Ride Along with Rhea


Assembled from 29 raw images taken by the Cassini orbiter on Monday, April 25, this animation brings us along an orbital ride with Rhea as it crosses Saturn’s nighttime face, the planet’s shadow cast across the ringplane. Sister moons Dione and Tethys travel the opposite lane in the background, eventually appearing to sink into Saturn’s atmosphere.

Rhea's heavily cratered surface, imaged by Cassini on October 2010. NASA/JPL/SSI

The exposure varies slightly from frame to frame due to the fact that they are not all taken with the same color channel filter.

Rhea (1,528 kilometers, or 949 miles, wide), Dione (1,123 kilometers, or 698 miles wide) and Tethys (1,066 kilometers, or 662 miles wide) are all very similar in composition and appearance. The moons are composed mostly of water ice and rock, each covered in craters of all sizes and crisscrossed by gouges, scarps and chasms. All three are tidally locked with Saturn, showing the same face to their parent planet in the same way that the Moon does with Earth.

The Cassini spacecraft was 2,227,878 km (1,384,339 miles) from Rhea when the images were taken.

(The original images have not been validated or calibrated. Validated/calibrated images will be archived with the NASA Planetary Data System in 2012.)

Image credit: NASA / JPL / Space Science Institute. Animation by Jason Major.

How Many Rings Does Uranus Have?

Here’s a question, how many rings does Uranus have? Well, as of 2008, the total number of rings circling Uranus is 13.

The rings of Uranus were first discovered in 1977 by astronomers James Elliot, Edward Dunham and Douglas Mink. When he first discovered Uranus 200 years before, William Herschel reported seeing rings around Uranus, but his telescope probably wasn’t powerful enough to reveal them. Additional rings were discovered in 1986 when NASA’s Voyager 2 spacecraft made its flyby, and then two more outer rings were turned up by the Hubble Space Telescope in 2003-2005.

The rings of Uranus are dark and opaque, with a very low albedo. Astronomers believe that they’re made of water ice mixed with organic molecules. Unlike Saturn’s rings, the rings of Uranus are very narrow; just a few kilometers wide.

Uranus’ rings consist of 3 major groups. There are the narrow main rings, the dusty rings, and the newly discovered outer ring system.

Astronomers think that the rings of Uranus are being shepherded by small moons in the ring system. Without these shepherd moons, the rings of Uranus would spread out radially and dissipate into space. It’s also believed that there’s some process that’s replenishing the ice particles in the rings; perhaps collisions between icy objects in the rings.

I mentioned at the beginning of the article that current ring count stands at 13; however, that’s for 2008. With improved technology and telescopes, astronomers could turn up more rings in the future, so stay tuned.

We have written many articles about Uranus for Universe Today. Here’s an article about the discovery of new rings and moons around Uranus, and here’s an image of a blue ring around Uranus.

Here’s NASA’s Solar System Exploration Guide on the rings of Uranus, and here’s NASA’s fact sheet on the rings.

We have also recorded an entire episode of Astronomy Cast just about Uranus. Check it out here.

How Many Rings Does Jupiter Have?

We’re familiar with the rings of Saturn, but did you know that Jupiter has rings too? The rings of Jupiter were first discovered by the Voyager 1 spacecraft when it passed by Jupiter in 1979. The rings were investigated in more detail by NASA’s Galileo spacecraft during the 1990s. It was during the 1990s that Galileo and ground-based observations made a complete count of the number of Jupiter’s rings. So, how many rings does Jupiter have?

Jupiter is known to have 4 sets of rings: the halo ring, the main ring, the Amalthea gossamer ring, and the Thebe gossamer ring.

The halo ring is closest into Jupiter starting at a radius of 92,000 km and extending out to a radius of 122,500 km. The halo ring has a total width of 12,500 km.

Next is the main ring. It starts at 122,500 km and extends out to 129,000 km. It has a total width of only 6,500 km.

