Cassini spies shy moon nestled among Saturn's rings

Cassini took the clearest images yet of Saturn's mini-moon Daphnis, shedding light on ring structure and behavior. 

Space Science Institute/JPL-Caltech/NASA
The wavemaker moon, Daphnis, as seen by NASA's Cassini spacecraft while making one of its ring-grazing passes over the outer edges of Saturn's rings on Monday. This is the closest view of the small moon obtained yet.
Space Science Institute/JPL/NASA
Vertical structures created by Saturn's small moon Daphnis cast long shadows across the rings as the planet approaches its mid-August 2009 equinox.

Mini-moon Daphnis is literally making waves, and NASA caught it on camera.

NASA probe Cassini snapped its crispest images yet of the five-mile wide moon Daphnis, as it performed another “ring grazing” pass of Saturn’s outer rings on Monday. The pictures provide a close-up look at some previously indirectly observed ring features as the spacecraft approaches the final phase of its two-decade mission.

Daphnis, named for a poet-shepherd in Greek mythology, inhabits the Keeler Gap. Fifty percent wider than the Grand Canyon, this dust-free band in the outermost A Ring of Saturn is thought to be kept clear by the moon’s orbit around the gas giant.

Due to Cassini’s low-to-the-ring perspective, a foreshortening effect makes Daphnis appear to take up most of the gap, but a top-down look would show the band to be five times wider than the moon, according to a NASA press release.

The unprecedented detail reveals features such as horizontal ridges circling the moon’s equator, paralleling Saturn’s rings. Scientists believe these bands could be a sign of the ring-particles that pile up on the surface as the body sweeps through the Keeler Gap, like dust on a car driving down a dry, dirt road.

Cassini may have even caught the planet in the act of dust disturbance, as a faint tendril appears to follow Daphnis, hugging its lower left boundary. NASA speculates the moon’s gravity may have pulled this wispy material from its ring out into the gap, which explains its nickname, wavemaker.

This gravitational disruption can be seen in the form of waves spanning the lower edge of the gap, which sends a reminder that Saturn’s rings are not the static 2-D disks they seem in pictures but rather full 3-D objects with their own internal structure and movement.

Daphnis’s gravity disturbs the 30-foot-thick rings surrounding it in more ways than one. In addition to the horizontal waves visible in this new image, another shot taken in 2009  shows vertical ones as well. Invisible against the bright backdrop, the up to one-mile tall waves revealed themselves indirectly as shadows cast against the outer rings during Saturn’s equinox.

And Daphnis isn’t the only ring-denizen to be caught by Cassini’s cameras. Also taking advantage of the 2009 equinox, when the low angles of sun rays cause out-of-ring objects to cast dramatic shadows, the probe discovered evidence of 100-meter wide moonlets hiding in the rings themselves. While not directly visible, the moonlets reveal themselves in the propeller-shaped swirls they produce moving through the rings, as well as the resulting shadows.

A joint project between NASA and the European Space Agency, among others, Cassini launched from Earth in 1997. Originally intended for a four-year mission, two extensions have brought discoveries such as methane rivers on Titan in addition to the unprecedented views of the rings.

After exploring the inner space between Saturn and its rings, the probe’s extraordinary 20-year career will come to a close on September 15 of this year, when it intentionally dives into the clouds of the gas giant, burning up.

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