CICLOPS: Cassini Imaging Central Laboratory for OPerationS

The Rays of Rhea
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The Rays of Rhea
PIA 09841

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  Cassini views Rhea and the bright, rayed crater that is likely one of the younger features on the moon's surface.

The impact excavated fresh material from beneath the ground, which spread out in this pattern as it fell back to Rhea.

This view, centered on 12 degrees south latitude, 133 degrees west longitude, predominately encompasses the anti-Saturn side on Rhea's leading hemisphere. North is up and rotated 28 degrees to the right.

Icy Rhea is 1,528 kilometers (949 miles) across.

The image was taken with the Cassini spacecraft narrow-angle camera on Jan. 17, 2008 using a combination of spectral filters sensitive to wavelengths of polarized green light centered at 617 and 568 nanometers. The view was obtained at a distance of approximately 538,000 kilometers (334,000 miles) from Rhea and at a Sun-Rhea-spacecraft, or phase, angle of 9 degrees. Image scale is 3 kilometers (2 miles) per pixel.

The Cassini-Huygens mission is 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, manages the Cassini-Huygens mission for NASA's Science Mission Directorate, Washington, D.C. The imaging team consists of scientists from the US, England, France, and Germany. The imaging operations center and team lead (Dr. C. Porco) are based at the Space Science Institute in Boulder, Colo.

For more information about the Cassini-Huygens mission, visit and the Cassini imaging team home page,

Credit: NASA/JPL/Space Science Institute
Released: February 20, 2008 (PIA 09841)
Image/Caption Information

Alliance Member Comments
Mercury_3488 (Mar 1, 2008 at 8:37 PM):
Hi Red_dragon,

It is so interesting to see how different the individual moons are within a particular system.

With Saturn's system, the difference between Titan & Mimas is most striking. Phoebe almost certainly is an interloper from the KBO, Hyperion is just odd, Enceladus, highly active, Dione also probably moderately active, Tethys, Rhea, & Mimas as dead as door nails.

Iapetus is extremely fascinating for other reasons.

I suspect the Uranian moons had more radiactive elements & it is interesting to see, how much denser Titania & Oberon are to Saturn's similarly sized moons Rhea & Iapetus.

Clearly the Saturn & Uranus systems have evolved very differently.

I like your comparison of the differences between Io & Callisto in the Jovian system. True, Jupiter's immensely gravity & tidal influences have much to do with this.

Even so, Io is poor in volatiles, but heavily differentiated & contains a large amout of heavy materials (Io has the highest density & surface gravity of the solar system's moons). Callisto appears to be a mixture of rock & ice, with a slight concentration of rock towards the centre, but not enough to form a core.

Probably much like Rhea, which appears to be much like a smaller Callisto.

Andrew Brown.
Red_dragon (Feb 29, 2008 at 7:40 AM):
Interesting the abstract you comment. I say it, because it seems there was diferentiation in Enceladus, or at least I've that impression seeing some renderings of the internal structure or that moon.

It's a pity we've so few data about the uranian moons, but it seems quite possible that having more dense (rocky) materials, more radiactive elements are present, thus there will be more internal heat and so more geological activity (ask Miranda; I'm skeptical about the theory of a "stitched moon" after being destroyed due to an impact).
Perhaps other possibility for the past geological activity of the uranian moons was the event that made Uranus to have its rotation axis so inclined -probably a HUGE impact-... but I can't see how that could affect the moons. It's unlikely for me that it could affect their orbits -in the sense of giving them a higher eccentricity, different distances, etc- and the rearrangement of those orbits to the uranian equator I think would be something that would be occurring relatively slowly and not from one day to other.

Anyway, it's really interesting to see how even within the same satellite system there're worlds so different as Rhea and Enceladus (and nothing to say about Io and Callisto in Jupiter). There's still much to see in the Solar System and I still think I'll see spacecrafts orbiting all planets of the Solar System (currently, the unique planets that lack of that are Mercury -until MESSENGER arrival at 2011-, Uranus, and Neptune).
Mercury_3488 (Feb 25, 2008 at 3:04 PM):
Hi Red_dragon,

Maybe the fact that Rhea is not so 'hemmed' in as the inner Saturn moons, but I still cannot help but think the Rhea's lesser density & over all mass than the Uranius similar sized moons may have something to do with this also.

