Saturn's Prometheus: Just Plain Weird!

By: J. Kelly Beatty December 30, 2009 7

Versions of the saying "All things come to those who wait" have been around for nearly 500 years.

NASA's Cassini spacecraft has been orbiting Saturn for only 5½ years, but in that time this remarkable craft has had occasion to pass near many of the planet's 61 moons (six of which were discovered in its images). It's an amazing bunch of bodies &mdash variously big and small, smooth and cratered, gas-gushing and quiescent.

The small Saturnian moon Prometheus, 74 miles (119 km) long, appears to be thickly mantled with dust from Saturn's F ring. NASA's Cassini orbiter snapped the frames for this composite image on December 26, 2009, from 36,000 miles (59,000 km) away.

NASA / JPL / Space Science Inst. / S. Walker

But the close-up of Prometheus seen here represents new territory, in a sense. This elongated body, just 74 miles (119 km) long, has an alien quality to it. Note how all the edges are rounded and the craters are filled in. We're just not used to seeing moons (or asteroids) with such muted features. Heck, I've seen potatoes with more detailed surfaces — but no mere spud has ever fascinated me nearly as much as this picture does!

Cassini captured this view on December 26th, from just 36,000 miles (59,000 km) away. We're not likely to see Prometheus with this kind of detail (about 1,150 feet, or 350 meters, per pixel) for a very long time, if ever again.

Discovered in 1980 by Voyager 1, Prometheus and like-sized Pandora are the F ring's gatekeepers, "shepherd satellites" that circle just inside and outside the faint ribbon of dust. Their gravitational influence confines the F ring, keeps its particles from dispersing, and gives a tortured appearance.

But all this gravitational jostling forces each moon into a very slightly eccentric orbit, one part in 450 (0.0022) for Prometheus and one in 240 (0.0042) for Pandora. The slight orbital eccentricity causes Prometheus to make headlong dives in the F ring once per orbit (every 15 hours) into Saturn's F ring. You'd lose the rough edges too if you endured all that peppering. Pandora, which got its Cassini close-up in 2005, looks somewhat the same — though not nearly as blanketed in ring dust.

I'll bet that Carolyn Porco, the lead scientist for Cassini's imaging team, finds this picture particularly satisfying. As a young scientist on the Voyager mission, she specialized in the gravitational interplay of rings and moons. While there's been no shortage of amazing sights to grab her attention throughout Cassini's long-running romp, I suspect that right now she's thinking long and hard about how this funny little moon, along with its buddies Pandora and Atlas (the A ring's shepherd), came to be.

Follow Cassini's ongoing adventures in imaging at the team's website, ciclops.org.

Comments

Carolyn Porco

December 31, 2009 at 3:39 pm

Hey Kelly,

Thanks for the great blog! Actually, I did think ...about 4 years ago...about how Prometheus and the other ring region moons came to be. And I even published a paper on it in 2007. My co-authors and I figured that since accretion can't occur spontaneously in the ring region, they must have started with a shard of material -- perhaps a left-over from the breakup of a pre-existing moon -- that is denser than the ring material itself. And then, accretion filled out the moon to its present size.

Your readers might want to read the paper at ...

http://ciclops.org/sci/papers.php

...under 2007 on this subject with the reference "Porco, C., Thomas, P., Weiss, J., Richardson, D. (2007). "Saturn's Small Inner Satellites: Clues to Their Origins." Science 318, 1602-1607."

It's one of my papers that I'm most proud of. It makes a nice story.

Happy New Year to you, Kelly, and all your readers,
Carolyn Porco