Posted on by Nancy Atkinson

Mystery of Saturn’s Wonky B Ring: Solved

Vertical structures, among the tallest seen in Saturn's main rings, rise abruptly from the edge of Saturn's B ring to cast long shadows on the ring in this image taken by NASA's Cassini spacecraft in 2009.

It has long been known that Saturn’s rings are not the perfect hoops they appear as in small amateur telescopes, and when the Cassini spacecraft entered orbit around Saturn, the wonky disorder of the massive B ring became even more apparent. Scientists were stunned by towering vertical structures, scalloped edges on the rings, and odd propeller-like features. But scientists have now found the cause of these strange features: The region is acting just like a spiral galaxy, said Carolyn Porco, team lead of the Cassini imaging team.

“We have found what we hoped we’d find when we set out on this journey with Cassini nearly 13 years ago,” said Porco, “(and have gotten) visibility into the mechanisms that have sculpted not only Saturn’s rings, but celestial disks of a far grander scale, from solar systems, like our own, all the way to the giant spiral galaxies.”

The B ring is one of the most dynamic areas in Saturn’s rings, and surprisingly, scientists say, the rings are behaving like a miniature version of our own Milky Way galaxy.

When the the Voyager spacecraft flew by Saturn in 1980 and 1981, scientists saw that the outer edge of the planet’s B ring was shaped like a rotating, flattened football by the gravitational perturbations of Mimas. But it was clear, even in Voyager’s findings, that the outer B ring’s behavior was far more complex than anything Mimas alone might do.

Vertical Structures Tower over Saturn's B-Ring. This image was rendered using Autodesk Maya and Adobe Photoshop. Credit: Kevin Gill.
Vertical Structures Tower over Saturn’s B-Ring. This image was rendered using Autodesk Maya and Adobe Photoshop. Credit: Kevin Gill.

Through the analysis of thousands of Cassini images of the B ring taken over a four-year period, Porco and her team have found the source of most of the complexity: at least three additional, independently rotating wave patterns, or oscillations, that distort the B ring’s edge.

The oscillations travel around the ring with differing speeds and the small, random motions of the ring particles feed energy into a wave that propagates outward across the ring from an inner boundary, reflects off the outer edge of the B ring (which becomes distorted as a result), and then travels inward until it reflects off the inner boundary. This continuous back-and-forth reflection is necessary for these wave patterns to grow and become visible as distortions in the outer edge of the B ring.

Watch a video of the oscillations.

And see more “movies” at the CICLOPS website.

These oscillations, with one, two or three lobes, are not created by any moons. They have instead spontaneously arisen, in part because the ring is dense enough, and the B ring edge is sharp enough, for waves to grow on their own and then reflect at the edge.

The ring particles’ small, random motions feed energy into a wave and cause it to grow. The new results confirm a Voyager-era predication that this same process can explain all the puzzling chaotic waveforms found in Saturn’s densest rings, from tens of meters up to hundreds of kilometers wide.

“This process has already been verified to produce wave features in Saturn’s dense rings that are of small scale…about 150 meters or so,” Porco wrote in her “Captain’s Log” feature on the CICLOPS (Cassini imaging)website. “That it now also appears to produce waves of large, hundreds-of-kilometers scale in the outer B ring suggests that it can operate in dense rings on all spatial scales.”

“These oscillations exist for the same reason that guitar strings have natural modes of oscillation, which can be excited when plucked or otherwise disturbed,” said Joseph Spitale, Cassini imaging team associate and lead author of a new article in the Astronomical Journal, published today. “The ring, too, has its own natural oscillation frequencies, and that’s what we’re observing.”

Astronomers believe such “self-excited” oscillations exist in other disk systems, like spiral disk galaxies and proto-planetary disks found around nearby stars, but they have not been able to directly confirm their existence. The new observations confirm the first large-scale wave oscillations of this type in a broad disk of material anywhere in nature.

