Animation made from raw Cassini image data acquired April 25, 2011
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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.
Using images obtained by NASA's Cassini mission, astronomers followed several of what are likely to be dozens of small moons orbiting within the outer edge of Saturn's A ring -- the outermost of the planet's large, dense rings -- from 2005 to 2009.
Scientists for NASA’s Cassini mission noticed some weird-looking propeller-like shapes in the outer edge of Saturn’s A ring. What could be creating these unusual contours? A closer look revealed they were being formed by dozens of moving moonlets. Normally, these kilometer-sized moons would have been almost impossible to see, since they are embedded within the rings. “However, their presence is betrayed by the large tell-tale ‘propeller’ structures they generate in the ring material on either side of them,” said Carolyn Porco, leader of the Cassini imaging team, and co-author on a new paper on these propeller moons. In an email, Porco said similar features had been seen earlier in other locations in Saturn’s rings, but were “much smaller, harder to see, and so numerous that there was no hope of following any one of them. The new propellers, and the moonlets that create them, are some ten times larger and much easier to identify and follow from image to image and year to year.”
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The team said the ability to watch as the embedded moons’ orbits evolve over time could give scientists valuable new clues about how planets form and grow around stars in young solar systems.
“What is outstanding about these new findings is the insight they ultimately will provide into the early stages of solar system formation,” said Porco, “when growing planets become large enough to open gaps in the ring material around them and ultimately truncate their own growth.”
The scientists have tracked eleven of these moons since 2006. Most are between one and several kilometers in diameter, too small to be imaged directly by Cassini’s cameras, but are only distinguishable by the unique double-armed propeller features.
The area in the middle of Saturn’s outermost dense A ring is now known as the “propeller belts,” and the new moonlets have been given appropriate names.
“You may find it amusing that these large propellers have unofficially been named after famous aviators,” Porco said. “Those flight enthusiasts among you will recognize Bleriot, Earhart, Santos-Dumont, and others.”
Cassini caught sight of Blériot (named after a French aviator) more than 100 times, allowing the researchers to map its path in detail. The propeller shape it created is several thousand kilometers long, or half the distance across the continental United States.
“You would expect any object that’s just orbiting Saturn on its own should stay in a constant path,” said lead author Matthew Tiscareno from Cornell University. “What we actually see is that the orbits are changing.”
The most likely explanation, he said, is that the moons are actually interacting with the disk: exchanging angular momentum with the ring particles around them either through gravity or by direct collisions.
Still, other explanations, like resonant interactions with more distant moons, have not been ruled out as causes.
Scientists will be keeping an eye on these wandering little moons in order to figure out if the disk itself is driving the changes, similar to the interactions that occur in young solar systems. If it is, Tiscareno said, this would be the first time such a measurement has been made directly.
The largest moon of Saturn is Titan, measuring 5,150 km across. In fact, Titan is the second largest moon in the Solar System, after Jupiter’s Ganymede. Titan is so big that it’s even larger than planet Mercury, which is only 4,879 km across. And it’s much bigger than the Earth’s moon at 3,474 km.
Astronomers used to think that Titan was actually the largest moon in the Solar System, but when NASA’s Voyager spacecraft first arrived at the moon in the 1980s, they were able to make detailed observations of the moon at its atmosphere. They proved that Titan’s atmosphere extended out for dozens of kilometers, and so the physical moon itself was actually smaller than previously thought, making it smaller than Ganymede.
Titan orbits Saturn at an average distance of 1,221,870 km, completing an orbit every 15.945 days. It’s tidally locked to Saturn, so it always presents the same face to Saturn. So a day on Saturn is also the same amount of time it takes to orbit Saturn.
Titan is the only moon in the Solar System known to have a thick atmosphere. In fact, the pressure of the atmosphere on the surface of Saturn is 1.5 times greater than the atmospheric pressure here on Earth. Of course, the atmosphere of Titan is almost entirely nitrogen, and the temperature is -179° C. So it wouldn’t be a comfortable place to visit without a spacesuit.
