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Planetary News: Cassini-Huygens (2008)Cassini Finds Enceladus Tastes Like a CometBy Emily LakdawallaMarch 26, 2008
As Cassini swooped upward from a death-defying 52-kilometer (31-mile) plunge past Enceladus two weeks ago, it skirted the edge of Enceladus' south polar plume, scooping up particles and gases to sample their composition. To the evident surprise of the Cassini science team, the gases that were tasted by Cassini's Ion and Neutral Mass Spectrometer (INMS) bore a strong resemblance to the gases that issue from comets. Whether the particles in Enceladus' vents also "taste like comet" cannot be said, because an error in new software uploaded for the flyby prevented the Cosmic Dust Analyzer (CDA) from collecting data during the close approach. Taking the PlungeThe March 12, 2008 flyby of Enceladus was Cassini's closest yet, and the deepest entry into the south polar plumes. Since the discovery of the plumes, the "fields and particles" instrument teams -- the ones that measure the abundance, compositions, and motions of plasma, ions, atoms, molecules, particles, and magnetic fields in situ, wherever Cassini travels -- have been hoping to be able to dive into the plume and sample it. However, such an encounter could not be executed unless the mission could be sure that there was no risk from collisions with any large particles that might be entrained within the plumes. At a press conference held today at NASA Headquarters in Washington, D.C., Ultraviolet Imaging Spectrograph (UVIS) principal investigator Larry Esposito explained that a key observation from UVIS cleared the way for the close encounter. Last year, "we watched a star as it passed behind the plume. When the star passes behind the plume, its starlight becomes dimmer, which allows it to measure the shape of the plume. We're able to measure, actually count, the number of molecules along the path of the star. This gives us the most detailed measurements of the detailed properties of the jets along the surface." The UVIS measurements proved that the particle sizes were small enough to pose no threat to Cassini, and the close flyby was given the go-ahead. Cassini approached Enceladus from the north and dove to within 52 kilometers (32 miles) of the southern midlatitudes, where it also began to encounter gas from the plume. As it receded from the encounter, its course took it further southward, dipping it into a denser region of the plume. Cassini reached the thickest point at an altitude of roughly 250 kilometers above the surface, but it was still just grazing the plume's edge. Future flybys will carry Cassini to even denser regions. Picking Apart the PlumesEnceladus' plumes have two main components: gases and tiny particles. The two components have very different compositions, and until this flyby it was assumed that they also had different spatial forms. It is the particle component of the plume that we can see in Cassini camera images of Enceladus that are backlit by the Sun; the particles separate into many distinct, collimated jets, which flow faster in a direction upward from Enceladus' surface than they spread out to the sides. INMS principal investigator J. Hunter Waite explained that the gases were expected to have quite different behavior, spreading out fairly evenly in all directions from the south polar sources through thermal expansion. However, the two components of the plume "are more similar in structure than we had imagined until now," he said. A gas plume that spread just by thermal expansion would have caused Cassini to see a slow rise in the density of the plume as it approached, but, Waite remarked, "When we got above that plume, it just hit us right in the face. What this argues for is a supersonic outflow, which implies higher pressure and temperature in interior," and the possibility that the plume is being fed by liquid water under pressure. INMS measured the gas plume to be 20 times denser than their earlier models, based on thermal expansion, had predicted.
Twenty times denser meant excellent signal for the INMS. A mass spectrometer measures the composition of a gas by measuring the mass of each molecule it samples. During the process of measurement, some of the molecules get "cracked," broken into smaller pieces, so a mass spectrum contains peaks for whole molecules as well as for their cracking products. Small molecules may be readily identified from diagnostic peaks in the mass spectrum. For instance, a peak at 16 Daltons, spreading down to 12 and up to 18, indicates the presence of methane, which is an atom of carbon (12 Daltons) joined to four hydrogen atoms (1 Dalton each, though some will be isotopes of heavy hydrogen that may mass 2 Daltons). Likewise, water vapor peaks at 18 Daltons, carbon monoxide and nitrogen both have peaks at 28 Daltons, and so on. Decoding a mass spectrum requires the INMS team to model the abundance of each molecule and its cracking products. The mass spectrum from the Enceladus encounter is below.