Self-excited waves on small, 100-meter (300-foot) scales have been previously observed by Cassini instruments in a few dense ring regions and have been attributed to a process called “viscous overstability.”

Oscillations in Saturn's B ring. Credit: Space Science Institute

“Normally viscosity, or resistance to flow, damps waves — the way sound waves traveling through the air would die out,” said Peter Goldreich, a planetary ring theorist at the California Institute of Technology. “But the new findings show that, in the densest parts of Saturn’s rings, viscosity actually amplifies waves, explaining mysterious grooves first seen in images taken by the Voyager spacecraft.”

“How satisfying it is to find at last one explanation for most, if not all, of the chaotic looking structure we first saw in Saturn’s dense ring regions long ago with Voyager,” said Porco, “and have since seen in exquisite detail with Cassini.”

Source: JPL, CICLOPS

Posted on by Nancy Atkinson

Saturn’s Rings Formed from Large Moon’s Destruction

Raw image of Saturn's rings. Credit: NASA/JPL/Space Science Institute

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The formation of Saturn’s rings has been one of the classical if not eternal questions in astronomy. But one researcher has provided a provocative new theory to answer that question. Robin Canup from the Southwest Research Institute has uncovered evidence that the rings came from a large, Titan-sized moon that was destroyed as it spiraled into a young Saturn.

Over the years, different theories have evolved on how the rings formed around Saturn. The two leading theories involve a small moon that was shattered by meteor impacts, or the tidal disruption of a comet coming too close to Saturn.

But Saturn’s main rings are about 90% water ice by mass, and because bombardment of the rings by micrometeoroids increases their rock content over time, Canup said the rings’ current composition implies that they were essentially pure ice when they formed.

However, disruption of a small moon would generally lead to a mixed rock-ice ring, while tidal disruptions of comets would occur much more often at Jupiter, Uranus and Neptune than at Saturn.

New insights into the nature of Saturn’s rings are revealed in this panoramic mosaic of 15 images taken during the planet’s August 2009 equinox. Image credit: NASA/JPL/SSI

Additionally, neither of these theories would explain Saturn’s inner moons, which have low enough densities that they too must be comprised of nearly pure ice.

Canup’s new alternative theory is that Titan-sized moon with a rocky core and an icy mantle spiraled into Saturn early in solar system history. Tidal forces ripped off part of the icy mantle, distributing it into what would become the rings. But the rocky core was made of more durable material that held together until it hit Saturn’s surface. “The end result is a pure ice ring,” Canup said in an article in Nature.

Over time the ring spreads out and its mass decreases, and icy moons are created. Due to changes in the evolving Saturn system, these “spawned” moons then spiraled outward rather than inward. In this way, ice rings and ice-enhanced inner moons originate as a primordial byproduct of the same process that produces Saturn’s regular satellite system, making the whole process simpler than if there were several events.

Canup studies formation events with detailed computer simulations, including studying how our own Moon formed.

She presented her findings at the American Astronomical Society’s Division for Planetary Science meeting this week, in Pasadena, California, and her presentation was detailed in an article in Nature.

Sources: Canup’s abstract, Nature

Posted on by Nancy Atkinson

Simply Astonishing: Enceladus, the Jet-Powered Moon

The plumes of Enceladus are highlighted in this Cassini image. Credit: NASA/JPL/Space Science Institute

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What an astonishing view of Saturn’s moon Enceladus, as seen by Cassini! At least four different plumes of water ice are spewing out from the south polar region, highlighted because of the black space behind the Moon. On Twitter, Carolyn Porco said that we see four jets because we’re looking down the four tiger stripe fractures crossing the south pole. “How lovely it is to know!” she added.

Cassini was about 617,000 kilometers (383,000 miles) away from Enceladus when it captured this image.

