Strange Moons: Prometheus, The Peanut-Shaped Ring Shepherd

Strange Moons: Prometheus, The Ring Shepherd

Prometheus was the actual target in a recent photo that may have revealed a new moon of Saturn forming - or an old moon disintegrating. But let's have a closer look at Prometheus itself, this strange peanut-shaped shepherd moon of Saturn's F Ring.

Cassini image of Prometheus, taken during a close flyby on 27 Jan 2010.


Prometheus is an inner satellite of Saturn. It was discovered in 1980 from photos taken by the Voyager 1 probe. In late 1985 it was officially named after Prometheus, a Titan in Greek mythology. This small moon is extremely elongated, measuring about 136 by 79 by 59 km. It has several ridges and valleys and a number of impact craters of about 20 km diameter are visible. From its very low density and relatively high albedo, it seems likely that Prometheus is a very porous icy body.

Prometheus acts as a shepherd satellite for the inner edge of Saturn's F Ring. Recent images from the Cassini probe show that the Promethean gravitational field creates kinks and knots in the F Ring as the moon 'steals' material from it.


The orbit of Prometheus appears to be chaotic, as a consequence of gravitational perturbations from the moon Pandora (which orbits just outside the F Ring). The most appreciable changes in their orbits occur approximately every 6.2 years, when the two moons approach to within about 1400 km of each other. Prometheus is itself a significant perturber of the moon Atlas.

Here's a video of Prometheus and Pandora "shepherding" the F Ring:


Here's a true-colour image I created from raw Cassini images taken through red, green, and blue filters:

That's an interesting colouration for an icy body, probably comes from hydrocarbons that cover it and that might also come from the material Prometheus "steals" from the F Ring.

Prometheus quite stirs things up in the F Ring!

reply to post by wildespace


Very cool!
The pattern of disturbance in the F ring is very interesting.
I wonder why it forms the way it does? I know gravity is drawing material from the rings but why is it forming that particular pattern? It seems like you would be seeing a different kind of disturbance that the one we are seeing?
It's almost looks like it goes against the actual rotation of the rings and cuts across the rings in lines then those lines don't seem to fill back in with the normal rotation we see.
I assume as the ring goes around the lines fill in? or does that pattern continue around the whole orbit?

My question is in the video at 11 seconds what is that object that moves from left to right in a straight linear path through the ring towards the planet?

reply to post by mark1167


You would think it would cause a bulg where the "moon" is closest to the ring, not a wake caused by its passing.. Strange indeed..

bkfd54
My question is in the video at 11 seconds what is that object that moves from left to right in a straight linear path through the ring towards the planet?

The YouTube information says that some background stars were visible moving across the field of view.

The stars were basically static, but the Cassini craft itself is in motion. Since Cassini is in motion, but keeping its camera pointed at the spot in the F Ring, the background stars appear to be moving when it is actually Cassini that was moving.

reply to post by Soylent Green Is People


BOOM!!! Great answer, thank you.

Hey cool! Someone else doing a "Strange Moons" thread other than me!



Thanks for the thread wildspace!

No worries, those moons are so peculiar it's hard to resist posting about them!

I just found this cool video of Prometheus racing around the F ring.


Raw, uncalibrated images from the Cassini spacecraft in sequence at 24 frames per second to get an idea of what Cassini photographs as it orbits Saturn. Most of the video is focused on Saturn's moon, Prometheus, as it orbits the planet. If you look closely, you can see how the moon disturbs Saturn's F-ring, tugging on the particles with its own gravity and creating a rippling effect.