Afterword to the Paperback Edition
Since we completed the text of the hardcover edition of Titan Unveiled in 2007, Titan has been a busy place, scientifically speaking. Not only has Cassini continued to make observations, but more and more scientists have been analyzing its data, pursuing related laboratory investigations, and working with computer models.
A plot of the number of papers in refereed scientific journals with the word “Titan” in the title dramatically illustrates the heightened pace of activity when compared with the same data for Jupiter’s moon Europa, another solar system object that attracts considerable interest. A decade ago Titan merited only a single session at the DPS conference; in 2009 it demanded four.
MAPPING
Every Titan flyby brings the opportunity for new observations. In some cases these have allowed study of phenomena that vary over time, such as clouds, or have teased out variations with latitude of the atmosphere’s composition. An obvious metric of the accumulation of knowledge, though, is the proportion of Titan’s surface that has been mapped. At the time of writing (November 2009), the high-resolution coverage by radar is about 35 percent. As the coverage grows, different kinds of science emerge. Now it is possible to consider the distribution and arrangement of features rather than just to recognize and explain the features themselves.
Figure A.01. The number of papers in refereed journals with “Titan” in the title has shot up to over a hundred a year, driven by the avalanche of Cassini data. “Europa” is plotted for comparison and shows a corresponding, although rather smaller, jump in response to Galileo’s arrival in 1995. Except for that jump, Titan attracts almost four times more interest than Europa since there is more to observe at Titan, such as seasonal changes, clouds, etc.
For example, Jani Radebaugh at Brigham Young University has painstakingly mapped out some 15,000 individual sand dunes in the radar data. Dunes are found only in a band about 30 degrees either side of the equator, yet cover almost 40 percent of this low-latitude area, equivalent to some 20 percent of Titan’s total surface. As mentioned in chapter 6, Titan’s dunes seem to be almost exclusively of the linear or longitudinal type, delineating an average wind direction. Ralph Lorenz and Radebaugh boiled down the statistics of the dune locations and orientations to produce a map showing the average direction in 2-degree (~100-km) square boxes. The dunes show intriguing local deviations, parting around Xanadu, for instance, but the overwhelming impression is of a general transport of sand eastward.
This poses an interesting puzzle. While global circulation models (GCMs) seem to predict low-latitude drying consistent with the concentration of dunes around the equator, all of them also suggest that the low-latitude winds near the surface should be toward the west—exactly the opposite of what the dunes indicate. This isn’t just an issue with any particular model. There is something fundamental about the balance of angular momentum in the atmosphere that is simply not understood. Indeed, one modeler noted that it was almost as strange as if “water flowed uphill.” Sooner or later, the modelers may find a way of making the winds they generate in cyberspace act the way Titan’s winds seem to in reality, but at present, the discrepancy is sufficiently perplexing to cause the modelers to ask whether the interpretation of the dune images is correct, or whether the images could somehow be back to front!
Figure A.02. A map showing the direction of sand transport indicated by dunes mapped in radar data, overlain on a basemap of near-infrared images from the Cassini ISS instrument.
Another dataset that has seen dramatic improvement over the last couple of years is high-resolution surface observations by VIMS. VIMS sees much less at a time than RADAR does but has now covered a few percent of the surface. The best VIMS observations yield information about the structure of the surface at a resolution comparable with the radar, allowing new insight into dunes, river channels, and craters. Furthermore, the higher-resolution observations are better able to pinpoint areas of specific composition, instead of diluting the spectral signatures of molecules over a wider, varied region. For example, there is an indication that aromatic molecules (such as benzene) are associated with the equatorial dune fields and that Ontario Lacus in the south contains liquid ethane.
Ontario was also found by a radar altimetry observation in December 2008 to be literally flat as a millpond. Not only was the whole structure flat to a few tens of meters or better across its 70-km width, but the statistics of the radar echoes, analyzed by Lauren Wye at Stanford, showed that the lake reflected radar like a perfect mirror, having a height variation over 100-m-wide patches of no more than 3 mm. This is something of a surprise given the thicker atmosphere, lower gravity, and lower density of Titan liquids, which should make waves easier to generate than they are on water on Earth (see chapter 7). Perhaps Ontario is tarry so that its viscosity damps waves out.
As Titan’s seasons march onward, the Sun is starting to rise over the northern lakes and seas, and indeed VIMS caught an iconic image of the Sun reflected on Kraken, the largest sea. Radar images made during the northern winter (2006–2007) showed that these lakes, too, seemed to be rather flat and smooth, but circulation models suggest that the winds should freshen as we move into summer. Maybe they’ll pick up to the point of generating waves.
