Saturn may be in the Cassini spacecraft's rear-view mirror, but the science team behind the international mission is continuing to take in data — including eye-opening readings of the planet's rings.
After a seven-year cruise, the bus-sized probe successfully fired its engine to enter Saturnian orbit last Wednesday night, and it's now heading away from the planet for its first full 116-day circuit.
Scientists are continuing to process data from last week's close encounters. The science team for Cassini's Ultraviolet Imaging Spectrograph, or UVIS, reported Wednesday that Saturn's rings were sparser and dirtier toward the inside, but denser and icier toward the outside.
Such findings could lead researchers to a better understanding of how the rings evolved, said Larry Esposito of the University of Colorado at Boulder, who leads the UVIS team.
Esposito sketched out two possible explanations for variations in the rings.
"One is that the different parts of the rings are fragments from different moons that had different compositions," he told MSNBC.com.
The other explanation, which he prefers, is based on the fact that "the rings are constantly bombarded by meteorites which are dark and dirty, compared with the clean ice that we'd see."
That dark material makes a big impact on the inner, sparser region of the rings. But toward the outside, where the rings are denser, the greater volume of ice "dilutes the meteorite input," Erickson said.
It's too early to determine which explanation fits the data better, he said.
"We're still working with our data to try to determine these answers ourselves," he said, by running the UVIS readings through various computer models for ring dynamics. Esposito and his colleagues are also looking forward to more observations of Saturn's rings.
As Cassini zooms away from Saturn, the scientific instruments can get a wide-angle perspective on the planet and its surroundings.
"We'll actually be making movies of Saturn as part of that," Linda Spilker, Cassini's deputy project scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif., told MSNBC.com Wednesday. "When you get a little bit farther away, that's a good time to make movies."
In Cassini's case, the "movies" are time-lapse sequences showing the dynamics of Saturn's atmosphere, its rings and its moons.
Spilker said the science team would also be observing the distribution of hydrogen and oxygen across the Saturnian system, as well as activity in the magnetosphere. On its way out from Saturn, Cassini already captured closer views of Iapetus and other moons.
"We got more hints of what Iapetus might look like," Spilker said. "I see this as a kind of teaser for what's coming up."
Mysteries of Titan
Titan — which is Saturn's largest moon as well as the only moon in the solar system with a thick atmosphere — was an important target during Cassini's first post-insertion pass on Friday, and it will be even more important this fall and winter.
Cassini is equipped with camera filters and other instruments that can pierce through Titan's obscuring smog, and even during Friday's flyby, scientists had hoped to see clear evidence that Titan possessed oceans or lakes of liquid methane and ethane.
They didn't get the readings they expected, but Spilker said that doesn't mean the liquid isn't there.
Video: Titan revealed "That's still a big question mark," Spilker said. "If there had been global oceans, we would have seen them by now, but there still could be some substantial lakes."
In addition to the question about liquid hydrocarbons, scientists are intrigued by the linear and circular features they saw in the Titan flyby imagery.
The lines could represent faults in Titan's surface, and the circles could be craters — all of which may well indicate that the icy moon is geologically active, like Jupiter's equally mysterious moon Europa.
This first round of data is only whetting Spilker's appetite for the much closer flyby of Titan on Oct. 26, when Cassini is due to come within 750 miles (1,200 kilometers) of the moon's surface.
"That will be 300 times closer than the one after Saturn orbit insertion," she said. "We know that with the camera and the Visible and Infrared Mapping Spectrometer we can see through to the surface, so we'll get just much more detail on what the surface looks like."
Yet another flyby will bring Cassini just 1,475 miles (2,360 kilometers) from Titan on Dec. 13, and on Christmas Eve, the spacecraft will send its piggyback Huygens probe toward Titan for a trip through Titan's atmosphere. Now that Saturn orbit insertion is out of the way, the release of the Huygens probe looms as the mission team's next nail-biting moment.
"We have a slight breather with this 116-day orbit," Spilker said, "but that gives us time to focus on the probe release activities."
As Cassini's four-year mission proceeds, the orbits get tighter — and the breathers get shorter.
"Once we get a couple of years into the mission, we fly by Titan every 16 days," Spilker said.
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