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Planetary News: Cassini-Huygens (2004)Cassini's First Close Flyby of Titan: So Much Information, So Few Answers (Yet)By Emily LakdawallaOctober 28, 2004
The Cassini spacecraft has pierced Titan's veil of haze to see its surface, but Titan is still holding on to its mysteries. Yesterday's successful flyby has not yet answered any of the outstanding questions about what's going on under Titan's haze. In particular, there is no conclusive evidence yet to either confirm or deny the presence of the putative lakes of liquid ethane on Titan's surface. In a press conference at the Jet Propulsion Laboratory yesterday morning, Carolyn Porco, Team Leader for Cassini's camera system, summed up the expectations and results: "What remains hidden under Titan's atmosphere and haze, the conditions on its surface and its geological history, are the solar system's last great mystery. The images [from yesterday's flyby] were great, but I have to report we are still mystified, and we aren't quite sure what we are looking at." The Cassini spacecraft has pierced Titan's veil of haze to see its surface, but Titan is still holding on to its mysteries. Yesterday's successful flyby has not yet answered any of the outstanding questions about what's going on under Titan's haze. In particular, there is no conclusive evidence yet to either confirm or deny the presence of the putative lakes of liquid ethane on Titan's surface. In a press conference at the Jet Propulsion Laboratory yesterday morning, Carolyn Porco, Team Leader for Cassini's camera system, summed up the expectations and results: "What remains hidden under Titan's atmosphere and haze, the conditions on its surface and its geological history, are the solar system's last great mystery. The images [from yesterday's flyby] were great, but I have to report we are still mystified, and we aren't quite sure what we are looking at." Does this sound like bad news? It's not. In fact, yesterday was "a very, very good day," enthused Earl Maize, deputy program manager for the mission. Despite concerns about rain in Spain interfering with the return of the data from Cassini to Earth, nearly all of the expected data was returned. And with a single exception, Cassini's instruments performed as expected, returning gigabits of data. The one "bump in the night" was caused by a software glitch on the Composite Infrared Spectrometer (CIRS), which forced mission engineers to shut the instrument off this morning. However, CIRS scientists were hopeful that data could be returned soon. The images were perfect, but it will take more than one day for the science team to figure out what they mean. The team was being very conservative in reporting their first impressions, particularly because most of them had stayed up all night attempting to make some sense of the gigabits of data returned from the encounter. "There isn't much we are definitively confident about right now," Porco said. But among the conclusions that scientists were ready to talk about were several outright surprises. Light Stuff and Dark Stuff Look Different, But Are They? And What About the Lakes, Anyway?
The high-resolution images from Cassini's cameras show a surface containing "boundaries between light and dark regions; linear features that are dark; and linear features that are light. There are things that look like islands of light material within dark material," Porco said, raising hopes that the dark areas could represent the putative liquid ethane lakes or oceans. Porco was quick to state, however, that the liquid question is still way up in the air. "There's no topography in our images. We do not see shadows in our images, so we cannot deduce what's up and what's down. Everything could be perfectly flat. We have not yet seen a specular reflection [a telltale glint of light from a smooth liquid surface] in any of our data. The jury is still out on whether there's any fluid on the surface of Titan. What we can say is that the surface seems to be geologically young. We see very few circular features that could be identified as craters." Still, there's no question that the scientists are imagining liquids, because they've begun to refer to a particular bright "island" in some dark material as "Great Britain." "You can almost see Loch Ness up there," Porco said of the image. "But we're really not sure what we're looking at." The best camera images of the surface are black-and-white, because there is only one camera filter available to the scientists that really allows Cassini to look through a "window" past the haze and atmosphere to see sharp images of the surface. Consequently, the cameras cannot tell what the bright and dark stuff are made of. But Cassini's Visual and Infrared Mapping Spectrometer, or VIMS, has access to five different such windows through which Titan's atmosphere appears to be mostly transparent. So VIMS can record information about the color of the surface, and color can give clues to composition. At least that's what scientists were hoping for from this encounter. "We see a lot of strong margins between light and dark regions," said Robert Brown, Team Leader for VIMS, "but the composition of the bright and dark regions is not all that different. This is not what we expected. Our first look indicates that their composition is very similar." He was not yet ready to say what that composition might be: "Within a few days or so, if the people who stayed up all night to produce this can get some sleep, we should be able to give some compositional information." The rain in Spain had affected the VIMS team, who lost one frame from a six-frame global mosaic of Titan to communication problems, but Brown was hopeful that the missing data would be returned today. One thing that shows up brightly in all of the VIMS and camera images is a shifting complex of clouds near Titan's south pole. These clouds were initially assumed to be methane, but Porco mentioned that the VIMS team had doubts. Brown confirmed that there was "something going on" in the south polar region, indicating that the clouds may not be methane. He said the VIMS team had noticed that the clouds became brighter as VIMS looked at them in longer wavelengths, indicating that the clouds were made of relatively large particles bigger than 5 microns in diameter.