THE SPIN ON TITAN’S INTERIOR
One major outstanding unknown is the nature of Titan’s interior. Does Titan possess an internal ocean of liquid water, perhaps containing some ammonia too? This is a question toward which Cassini’s radio science investigation has long been addressed, by measuring Titan’s gravity field at different points in its orbit around Saturn, to determine how much Titan is deformed by the changing tidal forces. Significant deformation would imply that Titan is flexible, and that its ice shell is thin. However, Titan’s gravity field has proved to be more complex than anticipated and a quick answer has not emerged.
Despite this challenge, another window into Titan’s interior unexpectedly opened up, one relating to how Titan rotates. Specifically, Titan should be in what is called a “Cassini state,” named after our mission’s eponymous Italian-French astronomer who worked out the theory. Put simply, what it means is this. Gravitational perturbations cause Titan’s orbit to precess. That is, the orientation of the orbit wobbles around. The easiest way to describe its orientation is by the direction in space of a line at right angles—or normal—to the orbit. This normal to the orbital plane sweeps around a cone in space. The place it points to on the sky, the pole of the orbit, moves around a little circle once every six centuries or so. The center of this circle is called an “invariant pole” (although, as we’ll discuss later, it isn’t really invariant). At the same time, the body of Titan is turning around its spin axis. That axis points to a spot on the sky called the “spin pole.” Titan would like to rotate so that its spin axis is aligned with the normal to its orbit. If that were the situation, the spin pole and orbit pole would coincide on the sky. But because of the precession phenomenon, it can’t happen that way. Both the spin pole and the orbit pole precess around the invariant pole at the same rate, as if they were both on the same spoke of a wheel, but one is farther from the invariant pole than the other. The size of the offset between them depends on how mass is distributed within Titan’s interior. If most of the mass were concentrated in a dense core, the offset would be small. So if we could measure the position of the spin pole, we could estimate the size of Titan’s core. At least that was the theory.
Figure A.03. A high-phase angle 5-micron image of Titan acquired by VIMS in summer 2009. One spot is much brighter than the rest of the crescent, where the Sun is reflected off the mirror-like liquid surface of Kraken Mare.
RALPH’S LOG, MAY 2007
The strange turn that the story of Titan’s spin took resulted from the collision of two improbabilities. Once the radar imagery accumulated with enough overlaps to match up features from different flybys, we found that there could be mismatches of several tens of kilometers that could not be accounted for by spacecraft navigation errors. These mismatches could be eliminated if one adjusted the pole position and rotation state of Titan from what that they had been assumed to be, namely, perfectly synchronous rotation (that is, always keeping exactly the same face toward Saturn) about an axis at right angles to the orbital plane. Specifically, features matched up best if Titan’s spin axis were tilted from the normal by 0.3 degrees, roughly the amount that had been expected from the Cassini-state theory. But when Bryan Stiles of the radar team reported his preliminary fits to the data, he noted that the best fit was with a rotation rate that was not quite synchronous. I was about to email him pointing out that this couldn’t possibly be right when I remembered a talk given a couple of years earlier by Tetsuya Tokano.
Tokano had been working with his Titan GCM and had been inspired to look at the seasonal variation of the angular momentum budget in the atmosphere. (He was working on Mars models at the time too.) He noted that the angular momentum of Titan’s atmosphere varied significantly over the course of a long Titan year, as the zonal winds near the surface changed direction. But this angular momentum has to go somewhere—it is communicated to the ground by friction and by pressure differences across mountain ranges. On Earth, this leads to a variation in the planet’s rotation rate causing the length of a day to vary by about a millisecond over the course of a year. But on smaller Titan, with a thicker atmosphere, the effect could be rather more significant. And in particular, if Titan had an internal ocean, then the atmosphere would drag only on the outer icy crust. Its rotation rate would change significantly, since the massive rocky core would be decoupled from the rotation of the crust. Tokano calculated that, if an ocean were present, the seasonal change in rotation rate might give displacements of some 100 km in apparent longitude— at the time an absurdly large-sounding anomaly.
The discrepancies we noted when matching the radar swaths up implied a nonsynchronous rotation not too different from what Tokano had suggested. The implication was, then, that Titan did have an internal ocean, and that the atmospheric rotation changed significantly over the course of a year. The interpretation of the rotation rate was published in Science. It attracted a fair degree of attention and stimulated new work. In the following year, papers came out exploring how different assumptions about the methane cycle might affect the rotation rate (i.e., how the rotation might act as a diagnostic of other aspects of the Titan climate), how the pole position might relate to the distribution of mass in Titan’s interior, and, for good measure, how there could be gravitational coupling between the crust and interior, such that nonsynchronous rotation shouldn’t occur after all.