More info: Cassini website

Posted on by Nancy Atkinson

Cassini Flies Through Saturn’s Aurora

Cassini crossed the radio aurora of Saturn on 17 October 2008, at a distance of 4 Saturn's radii above the atmosphere. These radio emissions, generated by fast electrons, are strongly beamed. They were characterised by simultaneous observations of three different experiments. Credit: NASA/JPL/University of Iowa/CNES/Observatoire de Paris

Saturn’s mysterious aurora has fascinated astronomers and space enthusiasts since it was first observed back in 1979. Now, the Cassini spacecraft has made the first observations from within the giant radio aurora of Saturn. The spacecraft flew through an active auroral region in 2008, and scientists say there are both similarities and contrasts between the radio auroral emissions generated at Saturn and those at Earth. Additionally, Cassini’s visual and infrared mapping spectrometer instrument (VIMS) took data to create a new movie (above) showing Saturn’s shimmering aurora over a two-day period. All this new data are helping scientists understand what drives some of the solar system’s most impressive light shows.

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“So far, this is a unique event,” said Dr. Laurent Lamy at the European Planetary Science Congress in Rome this week. “Whereas the source region of Earth’s radio aurora has been studied by many missions, this is our first opportunity to observe the equivalent region at Saturn from the inside. From this single encounter, we have been able to build up a detailed snapshot of auroral activity using three of Cassini’s instruments. This gives us a fascinating insight into the processes that are generating Saturn’s radio aurora.”

See an animation created from the radio instrument on Cassini at this link. On the left hand side are the radio sources as seen from Cassini. The right hand side shows the projection of the radio sources down onto the southern pole of the planet. Credit: NASA/JPL/University of Iowa/CNES/Observatoire de Paris

Separately, Tom Stallard, lead scientist on a joint VIMS and Cassini magnetometer collaboration, presented the VIMS movie at the conference.

Source: European Planetary Science Congress

In the movie, the aurora phenomenon clearly varies significantly over the course of a Saturnian day, which lasts around 10 hours 47 minutes. On the noon and midnight sides (left and right sides of the images, respectively), the aurora can be seen to brighten significantly for periods of several hours, suggesting the brightening is connected with the angle of the sun. Other features can be seen to rotate with the planet, reappearing at the same time and the same place on the second day, suggesting that these are directly controlled by the orientation of Saturn’s magnetic field.

Image of Saturn’s aurora seen at ultraviolet wavelengths. The spiral shape seen here is similar to the distorted radio aurora visualised by the team and also indicates enhanced auroral activity. Credit: ESA/NASA/Hubble

“Saturn’s auroras are very complex and we are only just beginning to understand all the factors involved,” Stallard said. “This study will provide a broader view of the wide variety of different auroral features that can be seen, and will allow us to better understand what controls these changes in appearance.”
Auroras on Saturn occur in a process similar to Earth’s northern and southern lights. Particles from the solar wind are channeled by Saturn’s magnetic field toward the planet’s poles, where they interact with electrically charged gas (plasma) in the upper atmosphere and emit light. At Saturn, however, auroral features can also be caused by electromagnetic waves generated when the planet’s moons move through the plasma that fills Saturn’s magnetosphere.

This false-color composite image shows Saturn’s rings and southern hemisphere. The composite image was made from 65 individual observations by Cassini’s visual and infrared mapping spectrometer in the near-infrared portion of the light spectrum on Nov. 1, 2008. Credit: NASA/JPL/University of Arizona
Posted on by Nancy Atkinson

Titan Weather Report for Spring: Still Cold, but Clearing Skies

Left: T43 flyby of Titan - 12 May 2008 – VIMS images a large cloud that caps the north pole of Titan (yellowish tones). Right: T63 flyby of Titan - 12 December 2009 – VIMS still observes a huge cloud system at 40°S (yellowish tones) and the north pole of Titan free of clouds, a few months after the equinox. Credit: NASA/JPL/University of Arizona/University of Nantes/ University of Paris Diderot

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The beauty of an extended space mission is that scientists can make long term observations and find out things we’ve never known before. The Cassini spacecraft’s Visual and Infrared Mapping Spectrometer (VIMS) instrument has been monitoring clouds on Titan continuously since the spacecraft went into orbit around Saturn in 2004, and a team led by Sébastien Rodriguez (AIM laboratory – Université Paris Diderot) has used more than 2,000 VIMS images to create the first long-term study of Titan’s weather. Are they ready to make a weather forecast? They say Titan’s northern hemisphere is set for mainly fine spring weather, with polar skies clearing since the equinox in August last year.

