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Planetary News: Cassini-Huygens (2004)Saturn's Rings Surprise Cassini As the Spacecraft Flies By Titan
By A. J. S. Rayl Cassini, to adapt an old adage, has hit the Saturnian orbit flying. Earlier today, the spacecraft flew by Titan -- Saturn's largest moon and one of the primary targets of the mission -- and was expected to beam down raw images from that encounter at some point late today. "We've waited 14 years to see these images," said an anxious Carolyn Porco, head of the imaging team, of the Space Science Institute, at a press conference held late this morning at the Jet Propulsion Laboratory (JPL) just about the time Cassini was flying by the hazy, orange moon. In the meantime, team members have already begun to analyze data collected during Cassini-Huygens' approach to Saturn a couple of weeks ago. Out of the gate Saturn's surprises are at once both delighting and mystifying to the scientists, who collectively have hundreds of years invested in this journey. In fact, barely two days into the mission, the preliminary science results are beginning to reveal a complex and fascinating planetary system -- not the least of which involves Saturn's atmosphere "eating away" at the rings. Today, Cassini passed within 300,000 kilometers (200,000 miles) of Titan, snapping images along the way, and taking measurements with specially designed spectral filters that can pierce the hazy veil that has forever clouded the surface. In true-color images taken in visible wavelengths, Titan's photochemical smog, rich in organic material, gives the moon a smooth, opaque, orange glow. Although too cold to support life now, Titan is thought to harbor simple organic compounds that may be important in understanding the chemical building blocks that led to life on Earth. The moon is, basically, a frozen vault that holds clues to what early Earth might have been like. If the flyby went as planned -- and since the spacecraft status "remains excellent," according to deputy project scientist Linda Spilker, there is no reason to believe otherwise -- Cassini, for the first time, will be returning close-up images of this mystery moon. In terms of resolution, these pictures will show Titan in a manner "roughly equivalent" to what a backyard astronomer would see when looking through a decent telescope at Earth's Moon. While the team -- and the world -- waited for those first close-up images of the big Saturnian moon, team members took the opportunity at the press conference to announce the first science results from Cassini's close encounter with Saturn's rings, as well as the results from measurements and observations taken of both Saturn and Titan by some of the spacecraft's sophisticated scientific suite of instruments captured during the spacecraft's approach to the system. Already, they have found that:
Cassini's examination of Saturn's atmosphere began a couple of weeks ago as the spacecraft was still approaching the planet. The Ultraviolet Imaging Spectrograph (UVIS) has detected large quantities of oxygen at the edge of the rings, announced Donald Shemansky, co-investigator of the instrument, of the University of Southern California (USC). Although the team is still working to analyze these results, they think the oxygen may be left over from a collision that occurred as recently as January of this year. "What we are seeing is an abundance of oxygen first rising then dropping in a mass quantity," said Shemansky. The timeline of this UVIS data began with observations on December 25, 200 through January 11, 2004, which was followed by a gap where no observations were taken, he explained. Then, observations picked back up on February 10 of this year and continued into March. "It's that second image which goes from the 10th of February into March that shows the broad increase in abundance [of oxygen] and so our inference is that something happened somewhere in the period of the last half of January when we were not looking, and when we did look we found this increased abundance," he explained. "What is surprising is the evidence of a strong, sudden event during the observation period causing substantial variation in the oxygen distribution and abundance," said Shemansky. Although atomic oxygen has not been previously observed, its presence is not a surprise because hydroxyl (OH-) was discovered earlier in Hubble Space Telescope observations, and these chemicals are both products of water chemistry.
So where, exactly, is the oxygen coming from? "Saturn has a hydrogen atmosphere and we don't see oxygen there, so it has to come out of the solids of the system, the ice in the rings, somehow," Shemansky theorized. "The issue is -- how does it happen? My argument is it has to happen with low energy ions interacting with grains, forming cluster ions on the grains, which then rapidly recombine and essentially tear neutral atoms out into the vacuum. So it has to be eating the ring system somehow." The total amount of oxygen measured, Shemansky said, was 4 times the mass of the 1-micron particles in the E Ring "or 500,000 million kilograms." Using that measurement and based on the amount of oxygen eroding through the system over a 4-month period, the team has determined that the abundance of oxygen equals the total mass of 1-micron particles in the E Ring structure, and that the E Ring, therefore, is about 100 million years old, which is precisely what the leading hypothesis predicted. Cassini team member Michael Flasar, of NASA Goddard Spaceflight Center, reported that findings from the composite infrared spectrometer -- known as CIRS -- showed that winds on Saturn near the equator decrease dramatically with altitude above the cloud tops. The winds fall off by as much 300 miles per hour [140 meters per second] over an altitude range of about 200 miles [300 kilometers] in the upper stratosphere. This is the first time winds have been measured at altitudes so high in Saturn's atmosphere. "We are finally defining the wind field in three dimensions, and it is very complex," said Flasar, principal investigator for CIRS. "Temperature maps obtained now that Cassini is orbiting Saturn are expected to show more detail, helping us to unravel the riddles of Saturn's winds above the cloud tops." The Cassini imaging team also observed Saturn's winds, Porco noted, and found "vertical shear" in the same equatorial region. [Vertical shear implies that the winds move at different speeds at different depths in the atmosphere.] The Visual and Infrared Imaging Spectrometer (VIMS) -- which can produce "an image similar to what you'd see [with