There is a very large peak at 39 to 41 Daltons, which requires relatively complex molecules, as Waite explained. "These contain two carbons and a nitrogen. There may be acetylene, ethane, perhaps, tentatively, some hydrogen cyanide, as well as formaldehyde." At higher masses, "we saw more complex compounds, like propyne, propane, maybe even acetonitrile, and then we saw things even more complex. But they were so weak in signal that we didn't venture an identification." These compounds are not labeled in the graph above because their abundances were too low to appear at the vertical scale of the graph. One identification that presents a challenge to mass spectrometers is whether the peak at 28 Daltons represents carbon monoxide (CO) or molecular nitrogen gas (N2). But the high density of the plume meant that the cracking products of the gases were abundant enough for INMS to readily measure them, Waite said. "If we used N2 in [our model] fit, we got a pretty big error; we have to put in more CO than N2. But that's something we'll look at more closely in later flybys." All in all, the plume mostly consisted of water vapor, as expected. But the minor constituents of the plume were very surprising, particularly the unexpectedly high abundance of organic materials. The graph below shows the abundance of each material that INMS measured, and a comparison to the range of compositional values measured for comets.
What does it mean that the composition of Enceladus looks cometary? "As a scientist, I take the simplest explanation for what I see," Waite says, by which he implies that Enceladus, or at least the material that makes up Enceladus, has its origins in the Kuiper belt, as do comets. But that "simple" explanation raises a host of problems. It has long been assumed that Enceladus originally formed within the Saturn system. Since the composition of the Saturn system as a whole evolved during the formation of the planet and its moons, the composition of the moons should be noticeably different from the composition of comets, which formed farther from the Sun, in the Kuiper belt. Furthermore, Enceladus itself has evidently experienced a great deal of geologic processing, and has spewed forth enough of its interior materials to create Saturn's E ring. Such geologic activity is unlikely to have allowed Enceladus to retain the same composition it had when it formed. "Rather than trying to come up with elaborate chemical schemes to generate such a surprising proportion of organics," Waite argued, "you should just look at cometary material, which already exists that way." But, he admitted, "it opens up a more complicated argument, which is: what the heck is a big [cometary] object like this doing in the middle of the Saturn system? And it doesn't provide an answer." In fact, it opens up a whole new mystery about what's going on with Enceladus and how it came to be. In a way, it's a mirror image of the surprising results of the Stardust mission, which found a comet to contain an unexpected amount of silicate mineral grains that must have formed close to the Sun and been transported to the outer reaches of the solar system before being incorporated into the nucleus of comet Wild-2. Waite laughed about this comparison: "Yes, comets are probably more complicated and less pristine than we imagined; and Enceladus is more pristine than we imagined. We're finding that solar system formation processes are a lot more complex than we've ever appreciated before." Plasma MysteriesINMS was not the only instrument actively sampling the plume. The Cassini Plasma Spectrometer, or CAPS, "is capable of seeing pretty high masses" as well, remarked CAPS principal investigator David Young. Data from his instrument were not presented at today's press conference because "We're still trying to figure out what it means." What he can report is that "as we flew through the plume our instruments picked up a huge signal. In fact, there were more counts in our electron spectrometer than we've ever seen before. It's beginning to look like what we're seeing are very small particles, from individual water molecules on up to nanogram-sized particles, essentially ice crystals. "There are a couple of funny things about the data. We measure both negatively and positively charged molecules, and they look different from each other; they're located in different places within the plume, and we don't understand why that is. If there are both positive and negative charges they ought to be all mixed up together, and they're not." Also, he said, "the ice particles that we're seeing are associated with the actual structure of the plume; we're tracing them back to where they came out of the surface, and we're seeing details of the plume structure." Although the CAPS data is generally consistent with the INMS data, there are some areas of disagreement: "We're seeing water ions, but we're also seeing more nitrogen than Hunter [Waite]'s instrument, and we're puzzled by that. He sees a couple of percent, and we see 10 percent."