Together with Saturn in its 30-years orbit around the Sun, Titan has seasons that last for 7 terrestrial years. The team has observed significant atmospheric changes between July 2004 (early summer in the southern hemisphere) and April 2010, the very start of northern spring. The images showed that cloud activity has recently decreased near both of Titan’s poles. These regions had been heavily overcast during the late southern summer until 2008, a few months before the equinox.

“Over the past six years, we’ve found that clouds appear clustered in three distinct latitude regions of Titan: large clouds at the north pole, patchy cloud at the south pole and a narrow belt around 40 degrees south. However, we are now seeing evidence of a seasonal circulation turnover on Titan – the clouds at the south pole completely disappeared just before the equinox and the clouds in the north are thinning out. This agrees with predictions from models and we are expecting to see cloud activity reverse from one hemisphere to another in the coming decade as southern winter approaches,” said Dr Rodriguez.

Fractional cloud coverage in Titan’s atmosphere integrated between July 2004 and April 2010. Black areas are cloud free and yellow are fully covered. Credit: NASA/JPL/University of Arizona/University of Nantes/ University of Paris Diderot

The team has used results from the Global Climate Models (GCMs) developed by Pascal Rannou (Institut Pierre Simon Laplace) to interpret the evolution of the observed cloud patterns over time. Northern polar clouds of ethane form in the Titan’s troposphere during the winter at altitudes of 30-50 km by a constant influx of ethane and aerosols from the stratosphere. In the other hemisphere, mid- and high-latitudes clouds are produced by the upwelling from the surface of air enriched in methane. Observations of the location and activity of Titan’s clouds over long periods are vital in developing a global understanding of Titan’s climate and meteorological cycle.

In Feburary 2010, the Cassini mission was extended to a few months past Saturn’s northern summer solstice in May 2017. This means that Rodriguez and his team will be able to observe the seasonal changes right the way through from mid-winter to mid-summer in the northern hemisphere.

“We have learned a lot about Titan’s climate since Cassini arrived in at Saturn but there is still a great deal to learn. With the new mission extension, we will have the opportunity to answer some of the key questions about the meteorology of this fascinating moon,” said Rodriguez.

Rodriguez presented the results at the European Planetary Science Congress 2010 in Rome.

Source: European Planetary Science Conference

Posted on by Nancy Atkinson

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”

Posted on by Nancy Atkinson

Amazing New Close-up Images of Enceladus

Caption: Looking down at a plume on Enceladus. Credit: NASA/Space Science Science Institute.

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Oh, wow! This is one of the best images yet from the Cassini spacecraft of the “tiger stripes” in the south polar region of Saturn’s moon Enceladus. Over the weekend, Cassini flew by Enceladus, and has sent back some incredible new images, such as the one above. The tiger stripes are actually giant fissures that spew jets of water vapor and organic particles hundreds of kilometers, or miles, out into space, and here, Cassini is staring right down into one of the fissures. See more great images of Enceladus below, plus images of the moons Dione and Tethys.


Close-up of the cracked, crevassed surface of Enceladus. Credit: NASA/Space Science Institute.

While the winter is darkening the moon’s southern hemisphere, Cassini has its own version of “night vision goggles” — the composite infrared spectrometer instrument – to track heat even when visible light is low. It will take time for scientists to assemble the data into temperature maps of the fissures.

