The Shortest of Shadows
March 2, 2010
Saturn's moon Daphnis casts a short shadow on the A ring in this image taken about six months after the planet's August 2009 equinox.
Daphnis (8 kilometers, or 5 miles across) appears as a tiny bright dot in the Keeler Gap of the A ring near the center top of the image. The moon's orbit is inclined relative to the plane of Saturn's rings. Daphnis' gravitational pull perturbs the orbits of the particles of the A ring that form the Keeler Gap's edge, and sculpts the edge into waves having both horizontal (radial) and out-of-plane components. Material on the inner edge of the gap orbits faster than the moon so that the waves there lead the moon in its orbit. Material on the outer edge moves slower than the moon, so waves there trail the moon. See Wavy Shadows to learn more about this process.
The Encke Gap, wider than the Keeler Gap, can be seen across the middle of the image.
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 Across Resplendent Rings), but also the shadows of newly revealed vertical structures in the rings themselves (see A Small Find Near Equinox).
This view looks toward the southern, unilluminated side of the rings from about 25 degrees below the ringplane.
The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Jan. 10, 2010. The view was obtained at a distance of approximately 801,000 kilometers (498,000 miles) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 147 degrees. Image scale is 4 kilometers (2 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.
Credit: NASA/JPL/Space Science Institute