What’s up with Iapetus?

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Although Saturn’s moon Iapetus was first discovered in 1671 by Giovanni Cassini, its behavior was extremely odd. Cassini was able to regularly find the moon when it was to the west of Saturn, but when he waited for it to swing around to Saturn’s east side, it seemed to vanish. It wasn’t until 1705 that Cassini finally observed Iapetus on the eastern side, but it took a better telescope because the side Iapetus presented when to the east was a full two magnitudes darker. Cassini surmised that this was due to a light hemisphere, presented when Iapetus was to the west, and a dark one, visible when it was to the east due to tidal locking.

With the advances in telescopes, the reason for this dark divide has been the subject of much research. The first explanations came in the 1970’s and a recent paper summarizes the work done so far on this fascinating satellite as well as expanding it to the larger context of some of Saturn’s other moons.

The foundation for the current model of Iapetus’ uneven display was first proposed by Steven Soter, one of the co-writers for Carl Sagans Cosmos series. During a colloquium of the International Astronomical Union, Soter proposed that micrometeorite bombardment of another of Saturn’s moons, Pheobe, drifted inwards and were picked up by Iapetus. Since Iapetus keeps one side facing Saturn at all times, this would similarly give it a leading edge that would preferentially pick up the dust particles. One of the great successes of this theory is that the center of the dark region, known as the Cassini Regio, is directly situated along the path of motion. Additionally, in 2009, astronomers discovered a new ring around Saturn, following Phoebe’s retrograde orbit, although slightly interior to the moon, adding to the suspicion that the dust particles should drift inwards, due to the Poynting-Robertson effect.

In 2010, a team of astronomers reviewing the images from the Cassini mission, noted that the coloration had properties that didn’t quite fit with Soter’s theory. If deposition from dust was the end of the story, it was expected that the transition between the dark region and the light would be very gradual as the angle at which they would strike the surface would become elongated, spreading out the incoming dust. However, the Cassini mission revealed the transitions were unexpectedly abrupt. Additionally, Iapetus’ poles were bright as well and if dust accumulation was as simple as Soter had suggested, they should be somewhat coated as well. Furthermore, spectral imaging of the Cassini Regio revealed that its spectrum was notably different than that of Phoebe. Another potential problem was that the dark surface extended past the leading side by more than ten degrees.

Revised explanations were readily forthcoming. The Cassini team suggested that the abrupt transition was due to a runaway heating effect. As the dark dust accumulated, it would absorb more light, converting it to heat and helped to sublimate more of the bright ice. In turn, this would reduce the overall brightness, again increasing the heating, and so on. Since this effect amplified the coloration, it could explain the more abrupt transition in much the same way as adjusting the contrast on an image will sharpen gradual transitions between colors. This explanation also predicted that the sublimated ice could travel around the far side of the moon, freezing out and enhancing the brightness on the other sides as well as the poles.

To explain the spectral differences, astronomers proposed that Phoebe may not be the only contributor. Within Saturn’s satellite system, there are over three dozen irregular satellites with dark surfaces which could also potentially contribute, altering the chemical makeup. But while this sounded like a tantalizingly straight-forward solution, confirmation would require further investigation. The new study, led by Daniel Tamayo at Cornell University, analyzed the efficiency with which various other moons could produce dust as well as the likelihood with which Iapetus could scoop it up. Interestingly, their results showed that Ymir, a mere 18 km in diameter, “should be roughly as important a contributor of dust to Iapetus as Phoebe”. Although none of the other moons, independently looked to be as strong of sources for dust, the sum of dust coming the remaining irregular, dark moons was found to be at least as important as either Ymir or Phoebe. As such, this explanation for the spectral deviation is well grounded.

The last difficulty, that of spreading dust past the leading face of the moon, is also explained in the new paper. The team proposes that eccentricities in the orbit of the dust allow it to strike the moon at odd angles, off from the leading hemisphere. Such eccentricities could be readily produced by solar radiation, even if the orbit of the originating body was not eccentric. The team carefully analyzed such effects and produced models capable of matching the dust distribution past the leading edge.