Indeed, two years later, with the benefit of additional data and independent analyses, the nonsynchronicity appears to be rather smaller than initially determined. However, it has not disappeared altogether. In the meantime it has been recognized that there are many subtleties to the problem. For example, it turns out that the perturbations to Titan’s orbit cause the orbit pole to trace out an epicyclic pattern—sort of whorls within whorls—not a simple circle. And so as Titan’s spin axis tries to follow this variation, sometimes it will line up with the orbit normal, sometimes it won’t. Certainly, the relationship between the pole position and the distribution of mass inside Titan is not as simple as we’d thought.
Whatever the outcome, the story underscores the interdisciplinary appeal of Titan science, which embraces issues more usually confronted on the terrestrial planets. After all, measuring rotation states is basically an astronomical problem, while the changing zonal winds on Titan is an issue largely for meteorologists. And yet these disciplines intersect in Titan’s interior, usually the exclusive preserve of theoretical geophysics.
SCHUMANN RESONANCE
Another, similarly esoteric, perspective on the interior came from one of the more arcane Huygens datasets, that from the Permittivity and Wave Analyzer (PWA) on the HASI instrument. This experiment recorded electrical activity, in the hope of detecting lightning discharges. While there was no signal that could obviously be attributed to lightning (al-though there were some spurious claims that lightning was indicated), some electrical activity was detected during part of the descent. This took the form of a weak electric field varying at about 36 Hz, which might have been a phenomenon called a “Schumann resonance.” This is a class of signal that results from a resonant cavity formed between the conductive ionosphere of a planet, and a conductive surface. On Earth this resonance is set off by lightning discharges—an electrical, planetary equivalent of blowing across the top of a bottle to generate a note. If the signal detected by Huygens is real, it appears to have two interesting implications. The first is that there is a cavity, which suggests a conductive lower boundary—most likely the upper surface of a water-ammonia internal ocean. The second is that the orientation of the electric field is different from Earth’s, suggesting a different excitation mechanism, which some people think might be Saturn’s magnetosphere rather than lightning.
Examining data from the radio emissions detector on Cassini collected during the first thirty-five flybys, Georg Fischer of the University of Iowa found no evidence of lightning. This analysis showed that, if Earth-like lightning were present on Titan, it must be a hundred million times rarer than lightning on Earth, with a flash no more frequent than once every ten days.
DEWDROP
Although the view of Titan’s surface from above and the scene after landing are what everyone remembers from the Huygens DISR, the full quantitative interpretation of the data—in particular, the derivation of the optical properties of the aerosols and how they vary with altitude— took a couple of years. And among the loose ends in the Huygens data were the 200-odd superficially identical images taken after landing.
A painstaking analysis of these images was undertaken by Erich Karkoschka. In principle, they all show the same scene, but very careful measurements of the horizon show the probe to be tilting by a fraction of a degree over an hour, and the auto-exposure algorithm changes the brightness of images. But eleven of the images also show small spots that are not present in other images, and the speculation had been offered that perhaps these were splashes of rain.
Delving deep into the statistics of the data, Karkoschka showed that all but one of these spots were similar in size and intensity to cosmic ray hits on the detector. But in one image, a bright feature spans some 20 pixels, corresponding to something about 2 cm across on the surface, or smaller if it were closer to the camera. By simulating the out-of-focus reflection from a spherical particle, Karkoschka matched the feature with a 4-mm-wide drop of liquid, about 8 cm in front of the camera, in the beam from the DISR lamp. The best explanation, since raindrops were not observed at any other time, seems to be that the heat from the lamp sweated methane vapor off the ground. A 20-watt lamp doesn’t sound like much, but the power density on the patch of ground beneath it was thousands of times higher than the strength of sunlight on Titan. Some of this vapor condensed on the cold camera baffle, dripping back down at the rate of a few drops a minute, and one of these drops just happened to be caught streaking at 0.5 m/s through the camera’s field of view during the 14-millisecond exposure of image #897. Even as we grapple to understand Titan on the planetary scale, these studies of the minutiae of Huygens’s interaction with the Titan environment give us vital real-world experience with which to plan future missions.
FLAGSHIP
In late 2006, NASA began to contemplate future missions to the outer solar system. A short study first established that any future mission to the Saturnian system had a tough act to follow. It would have to do much better than Cassini, at least in some respects. This meant that a follow-up mission to Titan would have to do more than just fly by.
