Science News | Alexandra Witze | 12 Dec 2010
Demise of large satellite may have led to the formation of Saturn's rings and inner moons[c]A shattered moon could have sprayed ice particles around the planet[/c] Saturn’s majestic rings are the remnants of a long-vanished moon that was stripped of its icy outer layer before its rocky heart plunged into the planet, a new theory proposes. The icy fragments would have encircled the solar system’s second largest planet as rings and eventually spalled off small moons of their own that are still there today, says Robin Canup, a planetary scientist at the Southwest Research Institute in Boulder, Colo.
“Not only do you end up with the current ring, but you can also explain the inner ice-rich moons that haven’t been explained before,” she says. Canup’s paper appears online December 12 in Nature.
The origin of Saturn’s rings, a favorite of backyard astronomers, has baffled professional scientists. Earlier ideas about how the rings formed have fallen into two categories: either a small moon plunged intact into the planet and shattered, or a comet smacked into a moon, shredding the moon to bits. The problem is that both scenarios would produce an equal mix of rock and ice in Saturn’s rings — not the nearly 95 percent ice seen today.
Canup studied what happened in the period just after Saturn (and the solar system’s other planets) coalesced from a primordial disk of gas and dust 4.5 billion years ago. In previous work, she had shown that moon after moon would be born around the infant gas giants, each growing until the planet’s gravitational tug pulled it in to its destruction. Moons would have stopped forming when the disk of gas and dust was all used up.
In the new study, Canup calculated that a moon the size of Titan — Saturn’s largest at some 5,000 kilometers across — would begin to separate into layers as it migrated inward. Saturn’s tidal pull would cause much of the moon’s ice to melt and then refreeze as an outer mantle. As the moon spiraled into the planet, Canup’s calculations show, the icy layer would be stripped off to form the rings.
A moon so large would have produced rings several orders of magnitude more massive than today’s, Canup says. That, in turn, would have provided a source of ice for new, small moons spawned from the rings’ outer edge. Such a process, she says, could explain why Saturn’s inner moons are icy, out to and including the 1,000-kilometer-wide Tethys, while moons farther from the planet contain more rock.
“Once you hear it, it’s a pretty simple idea,” says Canup. “But no one was thinking of making a ring a lot more massive than the current ring, or losing a satellite like Titan. That was the conceptual break.”
“It’s a big deal,” agrees Luke Dones, also of the Southwest Research Institute, who has worked on the comet-makes-rings theory. “It never occurred to me that the rings could be so much more massive than they are now.”
Another recent study supports the notion that today’s rings are the remnants of massive ancient rings of pure ice. In a paper in press at Icarus, Larry Esposito, a planetary scientist at the University of Colorado at Boulder, calculates that more massive rings are less likely to be polluted by dust, and hence could still be as pristine as they appear today even after 4.5 billion years.
Some questions still linger about Canup’s model, says Dones, like why some of Saturn’s inner icy moons have more rock in them than others.
The theory will be put to the test in 2017, when NASA’s Cassini mission finishes its grand tour of Saturn by making the best measurements yet of the mass of the rings. Researchers can use those and other details to better tease out how the rings evolved over time.
PhysOrg | Space Exploration | 12 Dec 2010
Violent Origin for Saturn's RingsSimulations performed at Southwest Research Institute may explain how Saturn's majestic rings and icy inner moons formed following the collision of a Titan-sized satellite with the planet, according to a paper published in Nature magazine's Dec. 12 Advance Online Publication.
Saturn's rings are at present 90 to 95 percent water ice. Because dust and debris from rocky meteoroids have polluted the rings, the rings are believed to have consisted of pure ice when they formed. This composition is unusual compared to the approximately half-ice and half-rock mixture expected for materials in the outer Solar System. Similarly, the low densities of Saturn's inner moons show that they too are, as a group, unusually rich in ice.
The previous leading ring origin theory suggests the rings formed when a small satellite was disrupted by an impacting comet. "This scenario would have likely resulted in rings that were a mixture of rock and ice, rather than the ice-rich rings we see today," says the paper's author, Dr. Robin M. Canup, associate vice president of the SwRI Planetary Science Directorate in Boulder.
The new theory links the formation of the rings to the formation of Saturn's satellites. While Jupiter has four large satellites, Saturn has only one, Titan. Previous work suggests that multiple Titan-sized satellites originally formed at Saturn, but that those orbiting interior to Titan were lost as their orbits spiraled into the planet.