Enceladus against Saturn's limb. Credit: NASA/Space Science Institute.
More plumes on Enceladus. Credit: NASA/Space Science Institute.
Close-up of Tethys. Credit: NASA/Space Science Institute

Dione from 115,370 kilometers away. Credit: NASA/Space Science Institute

See more amazing images from Cassini’s latest at the CICLOPS website.

Emily Lakdawalla at the Planetary Blog also has created some very cool movies from the flyby images.


Hat tip to Stu Atkinson

Posted on by Nancy Atkinson

Ring Around Rhea? Probably Not

Rhea, taken by the Cassini spacecraft in March, 2010. Credit: NASA/JPL/Space Science Institute

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Back in 2005, a suite of six instruments on the Cassini spacecraft detected what was thought to be an extensive debris disk around Saturn’s moon Rhea, and while there was no visible evidence, researchers thought that perhaps there was a diffuse ring around the moon. This would have been the first ring ever found around a moon. New observations, however, have nixed the idea of a ring, but there’s still something around Rhea that is causing a strange, symmetrical structure in the charged-particle environment around Saturn’s second-largest moon.

Researchers announced their findings in 2008 that there was a sharp, symmetrical drop in electrons detected around Rhea. This moon is about 1,500 kilometers (950 miles) in diameter, and scientists began searching for what could have caused the drop. If there were a debris disk around Rhea, it would have had to measure several thousand miles from end to end, and would probably be made of particles that would range from the size of small pebbles to boulders.

Testing the hypothesis, Cassini flew by the moon several times and took 65 images between 2008 and 2009, flying at what would be edge-on to the rings, where the greatest amount of material would be within its line of sight.

Using light angles to their advantage — and if the ring was there – the scientists should have been able to detect micron-sized particles up to boulder size objects.

But they saw nothing.

“There are very strong and interesting and unexplained electromagnetic effects going on around Rhea,” said Matthew Tiscareno from Cornell University, who led the imaging campaign. “But we’re making a pretty strong case that it’s not because of solid material orbiting the moon….For the amount of dust that you need to account for [the earlier] observations, if it were there, we would have seen it.”

While the ring hypothesis has been disproved, there’s still a mystery about the cause of the symmetrical structure in the charged-particles around the moon.

But the Cassini spacecraft and team are up for the challenge.

Source: Cornell University

Posted on by Nancy Atkinson

Prometheus: the Michelangelo of Saturn

Saturn's moon Prometheus creates streamer channels in the planet's rings. Credit: NASA/JPL/Space Science Institute

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I’ve frequently said the Cassini spacecraft is an artist, so when Carolyn Porco, the mission’s imaging team lead, mentioned on Twitter that Saturn’s moon Prometheus is akin to Michelangelo, I had to take a look. Wow, this gorgeous image is suitable for framing! Visible in the perturbed, thin F ring, is the potato-shaped Prometheus, and having performed the perturbing, it continues in its orbit. Click the image for the super-huge version.

Prometheus (148 kilometers, 92 miles across) periodically creates streamer-channels in the F ring, and the moon’s handiwork can be seen as the dark channels. Here’s a movie made from Cassini images showing this process:

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on June 1, 2010. A star is also visible through the rings near the center right of the image.

There are also some additional features in the F ring, courtesy of Prometheus.

This Cassini image shows icy particles in Saturn’s F ring clumping into giant snowballs as the moon Prometheus makes multiple swings by the ring. Scientists say that the gravitational pull of the moon sloshes ring material around, creating wake channels that trigger the formation of objects as large as 20 kilometers (12 miles) in diameter.

“Scientists have never seen objects actually form before,” said Carl Murray, a Cassini imaging team member based at Queen Mary, University of London. “We now have direct evidence of that process and the rowdy dance between the moons and bits of space debris.”

Read more about these fans and snowballs in this JPL article.

Source: CICLOPS

Posted on by Nancy Atkinson

Mini Moons Are Buzzing Through Saturn’s Rings

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.

Read the team’s paper.

More images at CICLOPS

Sources: NASA, Cornell, Porco email