Young sees a family resemblance to comets in more than just Enceladus' composition, having been the principal investigator for a similar instrument on the Deep Space 1 mission to comet Borelly. "The ices may have all formed the same way [on Enceladus and comets], so it wouldn't be too surprising if the ices had the same composition. What we see coming out of Enceladus, in imaging, reminds me of Borelly in a lot of ways. Why they should be so similar, I don't know. But if you've got a hole, and you've got supersonic flow coming out of it, then the walls tend to collimate the flow" into jets as seen in both comet Borelly and Enceladus' south pole. The loss of the data from the Cosmic Dust Analyzer (CDA) was keenly felt by both Waite and Young. CDA can measure the sizes and compositions of particles that are much larger than the gases measured by INMS and ions measured by CAPS. A new version of the software that drives CDA had been uploaded to Cassini especially for this encounter, to allow the instrument to increase the rate of particle hits that it could record. The instrument was switched to the new software shortly before closest approach, and switched back shortly after. For as-yet-unexplained reasons, the new version of the software failed to work, and the instrument did not collect any data during closest approach, although it did collect data before and after. Presumably this problem will be corrected for future flybys. "We're going to be interested in seeing if we can map a whole continuum of sizes; that tells you the process of how the particles get lifted off the surface," Young said. It's Hotter Than We ThoughtAlthough the main science focus of this flyby was for CDA and INMS to collect in situ data on the composition of the plumes, the remote sensing instruments were also hard at work. In particular, the Composite Infrared Spectrometer (CIRS) took a close look at the south pole of Enceladus once the in situ measurements were complete, mapping the thermal radiation from the fissures in Enceladus' crust that were determined during the 2005 flyby to be the sources of the plumes. CIRS had acquired a similar map during that 2005 flyby, but the March 12 map has four times better resolution, permitting a much more detailed analysis of where are the hottest places on Enceladus, explained CIRS team member John Spencer. A comparison of the old and new maps is below.
The old CIRS map averaged out temperature variations between the fissures, or sulci, and the colder icy plains between them. The newer map, with higher resolution, does a better job of separating the hot temperatures of the sulci from the plains. The better separation allowed the CIRS team to measure even higher temperatures along the sulci than they had reported before. The temperatures are now reported to be at least 180 Kelvin (minus 93 Celsius / minus 135 Fahrenheit), in sharp contrast to the colder-than-72 Kelvin (minus 201 Celsius / minus 330 Fahrenheit) measured elsewhere on the moon. "The highest temperatures we'd seen on the previous flyby in July 2005 were much lower, around 145 Kelvin (-198 Fahrenheit) -- not because Enceladus' fractures were cooler then, but because the older, more distant, scans were less sensitive. If it's 180 Kelvin on the surface it must be even warmer down below, perhaps approaching the magic number of 273 Kelvin or +32 Fahrenheit, where Enceladus' ubiquitous ice can melt to form the holy grail of astrobiology, liquid water," wrote Spencer in a weblog entry on the results.
The new scan revealed the temperatures to vary sharply as you cross the south polar sulci, but the temperatures are "remarkably smoothly varying along the tiger stripes," Spencer said. So, despite the jet-like appearance of the plumes, heat is radiating out of the sulci more or less evenly along their lengths.
This new temperature map of the south pole "is going to provide us a nice road map for the future, where to concentrate for the next flyby," Spencer said. He explained that this map is the most complete scan they'll get of the south pole, but that future flybys will allow them to view selected areas at higher resolution. The next flyby, "Rev 80," which will take place on August 11, will allow CIRS to get an even higher-resolution map of Damascus sulcus, the hottest area on the map. He also remarked that he hoped to get a better look at the unexpectedly warm region away from the pole near longitude 180, near the top of the CIRS map, at the ends of Alexandria and Cairo sulci. But Is There Water?At today's press conference, Larry Esposito discussed the elephant in the room: the question of Enceladus' astrobiological potential. "We see on Enceladus the three basic ingredients necessary for the origin of life. We see water, although it may not be liquid. We see organic molecules. And we see heat. These three ingredients are a minimum for the origin of life," though, of course, the question of whether any of Enceladus' water really is liquid could be a showstopper. "We cannot yet tell or state whether the interior of Enceladus contains liquid water or whether it might be a habitat for life. These are the questions that Cassini will focus on in coming flybys, and what the connection might be to a possible habitation for life." In a feature posted today on the Cassini website, astrobiologist Christopher McKay also addressed the question: "Could microbial life exist inside Enceladus, where no sunlight reaches, photosynthesis is impossible and no oxygen is available? The answer appears to be, yes, it could be possible. It is this tantalizing potential that brings us back to Enceladus for further study. The first step toward answering the question of whether life exists inside the [putative] subsurface aquifer of Enceladus is to analyze the organic compounds in the plume. Cassini's March 12 passage through the plume provided some measurements that help us move toward an answer, and preliminary plans call for Cassini to fly through the plume again for more measurements in the future." However, Cassini is not capable of answering the question of whether life is -- or was ever -- there. Ultimately, it will take a future mission, one that could sample the surface or even interior of the moon, to begin to seek real clues to the answer.
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