As the final lost satellite neared Saturn, heating caused by the flexing of its shape by the planet's gravity would cause its ice to melt and its rock to sink to its center. Canup uses numerical simulations to show that as such a satellite crosses the region of the current B ring, planetary tidal forces strip material from its outer icy layers, while its rocky core remains intact and eventually collides with the planet. This produces an initial ice ring that is much more massive than Saturn's current rings.
Over time, collisions in the ring cause it to spread radially and decrease in mass. Inwardly spreading ring material is lost, while material spreading past the ring's outer edge accumulates into icy moons with estimated masses consistent with the inner moons seen today.
"The new model proposes that the rings are primordial, formed from the same events that left Titan as Saturn's sole large satellite, " says Canup. "The implication is that the rings and the Saturnian moons interior to and including Tethys share a coupled origin, and are the last remnants of a lost companion satellite to Titan."
During its extended mission, the Cassini spacecraft will measure the rings' current mass and will indirectly measure the pollution rate of the rings. This should provide an improved estimate of the rings' age and a test of the new ring origin model.
Source: Southwest Research Institute (SwRI)
Science NOW | Sid Perkins | 12 Dec 2010
Origin of Saturn's Rings and Inner Moons by Mass Removal from a Lost Titan-Sized Satellite - RM CanupA centuries-old astronomical mystery may be finally solved. A scientist says she has figured out how Saturn's spectacular rings formed. The dramatic process could help explain other solar system mysteries as well.
Saturn's rings have mystified scientists since they were discovered in the mid-1600s. In particular, none of the hypotheses about their origin explain why individual ring particles, which range in size from hailstones to small boulders, average between 90% and 95% ice. If a moon disintegrated in Saturn's orbit, as some astronomers have suggested, the rings should be about half ice and half rock. That's the composition of most moons this far from the sun.
The new theory, set forth by planetary scientist Robin Canup of the Southwest Research Institute in Boulder, Colorado, and published online today in Nature, explains the ice-rich composition of the rings and accounts for the odd characteristics of some of Saturn's smaller moons.
Canup created detailed computer simulations, which suggest a violent origin for Saturn's rings. As the planet coalesced during the birth of the solar system more than 4.5 billion years ago, the swirling disk of gas surrounding it included several moons about the size of Titan, Saturn's largest remaining satellite, which is about 50% larger than Earth's moon. But gravitational interactions with the gas caused the moons' orbits to shrink, and one by one the satellites entered death spirals and plunged into the planet.
Before each moon collided, immense tidal forces produced by Saturn's gravity stretched and contracted it, stripping off much of its ice. Subsequent moons gravitationally captured this ice, but they were eventually stretched and contracted until they too shed their ice and plunged into Saturn. Today's ring system is the fossil remains of the last moon to fall prey to Saturn's immense gravity, Canup contends. This moon was basically a giant ice ball with a rocky center. After its ice-rich veneer was stripped away in large chunks, its rocky core disappeared beneath the saturnian clouds.
The fragments of that final doomed moon, each originally between 1 and 50 kilometers across, formed an icy ring system as much as 1000 times as massive as today's rings. In the subsequent 4.5 billion years, innumerable collisions between these large chunks produced the much-smaller ring particles now orbiting Saturn. What little rocky material occurs in today's ring system probably is the debris of collisions between icy ring particles and asteroids and comets swept up by the planet's huge gravitational field, says Canup.
The new hypothesis also explains how Saturn's moons that orbit just beyond the edge of today's ring system might have formed. Over time, the rings spread out, and the icy bits that drifted farthest from Saturn eventually reached distances where their gravitational attraction for each other could overcome the planet's tidal forces that tended to rip them apart—a process that is still happening today, according to observations by the Cassini spacecraft now touring the Saturn system. In particular, says Canup, the findings offer a good explanation for why the moon Tethys is apparently almost pure ice.
"This is a pretty impressive piece of work," says planetary scientist Joseph Burns of Cornell University. It's more comprehensive than previous theories and consistent with Cassini observations, he says, and "it tells a fun and convincing story."
The new study "provides a very compelling narrative," says planetary scientist Matthew Hedman, also of Cornell. "This is the first reasonably plausible scenario about how the rings could have formed."
- Nature (online 12 Dec 2010) DOI: 10.1038/nature09661
- JP Elliott, LW Esposito, Icarus (online 11 Dec 2010) DOI: 10.1016/j.icarus.2010.10.031