Like Titania & Oberon are nearly 20% more massive than Rhea, yet despite being of very similar sizes. I wonder if Titania & Oberon may have had more in the way of radioactive elements in their youths than Rhea also?

I think this will only ever be answered when a Uranus orbiter arrives in Sector 7 & closely rendezvous with them.

Of course, it is true that rhea may have had its original features erased with subsequent cratering.
What would help, would be if Rhea's gravitaional field was measured to determine if Rhea is differentiated like Titan & Ganymede, or is homogenous like Callisto?

In an Icraus abstract I found, the gravity data did suggest that Rhea is more like Callisto, virtually no rock in the outer lars, but slowly incraesing towards the centre, but not forming a well defined core, so in effect Rhea appears undifferentiated.

Rhea is far too massive to be pure ice, but the density fits in well with roughly two thirds ice, one third heavier materials.

Titania & Oberon on the other hand appear to be almost 50 / 50 rock & ice & I think that this has a huge bearing on what happened & they unlike Rhea, do appear to be differentiated, which may explain the greater geological variety that Voyager 2 saw whilst traversing Sector 7.

Fascinating discussion this.

Andrew Brown.
Red_dragon (Feb 25, 2008 at 7:17 AM):
Yes, it's a possibility. But note also that Rhea's distance to Saturn may have something to see with the apparent lack of geological activity. Titan is at roughly 750'000 kilometers of her and Dione at 150'000 kilometes; she's not so "tightly packed" as inner moons.

Or perhaps Rhea was very active after her formation, but *heavy* bombardment destroyed the geological features she could had... who knows.
Mercury_3488 (Feb 24, 2008 at 3:31 PM):
Hi Red_dragon.

I suspect in the case of Rhea, it is lighter less space weathered ice being deposited over older more space weathered ice.

Rhea appears to have had no internal geological activity, the entire surface appears to have been effected by impact events.

I use the word appears, as in the images I have & have seen, can see no evidence for anything else other than impacts.

Many of Saturn's moons appear to have had varying degrees of geological activity to varying degrees.

Enceladus & Titan: extensive.

Dione: moderate overall, extensive in a few locations.

Iapetus: Some, the giant equatorial ridge of the Voyager Mountains.

Tethys: Little, smoother equatorial regions, Ithaca Chasma, little else.

Rhea & Mimas: None as far as I can tell, only impact cratering.

Rhea appears to be a bit of an odd ball in this respect. Rhea is a large object that appears to have had little other than impacts happen to it.

Rhea is similar in size to the Uranus moons Titania & Oberon, as well as Iapetus.

In descending diameter size order below:

Titania: 1,578 KM. Largest moon of Uranus. Density: 1.70 Grammes cm/3
Comments: Giant canyons, smoother subdued craters in places, possibly previous cryovolcanism, possibly very tenuous atmosphere.

Rhea: 1,528 KM. Second largest moon of Saturn. Density: 1.33 Grammes cm/3.
Comments: Little has happened apart from impact cratering.

Oberon: 1,522 KM. Second largest moon of Uranus. Density: 1.64 Grammes cm/3.
Comments. Like Rhea has experienced extensive cratering. Unlike Rhea, has tall mountains & has had experienced cryovolcanism & like Titania possible limited tectonic activity. Possibly a very tenuous atmosphere. Note Oberon's average diameter is only 6 KM less than Rhea's, but is somewhat more massive due to higher density.

Iapetus: 1,436 KM. Third largest moon of Saturn. Density: 1.21 Grammes cm/3.
Comments. Odd ball moon. Dark leading, bright trailling hemispheres. Huge equatorial mountain range, some peaks almost as tall as those on Mars & Io.

I suspect Rhea's lesser density (greater ice to rock ratio) may explain lack of geological activity.

Andrew Brown.
Red_dragon (Feb 21, 2008 at 3:36 AM):
it's curious how, even considerating the difference of composition of the bedrock -ice or rock-, the effect of rays and the excavation of fresh, brighter material occurs in worlds so different as Rhea, Mercury, or our Moon.

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