Saturn at Equinox | NASA Cassini

Saturn near equinox on August 3, 2009. Color-composite made from calibrated raw images captured by Cassini in visible-light filters from 2.2 million kilometers.

Once every 15 years, Saturn's ring system is plunged into a four-day-long night, throwing the orbiting dust particles into startling relief. NASA's Cassini spacecraft measured the coldest temperatures ever observed for the ring system.

The equinox observations can help astronomers better understand the structure and evolution of Saturn's rings, as well as the origin of the solar system.

A section of Saturn's rings are typically in shadow at any given time, experiencing a brief night that lasts from six to 14 hours. But during equinox, that night lasts for four whole days and affects the entire ring system.

Like Earth, Saturn experiences two equinoxes per orbit. During an equinox, each planet's equator lines up edge-on to its orbital plane, causing the sun to appear directly over the equator.

Of course, because Saturn is more distant from the sun, it takes much longer to complete an orbit—29.5 Earth years, to be exact—so there is much more time between equinoxes than on Earth.

As equinox approaches, sunlight fades as the top edge of the solar disk appears to touch the ring (from the perspective of a viewer embedded in the rings). As the solar disk slowly crosses the rings, there is full darkness. Then the bottom edge of the sun rises above the ring planes, about four days after the sunlight originally began to fade.

During equinox, light from the sun hits the ring particles at very low angles, which accentuates their topography, giving a 3-D view.

The Cassini spacecraft ended its mission on Sept. 15, 2017.

For more information about the Cassini-Huygens mission visit: https://saturn.jpl.nasa.gov and http://www.nasa.gov/cassini

The Cassini-Huygens mission was 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, California, managed 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 was based at the Space Science Institute in Boulder, Colorado.

Image Credit: NASA/JPL-Caltech/Space Science Institute/Jason Major
Caption Credit: http://www.space.com /Jason Major
Image Date: August 3, 2009
Release Date: May 14, 2018

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