Nature: Mystery of Saturn's midget moons cracked

[bystander]

Mystery of Saturn's midget moons cracked
Nature News - 09 June 2010

Their recent birth in rings may explain why moons were not pulverized by comets.

For decades, researchers have puzzled over the origin of Saturn's baby moons. According to conventional models, these moons are so small that collisions with comets should have blown them to pieces long ago. Now a group of researchers in France and Britain think they have the answer — and it lies in the planet's icy rings.

Accepted theory says that the giant planets, and their moons, slowly accreted out of a gaseous 'protoplanetary disk' around the Sun some 4.5 billion years ago. Yet Saturn's baby moons never quite fitted this picture. At less than 50 kilometres across, they ought to have been destroyed by comets over that period. And over time, moons tend to recede from the planets they orbit, as indeed our Moon is receding from Earth. But Saturn's moons are in such a close orbit that they would have had to have formed virtually inside the giant planet.

About six years ago, the Cassini spacecraft relayed images that hinted at an alternative origin. Sailing past Saturn's outer rings, it found lumps of ice up to 100 metres across, ten times bigger than the rings' other icy particles. For some researchers, the discovery called to mind another intriguing fact: that the moons and the rings share a composition of the purest ice in the Solar System.

Ring-derived moons
Nature 465 (10 June 2010), Editor's Summary

A population of Saturn's smaller moons contrasts dramatically with the regular moons of the giant planets. Moons like Saturn's Enceladus and Jupiter's Europa orbit in the equatorial plane of their host planet and are thought to have finished their accretion at about the same time as the planets, 4.5 billion years ago. Saturn's icy moonlets are much younger, formed less than 10 million years ago, and as their spectra resemble those of the main rings it has been suggested that they formed by accretion at the rings' edges. A new hybrid numerical simulation of the Saturn system supports this idea, suggesting that the moons formed via viscous spreading of Saturn's main rings beyond the Roche limit (the distance beyond which rings are gravitationally unstable, about 140,000 km from Saturn). After the moons' formation, the edge of the ring migrated inward.

Planetary science: The birth of Saturn's baby moons

• Nature 465, 701-702 (10 June 2010) | doi: 10.1038/465701b

Simulations show that Saturn's nearby moons, after forming on the outskirts of the planet's main rings, get pushed clear of them. This model reproduces the moons' orbital locations and remarkably low densities.

The recent formation of Saturn's moonlets from viscous spreading of the main rings

• Nature 465, 752-754 (10 June 2010) | doi: 10.1038/nature09096

The regular satellites of the giant planets are believed to have finished their accretion concurrent with the planets, about 4.5 Gyr ago. A population of Saturn’s small moons orbiting just outside the main rings are dynamically young (less than 10⁷ yr old), which is inconsistent with the formation timescale for the regular satellites. They are also underdense (~600 kg m⁻³) and show spectral characteristics similar to those of the main rings. It has been suggested that they accreted at the rings’ edge, but hitherto it has been impossible to model the formation process fully owing to a lack of computational power. Here we report a hybrid simulation in which the viscous spreading of Saturn’s rings beyond the Roche limit (the distance beyond which the rings are gravitationally unstable) gives rise to the small moons. The moonlets’ mass distribution and orbital architecture are reproduced. The current confinement of the main rings and the existence of the dusty F ring are shown to be direct consequences of the coupling of viscous evolution and satellite formation. Saturn’s rings, like a mini protoplanetary disk, may be the last place where accretion was recently active in the Solar System, some 10⁶–10⁷ yr ago.

[bystander]

Saturn's Rings May Have Birthed Its Small Moons — and More Could Be Coming
Discover Blogs | 80beats | 09 June 2010

They're new, they're small, and they didn't make sense.

That's what could be said for five of the littlest members of Saturn's expansive satellite family. The largest of this group, Janus, measures barely more than 100 miles in diameter, but it's the age of these little moons that's the odd bit. Their clean, crater-free surfaces help reveal that they're only 10 million years old, meaning they didn't form the way the planet's other moons did—from the accretion disk that formed mighty Saturn itself billions of years ago. This week in Nature, astronomers published evidence to support an explanation for that oddity: Those moons formed from Saturn's rings.

Like so much new knowledge about the sixth planet and its moons, including Titan and Enceladus, the research team's findings come from the Cassini mission:

Sailing past Saturn's outer rings, it found lumps of ice up to 100 metres across, ten times bigger than the rings' other icy particles. For some researchers, the discovery called to mind another intriguing fact: that the moons and the rings share a composition of the purest ice in the Solar System. “When you put all this together, you had the strange feeling that something is going on in the rings' outer edge,” says Sébastien Charnoz at Paris Diderot University, who was involved in the latest research [Nature].

Some scientists had suspected this explanation, but they lacked the computer power to model how it could happen (even the most powerful machine would struggle to model the trillions of orbits in the solar system's history). So the team created a simplified model with the ring as a single dimension, tested it out on our own planet and moon's history, and then applied it to Saturn and its rings. At the out edge of the rings, it works: material can clump together.

“Disks in astrophysics are like pancakes—they spread,” [Charnoz] says, adding that collisions within the disk or ring drive the spreading detritus outward. Once the icy ring particles venture beyond about 140,000 kilometers from Saturn's center, they become unstable, clumping into tiny protomoons and then moonlets [Scientific American].

As the clumps get bigger, Saturn's gravity pushes them further out (our own moon is slowly receding from us). While it's nice to have a workable answer to the puzzle, the bigger implication is that the solar system is alive. Says Charnoz:

“There are still new objects forming in the solar system today. We used to think everything was formed four, five billion years ago, but no! New objects are still forming today” [Space].

