NASA/JPL/Space Science Institute
Cassini Gets Up Close and Personal With Saturn's 'Wavemaker' Moon Daphnis
As NASA's Saturn mission skims the gas giant's rings, it's seeing incredible detail in the interactions between ring particles and a tiny moon's gravity.
Cassini's daring ring-skimming orbits of Saturn are already paying off, producing some beautiful and awe-inspiring views that have, until now, been too far away to see. But now, as this almost surreal observation of Saturn's tiny moon Daphnis shows, we're finally getting a really good look at the small-scale processes that are at work in Saturn's rings.
Orbiting the Saturnian system since 2004, NASA's Cassini mission has enriched us incredible views of the seemingly flat ring plane. Beyond the robotic probe's camera resolution, however, are the ripples and waves that are inevitably caused by the gravities of small moons embedded in the many ring gaps. In one 26-mile-wide gap, called the Keeler Gap, a 5-mile-wide moon roams and it has a pretty dramatic effect on the tiny particles at the gap's borders.
The oblique viewing angle is a little misleading; we're not looking directly down on the ring plane, we're actually looking at the moon from the side. The waves in the foreground are therefore rippling up and down as the moon goes about its orbit. The ring gap also looks more narrow than its 26-mile width, an optical effect known as foreshortening. Cassini was 17,000 miles from the moon when this image was captured on Jan. 16.
Previously, in 2009, Cassini was able to spot these waves in Saturn's rings, albeit from afar, when the ringed gas giant was passing through its equinox. At this time, the ring plane was parallel to the direction of sunlight, allowing any vertical structures in the rings to cast a long shadow:
Cassini's view of Daphnis and the waves it creates in 2009 when the spacecraft was 414,000 miles from the moon (NASA/JPL/Space Science Institute)
Viewing Daphnis so close means that previously unseen details pop into view. Of particular note is the narrow ridge that seems to run around the moon's equator and the smooth layer that covers its surface — traits shared by other ring moons Atlas and Pan. These features are likely a build-up of ring particles that have collided with the moon and accumulated during its orbits. Also, small craters are evident, proving that even the smallest of moons are not immune from impacts.
Another feature is a thin wisp of dusty material to the left of the moon, likely a clump of dust pulled from the gap's edge, which is now trailing the moon and spreading out.
Although Cassini's mission will end in September, with a fiery farewell into Saturn's atmosphere, as this latest observation shows, there's a lot more surprises in store as it sets itself up for a sequence of polar orbits that will take the spacecraft through the ring plane — a feat that has never before been attempted.
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