Outside these two major rings are the gossamer rings. These are very faint rings that are shepherded by two of Jupiter’s moons. The first is the Amalthea gossamer ring, which is shepherded by Jupiter’s moon Amalthea. It starts at a radius of 129,000 km from Jupiter and goes out to the orbit of Amalthea at 182,000 km.

Overlapping the Amalthea ring is the Thebe gossamer ring. It starts at a radius of 129,000 and goes out to a radius of 226,000 km.

How many rings does Jupiter have? The answer is four. Of course, it’s always possible that new rings will be discovered around Jupiter as new and better spacecraft and telescopes examine the planet.

We have written many articles about Jupiter and its rings for Universe Today. Here’s an article about how Jupiter’s rings are made in the shade. And here are some interesting discoveries made by NASA’s New Horizon’s spacecraft when it arrived at Jupiter.

Windows to the Universe has more information about Jupiter’s rings. And here’s more information on the rings from NASA.

We have recorded a whole episode of Astronomy Cast just about Jupiter, and in that episode we talked about the planet’s rings.


Vertical Structures Tower Above Saturn’s Rings

Cassini has imaged towering vertical structures in the planet’s otherwise flat rings that come from the gravitational effects of a small nearby moon. This is the first time these structures have been seen. They reach up over one kilometer high, and are visible now as the sun nears “high noon” directly overhead at the planet’s equator, as Saturn approaches its equinox.

The search for ring material extending above and below Saturn’s ring plane has been a major goal of the imaging team during Cassini’s “Equinox Mission,” the two-year period containing the exact equinox. This novel illumination geometry, which occurs every half-Saturn-year, or about 15 Earth years, lowers the sun’s angle to the ring plane and causes out-of-plane structures to cast long shadows across the rings, making them easy to detect.

Images taken in recent weeks have demonstrated how small moons in very narrow gaps can have considerable and complex effects on the edges of their gaps, and that such moons can be smaller than previously believed.

Looming vertical structures, seen here for the first time and created by Saturn's moon Daphnis, rise above the planet's otherwise flat, thin disk of rings to cast long shadows in this Cassini image.  Credit: CICLOPS
Looming vertical structures, seen here for the first time and created by Saturn's moon Daphnis, rise above the planet's otherwise flat, thin disk of rings to cast long shadows in this Cassini image. Credit: CICLOPS

The 8-kilometer-wide (5-mile) moon Daphnis orbits within the 42-kilometer-wide (26-mile) Keeler Gap in Saturn’s outer A ring, and its gravitational pull perturbs the orbits of the particles forming the gap’s edges. Earlier images have shown “waves” in the rings from Daphnis eccentric orbit.

But new images show the shadows of the vertical waves created by Daphnis cast onto the nearby ring. These characteristics match what was predicted by scientists.

Scientists have estimated, from the lengths of the shadows, wave heights that reach enormous distances above Saturn’s ring plane – as large as 1.5 kilometers (1 mile) — making these waves twice as high as previously known vertical ring structures, and as much as 150 times as high as the rings are thick. The main rings — named A, B and C — are only about 10 meters (30 feet) thick.

“We thought that this vertical structure was pretty neat when we first saw it in our simulations,” said John Weiss, lead author of a paper reporting on these images. “But it’s a million times cooler to have your theory supported by such gorgeous images. It makes you suspect you might be doing something right.”

Click here to watch a movie of the vertical structures and waves in motion.

Also presented in the paper is a refinement to a theory used since the Voyager missions of the 1980s to infer the mass of gap-embedded moons based on how much the moons affect the surrounding ring material. The authors conclude that an embedded moon in a very narrow gap can have a smaller mass than that inferred by earlier techniques. One of the prime future goals of the imaging team is to scour the remaining gaps and divisions within the rings to search for the moons expected to be there. “It is one of those questions that have been nagging us since getting into orbit: ‘Why haven’t we yet seen a moon in every gap?’” said Carolyn Porco, lead for the Cassini imaging team. “We now think they may actually be there, only a lot smaller than we expected.”