your own eyes] if you were standing on the space craft," says team member Roger Clark, of the U.S. Geological Survey -- can see wavelengths from ultraviolet to the infrared and is being used to map materials through the Saturnian system, especially in the intriguing rings. With the detail that the VIMS offers, says Clark, "we can see the detail of the ice and map the gradient through the system." The VIMS data confirmed that Saturn's rings are composed mostly of ice. Planetary ring scientists had long thought Saturn's rings were probably 98% ice. VIMS confirmed that one better, finding them to be closer to 99% ice with a 1% contaminant they're calling 'dirt.' The rings are, basically, boulders of ice and "we're seeing the frost on the boulders," Clark elaborated. But don't get confused -- that does not mean they have rocky cores. "All the wavelengths that we've probed the rings with have indicated [the rings are made up of] relatively pure ice, so we don't believe there are rocky cores to these things," Clark explained. "There have probably been enough collisions to see into those cores [if they existed] because they'd be broken up." Given that caveat, he added, they believe they are now seeing "the full extent of the composition of the rings." VIMS found 'dirt' mixed with the ice in the Cassini Division, the large gap between the A and B rings, and in other small gaps in the rings, as well as in the F ring. While Saturn's rings are almost exclusively composed of water ice, the new findings show the Cassini Division contains, relatively, more 'dirt' than ice. Further, the particles between the rings are similar to the dark material that scientists saw on Phoebe, another one of Saturn's 31 moons, which they measured last week. "The surprising fingerprint in the data is that the 'dirt' appears similar to what we saw at Phoebe. In the next several months we will be looking for the origin of this material," Clark said. Meanwhile, these dark particles are refueling the theory that the rings may be the remnants of a moon. "What we haven't found yet -- because we haven't had to time to thoroughly analyze the data -- are some of the other components that we saw on Phoebe like the organics and stuff," Clark offered. "We found carbon dioxide, for example, in the Jupiter system on the satellites and we haven't found any carbon dioxide in the rings, but we have a lot more data to analyze and collect." The VIMS measurements of two nights ago also actually showed a change in the ice in the rings, the kind of "secular change" the scientists had been hoping to see, as Porco mentioned yesterday. That they have detected a change in the rings so early in the mission can only be seen as a omen of many ring discoveries to come. In the imaging analysis lab, team members have been further analyzing the first 61 pictures that Cassini took as she passed close by the rings Wednesday night. From those pictures, they have determined that Prometheus -- the tiny Saturnian moon that can be seen as a bright spot in one of those images [see above] -- "perturbs the ring structure," Porco announced today. "Prometheus moves in and out and perturbs the material in the F Ring and creates these streamers," she said. "This is just an example of the kind of phenomenon we have captured and probably will capture over and over again in the next four years." Porco also presented a movie of the storms in Saturn's southern hemisphere created from infrared images comprised of 45 frames captured between February 6 and March 30, 2004, as Cassini-Huygens coasted toward Saturn. The movie shows a banded appearance due to winds, as well as dark cyclonic ovals. "The storms [and winds] are moving in opposite directions," she noted. "We can see only dark ovals, which seem to die by merging," offered Porco. It is not yet clear whether all the ovals visible here are cyclonic, meaning that their winds flow in a counter-clockwise direction. However, the ovals resemble the cyclonic ovals Cassini spied on Jupiter. Interestingly, a few of the ovals seem to change their apparent direction of travel, from westward to eastward, as they change their latitude. This fascinating behavior was also noted by Cassini imaging scientists in ovals on Jupiter. From this data, they hope to be able to characterize the storms and learn more about the atmosphere of this Ringed Planet. The images to come from Titan should be tantalizing. From the images of this moon taken just a couple of weeks ago during Cassini's approach, scientists have already discovered dark, linear markings that look "suspiciously like tectonic features," says Porco. Tectonic features, such as grooves on icy satellites, wrinkle ridges on Mars, and mountain chains on Earth, generally indicate a complex and interesting geologic history. The detail, such as it is for the moment, is better than anything scientists ever gotten before, so, not surprisingly, the team is viewing them as "obviously very exciting." Porco declined to say unequivocally that these dark linear markings are tectonic features. But generally on Earth and on other solar system bodies, "where you see linear features, that generally means tectonic," she pointed out. "That's the basis of our speculation now." The finding is significant, if only because we don't really know anything about the surface of Titan, and tectonic features would imply internal processes. "They're something other than just craters and surface features which are formed by exogenic processes. This is an internal process and so that would be interesting," she explained. "If you don't see the surface you can't read the story of the geology. So this is a glimmer of what's going on -- and it may not turn out to be the case, but it's what we're seeing now." Cassini's encounter with Titan today was, in effect, a "Voyager-class" flyby, according to Porco. What they'll specifically be looking for in these, most recent images, she said, are specular reflections, clouds in the polar regions, and "any features at the smallest scales." From these images, and the thousands more to come during the next four years, mission scientists are hoping to lift the hazy veil and get a glimpse of the surface of the moon. Today's flyby is the first of more than 40 visits to Titan that the orbiter it will make in the next four years. On Christmas Eve, the European Space Agency's Huygens probe, which is piggybacked on Cassini, will be released, to descend through Titan's atmosphere in early 2005, giving an even closer look at this mystery moon. "Life," mused Porco, "is good right now." |
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