Related Content:

• DISCOVER: Hello, Saturn
  80beats: Cassini Sends Back Ravishing New Photos of Saturn's Rings
  80beats: Cassini Probe Finds “Ingredients For Life” on Saturn’s Moon Enceladus
  80beats: Cassini Spacecraft Snaps Pictures of Saturn's Geyser-Spouting Moon

Moon Mill: Saturn May Still Be Producing New Satellites
Scientific American - 09 June 2010

A model of recent formation processes near Saturn's rings fits well with the planet's observed population of so-called moonlets

Saturn is perhaps best known for its intricate ring system, but the giant planet also boasts a collection of moons, numbering in the dozens, that is nothing to sniff at. The largest, Titan, has helped draw a bit more attention to the Saturnian satellites in recent days, following an announcement that various chemical abundances on Titan were consistent with but not necessarily indicative of the presence of methane-dwelling, hydrogen-breathing life.

Now, research in the June 10 issue of Nature deals with a population of smaller Saturnian satellites — Atlas, Prometheus, Pandora, Janus and Epimetheus — linking the origin of those so-called moonlets to the celebrated rings themselves. The study presents the result of a computer simulation of Saturn's dynamic environment, demonstrating how the moonlets, which dwell just beyond the planet's famed main rings, could have formed from material oozing out of the rings and accreting into clumps. What is more, the moonlets appear to have coagulated in recent astronomical time, implying that more moonlets may be forthcoming.

Saturn's Oddball Moons Born From Rings, Study Finds
Space.com - 09 June 2010

The rings of Saturn might have given birth to the giant planet's odd, small moons, scientists now reveal.

These unusual moons, some of which resemble flying saucers, might have clumped together from the bits of ice and dust that make up Saturn's majestic bands.

The large moons that orbit the giant planets are thought to have finished forming roughly about when their hosts did, some 4.5 billion years ago. [Photos: Saturn's rings and moons.]

However, calculations of the orbits of five small moons of Saturn that gather just within and beyond the periphery of the planet's main bright rings revealed they are far too young for this to have been the case. These must be less than 10 million years old — for instance, they have bright, nearly pure ice surfaces largely unmarred by the impacts expected from meteoroids.

Is Our Solar System Still Making Moons?
Science NOW - 09 June 2010

Don't close down the lunar maternity ward just yet. New simulations suggest that seven of Saturn's moons were formed as recently as 10 million years ago—over 4 billion years later than the 55 other major bodies orbiting the planet. Researchers think even more new moons could be in prospect because the processes that produced the most recent examples are still active.

Even before the Cassini spacecraft arrived at Saturn 6 years ago this month, scientists wondered about the origins of the seven tiny moons orbiting either just beyond or inside the planet's magnificent rings. All seven, including Pandora and Epimetheus (pictured), are peanut-shaped like asteroids, suggesting that they formed at the beginning of the solar system and were grabbed by Saturn's gravity. But Cassini's instruments discovered that the density of the ring moons was closer to that of Swiss cheese than asteroid rock: less than 1 gram per cubic centimeter. That difference means that unlike the sun, planets, and other moons in the solar system, the ring moons didn't condense from a huge primordial disk of gas and dust. So how were they born?

The most obvious answer is that material from Saturn's rings clumped to produce the moons, but no one could develop a coherent computer model that mimicked the process. Now a team of researchers has done exactly that. By combining and adapting computer models designed to simulate solar-system formation and the orbital migration of planets, the researchers were able to show that the seven moons could condense directly from the rings and retain their wispy consistency.

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Saturn's rings gave birth to mini-moons
ars technica | 09 June 2010

Ever since the Voyager missions, it has been obvious that Saturn's rings are a dynamic system, with tiny shepherd moons and orbital resonances producing a complex structure. Thanks to the arrival of Cassini, we've now gotten a much closer look at some of the tiny moons that reside just outside Saturn's outermost dense ring, the A Ring. And, apparently to the surprise of some scientists, they look much younger than the solar system's 4.5 billion years. Low density, recent surfaces, and somewhat oblong shapes all hint that some of these moons are likely to be less than 100 million years old.

Researchers suspected that the moons might have originated through some sort of interactions within the A Ring, but the number of bodies involved made modeling the system too computationally challenging. Fortunately, Moore's Law caught up with Cassini, and today's issue of Nature contains a paper that describes a model that successfully reproduces the pattern of moons we now observe.

The model has two components. The A Ring itself is modeled as a viscous liquid that spreads slowly, restrained by Saturn's gravity. At some point in its history, it would spread past the Roche Limit, the point in orbit where the presence of Saturn ensures that material can't aggregate under its own gravitational influence. Once past the Roche Limit, small bodies can start to aggregate within the ring material. That's where the second component of the model takes over: it tracks the orbital interactions among the aggregating bodies, which may gain further mass via collisions.

Once a significant body forms, the ring transfers some of its angular momentum to it. As a result, the newly-formed moonlet rapidly migrates outward; having lost momentum, the ring contracts, stopping when it reaches an orbital resonance point with the moon. Once the system stabilizes, the ring can begin to expand again, allowing the process to repeat. In the model, the formation of moons takes place rapidly, on the order of 10 million years. Their orbital location and the distribution of mass between moons and the ring, however, continues to evolve for hundreds of millions of years.

The results produced by the model seem to line up nicely with reality. More momentum gets transferred to larger moons, which ultimately ends up sorting them by size; this is precisely the pattern seen at Saturn, where Janus is both the heaviest and furthest out. Collisions among the moons should also provide material for Saturn's sparse F Ring, which resides in the neighborhood of Janus. The simulation explains how Saturn wound up with moons that are much younger than the solar system, but doesn't necessarily explain why so much material currently resides in the rings or how it originated.