The combination of these revisions seem to secure Soter’s basic premise. A further test would be to see if other large satellites like Iapetus also showed signs of dust deposition, even if not so starkly divided since most other moons lack the synchronous orbit. Indeed, the moon Hyperion was found to have darker regions pooling in its craters when Cassini few by in 2007. These dark regions also revealed similar spectra to that of Cassini Regio. Saturn’s largest moon, Titan is also tidally locked and would be expected to sweep up particles on its leading edge, but due to its thick atmosphere, the dust would likely be spread moon-wide. Although difficult to confirm, some studies have suggested that such dust may help contribute to the haze Titan’s atmosphere exhibits.

Stunning Flyover Videos of Saturn’s Moons

Saturn’s moons as you’ve never seen them before! By day, Dr. Paul Schenk works at the Lunar and Planetary Institute mapping the topography and geology of the moons of Saturn and Jupiter, as well as the icy bodies of the outer solar system. But because “it’s just plain cool,” he has created some flyover videos of Saturn’s satellites, using data from the Cassini spacecraft. Very cool, indeed! Above is a close-up, 3-D look at the walnut-shaped moon Iapetus. Scientists don’t know why there is a ridge along the moon’s equator, but in 2007, Cassini acquired a strip of color and stereo images along the ridge, and Schenk has created a flyover which shows the contrast in color and topography. There are “sharp peaks 15 to 20 kilometers above the surrounding dark cratered plains,” Schenk writes. “These are among the highest peaks in the Solar System. Patches of bright pure water ice can be seen flanking these dark peaks, which have the brightness of soot.”

And there’s more! Below is one of my favorites from Schenk’s collection of flyover videos, 3-D views of Inktomi, a very young crater on the moon Rhea.

Continue reading “Stunning Flyover Videos of Saturn’s Moons”

Pacman Eats The Death Star!

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Mimas has drawn a fair amount of attention with its “Death Star”-like appearance, but with new images from the Cassini spacecraft, this icy moon of Saturn has just gotten a lot more interesting. The highest-resolution-yet temperature map and images of Mimas reveal surprising patterns on the surface of the small moon, including unexpected hot regions that resemble “Pac-Man” eating the Death Star crater (officially known as Herschel Crater), as well as striking bands of light and dark in crater walls. “After much deliberation, we have concluded: Mimas is NOT boring,” said Carolyn Porco, Cassini imaging team leader, in an e-mail about the new images. “Who knew?!” And best of all, Porco added, “be sure you have a pair of red/green glasses handy ’cause you won’t want to miss peering into gigantic Herschel crater in 3D!”

Cassini collected the data on Feb. 13, and Porco said the team has spent some quality time poring over the images. She said the details in the moon’s craters that reminded the imaging team of features seen on Phoebe and Hyperion, plus the thermal signature is very peculiar and the team can’t yet explain it.

Herschel Crater in 3-D. Credit: NASA/JPL/Space Science Institute. Click for larger version.

“Other moons usually grab the spotlight, but it turns out Mimas is more bizarre than we thought it was,” said Linda Spilker, Cassini project scientist at JPL. “It has certainly given us some new puzzles.”

Scientists were expecting smoothly varying temperatures peaking in the early afternoon near the equator. Instead, the warmest region was in the morning, along one edge of the moon’s disk, making a sharply defined Pac-Man shape, with temperatures around 92 Kelvin (minus 294 degrees Fahrenheit). The rest of the moon was much colder, around 77 Kelvin (minus 320 degrees Fahrenheit). A smaller warm spot – the dot in Pac-Man’s mouth – showed up around Herschel, with a temperature around 84 Kelvin (minus 310 degrees Fahrenheit).

The warm spot around Herschel makes sense because tall crater walls (about 5 kilometers, or 3 miles, high) can trap heat inside the crater. But scientists were completely baffled by the sharp, V-shaped pattern.

“We suspect the temperatures are revealing differences in texture on the surface,” said John Spencer, who works with Cassini’s composite infrared spectrometer. “It’s maybe something like the difference between old, dense snow and freshly fallen powder.”

Denser ice quickly conducts the heat of the sun away from the surface, keeping it cold during the day. Powdery ice is more insulating and traps the sun’s heat at the surface, so the surface warms up.

Even if surface texture variations are to blame, scientists are still trying to figure out why there are such sharp boundaries between the regions, Spencer said. It is possible that the impact that created Herschel Crater melted surface ice and spread water across the moon. That liquid may have flash-frozen into a hard surface. But it is hard to understand why this dense top layer would remain intact when meteorites and other space debris should have pulverized it by now, Spencer said.

