also visible on
the left of the image.
This view looks toward the sunlit, northern side of the rings from about
12 degrees above the ringplane.
The image was taken with the Cassini spacecraft wide-angle camera on Sept.
22, 2009 using a spectral filter sensitive to wavelengths of near-infrared
light centered at 890 nanometers. The view was acquired at a distance of
approximately 1.3 million kilometers (808,000 miles) from Saturn. Image
scale is 71 kilometers (44 miles) 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 operations center is based at the Space
Science Institute in Boulder, Colo.
For more information about the Cassini-Huygens mission visit
http://saturn.jpl.nasa.gov/. The Cassini imaging team
homepage is at http://ciclops.org.