Titan's Atmosphere Is "Off the Scale"One of the most exciting aspects of this first flyby was the fact that Cassini flew so close past Titan that she "dipped her toes" into the thin outer regions of Titan's atmosphere. For a period near the spacecraft's closest approach, the collecting end of Cassini's Ion and Neutral Mass Spectrometer (INMS) was pointed in Cassini's direction of flight so that it could scoop up particles and measure their compositions. The results were quite surprising. INMS Team Leader Hunter Waite reported the detection of molecules that scientists already knew were in Titan's upper atmosphere, namely nitrogen and methane. But they were shocked to see more complex hydrocarbon species, including benzene (C6H6), diacetylene (C4H2), and propyne (C3H4). It's long been known that such molecules form in Titan's atmosphere when ultraviolet rays from the Sun cause reactions that break down methane gas, with the resulting ions recombining into more complex molecules consisting of chains of carbon atoms. But these substances are relatively heavy and should sink in Titan's atmosphere -- in other words, Titan's upper atmosphere should be stratified by mass. "Titan seems to be mixed all the way to the top. The fact that we saw these molecules at 1200 kilometers is quite surprising." The detection "bodes well for the mission," Waite said. "At lower altitudes, we expect to do even better."
The INMS team was also able to perform a more exacting measurement, examining the proportion of two isotopes of nitrogen, nitrogen-15 and nitrogen-14, in the atmosphere. Their measurements indicate that "up to three-fourths of Titan's atmosphere may have been lost over geologic time," Waite said. Here's how they came to that conclusion: Nitrogen atoms always contain the same number of protons (seven) but may have different numbers of neutrons. The most common isotope of nitrogen contains seven neutrons, but nitrogen atoms may rarely contain eight neutrons. Atoms of the heavier isotope participate in the same kinds of chemical reactions that atoms of the lighter isotope do, but the resulting molecules are heavier by the weight of the extra neutron. Molecules of gas are often lost from the top of a planet's atmosphere under the influence of the solar wind; and in this process, a planet is more likely to lose a lighter-weight molecule than a heavier-weight molecule. So the more gas an atmosphere loses, the more enriched it should be in heavier isotopes. Using the technique of comparing nitrogen-15 to nitrogen-14 on other planets, Waite explained, has shown that "Mars departs from the Earth and Venus; it has a heavier atmosphere. This has been used as a mark that Mars has lost much of its atmosphere." And today Mars has a very thin atmosphere, the result, presumably, of atmospheric loss over time. But "for Titan we've had to change the scale. It's off the scale" of nitrogen-15 to nitrogen-14 ratios seen on Earth, Venus, Mars, even Jupiter. "This is a strong indication that Titan has lost as much as three fourths of its atmosphere." That makes the source of Titan's thick atmosphere -- which generates 1.5 times Earth's air pressure on the surface -- more mysterious. Waite has not yet seen signs of the carbon-nitrogen compounds known as "nitriles" that are also known to form in Titan's atmosphere. These compounds are of interest to scientists because they are chemical precursors to very interesting prebiotic compounds such as amino acids. But Waite expects to see those materials in future, closer flybys with altitudes nearer 950 kilometers (590 miles). "Benzene and acetylene were easy to spot," he explained. But common nitriles, such as hydrogen cyanide (HCN) have masses more similar to nitrogen or methane, making them harder to pick out of the INMS results with only one night's study.
What Hope For Answers To All of the Questions?The images released today were so inexplicable -- for the present -- that one reporter asked wistfully if the whole mission was going to be "like this," with no explanations forthcoming, just lots of fuzzy images (a question that made VIMS Team Leader Robert Brown mime the twist of a dagger in his heart). Porco reassured the world that it was no surprise to the science team that the images were difficult to interpret. This is an environment we've never seen before," she explained. "It's not an airless body. We have a lot of experience looking at airless bodies. [With its atmosphere] Titan is more similar to the environment on Earth -- but the materials that Titan is made of are alien." One night's worth of analysis is not enough to answer Titan's mysteries. Porco predicted that it will take "several flybys, or even several years" to come to any kind of scientific consensus on what the images represent. Cassini's nominal mission involves four years spent in the Saturn system, including 43 more Titan flybys, so there will be ample opportunity for more puzzling pictures to be returned. And today's images came only from two instruments, the camera system and VIMS. Still to come will be CIRS data; if the data from this flyby proves unrecoverable, there will be many more chances. Tomorrow the science team will release images created using active sounding from Cassini's RADAR instrument. Over time, as these differing types of imagery are compared, scientists should be able to begin developing a coherent story for the complicated world. Now that Titan has revealed itself to be as complex and mysterious as advertised, there is even more anticipation among the science team for the descent of the European Space Agency's Huygens probe. The images captured during this flyby included the area in which Huygens is to descend on January 14, 2005. Prior to Cassini's approach to the Saturn system, there was little information on which to base a careful landing site selection process, so the team "did not select the Huygens landing site," said Jean-Pierre Lebreton, ESA's Project Manager for Huygens. But now that Lebreton has had a first look at Cassini's high-resolution images of the landing site, he said "I think it's the best place we could have selected, because of its diversity. I am really looking forward to visiting it!" In fact, before the press conference began, the science team was buzzing about how this taste of Titan's surface has them anticipating Huygens' landing. "It's going to be so exciting to be there!" said Linda Spilker, Deputy Project Scientist for Cassini. And Claudio Sollazzo, Head of Huygens Operations, could not even put his excitement about the Huygens descent into words. "I am so..." he began, trailing off, gesticulating with his hands to indicate being overcome; in the end he could only say "we have spent so many years waiting for this.... After that, I can retire." |
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