Dark regions below bright crater walls and streaks on some of the walls are seen in this mosaic of Saturn's moon Mimas. Credit: NASA/JPL/Space Science Institute. Click for larger version.

Icy spray from the E ring, one of Saturn’s outer rings, should also keep Mimas relatively light-colored, but the new visible-light images from the flyby paint a picture of surprising contrasts. Cassini imaging team scientists didn’t expect to see dark streaks trailing down the bright crater walls or a continuous, narrow pile of concentrated dark debris tracing the foot of each wall.

The pattern may appear because of the way the surface of Mimas ages, said Paul Helfenstein, a Cassini imaging team associate based at Cornell University, Ithaca, N.Y. Over time, the moon’s surface appears to accumulate a thin veil of silicate minerals or carbon-rich particles, possibly because of meteor dust falling onto the moon, or impurities already embedded in surface ice.

As the sun’s warming rays and the vacuum of space evaporate the brighter ice, the darker material is concentrated and left behind. Gravity pulls the dark material down the crater walls, exposing fresh ice underneath. Although similar effects are seen on other moons of Saturn, the visibility of these contrasts on a moon continually re-paved with small particles from the E ring helps scientists estimate rates of change on other satellites.

“These processes are not unique to Mimas, but the new high-definition images are like Rosetta stones for interpreting them,” Helfenstein said.

See more images of Mimas that were released today at the CICLOPS website.

Sources: JPL CICLOPS, email from Carolyn Porco

Cassini the Artist: Shadows, Ringshine, Double Crescent Moons

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I often ponder whether the Cassini spacecraft is a better scientist or artist. I found three recent images from Cassini that definitely give the nod to artist, but surely there’s lots of great science here as well. In this image, Saturn casts its shadow on the rings, but it also shows how the rings reflect sunlight onto the dark side of the planet. Here Saturn appears dimly illuminated by this ringshine. This view looks toward the southern, unilluminated side of the rings from about 10 degrees below the ringplane, and was taken on Jan. 2, 2010 when Cassini was about 2.3 million kilometers (1.4 million miles) from Saturn. Below: beautiful moons.

Two moons, with Saturn's rings. Image Credit: NASA/JPL/Space Science Institute

While this image is stunningly gorgeous, perhaps the most amazing thing is that it was snapped by Cassini’s cameras just yesterday (March 15, 2010) and beamed back to Earth today! This is a raw, uncalibrated image and the only details posted about it is that the camera was pointing toward Tethys at approximately 2,410,546 kilometers away. Can anyone guess what the second moon is?

Double crescent moons. Credit: NASA/JPL/Space Science Institute.

Another beauty, Dione and Titan make a smiling pair of crescent moons. This image was taken on March 12, 2010 and received on Earth March 13, 2010. The camera was pointing toward Dione at approximately 2,211,699 kilometers away.

For more great images see the Cassini website, or the CICLOPS website

Largest Moon of Saturn

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.

We’ve written many articles about Titan for Universe Today. Here’s an article about seasonal changes on Titan, and here’s an article about how Titan’s haze acts like an ozone layer.

If you’d like more info on Titan, check out Hubblesite’s News Releases about Saturn. And here’s a link to the homepage of NASA’s Cassini spacecraft, which is orbiting Saturn.

We’ve also recorded an episode of Astronomy Cast just about Saturn’s moons. Listen here, Episode 61: Saturn’s Moons.

May Visions of Saturn’s Moons Dance in Your Head

Moons dancing around Saturn. Credit: NASA/JPL/Space Science Institute

The Cassini CICLOPS imaging team has released some new movies of several moons orbiting Saturn as if in a cosmic ballet around the ringed planet. In one scene that blends 12 images taken over the span of 19 minutes, Rhea skates in front of Janus, as Mimas and Pandora slide across the screen in the opposite direction.

“As yet another year in Saturn orbit draws to a close, these wondrous movies of an alien place clear across the solar system remind us how fortunate we are to be engaged in this magnificent exploratory expedition,” said Carolyn Porco, Cassini imaging team leader.


Click here to see the movies.

While the dance appears leisurely on screen, Rhea actually orbits Saturn at a speed of about 8 kilometers per second (18,000 mph). The other moons are hurtling around the planet even faster. Mimas averages about 14 kilometers per second (31,000 mph), and Janus and Pandora travel at about 16 kilometers per second (36,000 mph).

Sources: JPL, CICLOPS