Then, in early 2007, NASA commissioned a 
1-million study to define a “flagship” mission, meaning a mission with a “soft” budget cap of 3 billion. This was serious. They commissioned similar studies for Europa and Ganymede, and for Enceladus. Science definition teams (SDTs) were appointed from the community. The Europa team, reflecting the fact that most published Europa science has centered on ice physics and structural geology, was chaired by two geologists, Ron Greeley and his former student Bob Pappalardo. The Titan team, reflecting the much broader range of science at that world, was chaired by a much “odder” couple, Ralph Lorenz and Hunter Waite, the latter a chemist and the leader of the INMS team on Cassini.
More so than for the other targets, perhaps, a plethora of options presented itself for Titan. A flagship mission must enjoy the support of a broad cross-section of the scientific community, and so it must address a wide range of scientific goals. It would include an orbiter, of course, to act as a communication relay for other elements, to perform global mapping, and to study the interaction of Titan with the Saturnian magneto-sphere and the solar wind. But what else?
A technical team, led at the Johns Hopkins University’s Applied Physics Lab (APL), developed designs for the various elements, with help from JPL and NASA Langley. This “wasn’t study land” anymore, as one APL engineer put it—the task was to present a detailed mission concept that was credible for a launch in the late 2010s. Airships and helicopters were out. Using clever new technology like nuclear hot air balloons was all very well, but a development plan, schedule, and costing would need to be laid out.
The consensus of the SDT was that an orbiter, a balloon, and a lander would meet the broad range of objectives that the team defined. Actually, two balloons and two landers would have been nice, but would break the bank. The key was that the “one-of-each” mission could fit onto a single launch vehicle.
The orbiter would be a central element. It is no coincidence that most of the missions launched to Mars in the last twenty years have been orbiters. They offer high science return for modest risk, although of course there are some things they just can’t do. Cleverly, the orbiter could exploit Titan’s atmosphere to brake into orbit, a technique called aerocapture. This would allow the orbiter to carry an arsenal of a dozen instruments and enough fuel to orbit Titan for four years. Even within the first three days, this orbiter would spend more time close to Titan than Cassini would in its extended mission, and its novel, more powerful instruments would explore Titan in new ways, for example, using a long-wavelength radar to probe the subsurface and a microwave spectrometer to measure the upper atmosphere winds.
The SDT was charged with assigning a priority to the different parts of the mission, and scores to each instrument in relation to eight major mission goals and many measurement objectives. The scores were compiled into a document that got nicknamed “the mother of all spreadsheets.” Surprisingly, although everyone instinctively liked the idea of a Titan balloon, the lander (destined to land on the dune fields, where it would be safe from the risk of encountering rocks or a gully) scored higher scientifically, its long-duration meteorology and seismology measurements and a powerful chemical analyzer having the potential to answer more fundamental questions than a balloon.
In early 2008, NASA instructed that two of the four studies—Europa and Titan—would be pursued further, but initially with a 2-billion cap. (This was later relaxed after staff changes at NASA.) Further, the studies were to be conducted in cooperation with the European Space Agency, which was itself considering future mission possibilities.
But the terms of the new study did Titan few favors. Aerocapture was ruled out, negating a major advantage of Titan as a destination. Moreover, because ESA was not committed to any outer solar system mission, the main objectives had to be accomplished with the NASA-only element, which was specified by NASA headquarters to be the orbiter. This left the “sexier” lander and balloon for ESA—hardly an arrangement likely to appeal to NASA centers.
Jonathan Lunine and Athena Coustenis bravely led the SDT for this new study, named Titan Saturn System Mission (TSSM), which NASA HQ had directed should also study Enceladus. But the 2-billion TSSM without aerocapture could afford no more than a handful of instruments on the orbiter, which would spend only two years at Titan, and instead of a long-lived lander on the dunes, just a battery-powered lake lander, much like Huygens. The Montgolfie balloon, however, was retained.
The other study that went forward was the Europa Jupiter System Mission (EJSM), with a NASA Europa orbiter and an ESA orbiter to visit Ganymede. TSSM and EJSM were evaluated in detailed reviews in late 2008. The science review panel was directed not to make any comparative judgment about Europa versus Titan, and concluded that strong science would result from either mission. The technical review determined that the Europa mission was more mature, which was hardly a surprise since Europa orbiters have been studied for almost a decade, and that somehow the radiation challenges that confront Europa missions would be magically solved. Politics was doubtless at work too.
It was a bitter blow for the Titan community, although not entirely unexpected. Some cynics were even at work on a backup plan . . .
CRYOVOLCANISM
We alluded in chapter 6 to the possibility that Titan might have active cryovolcanism. The main evidence offered early on, that there were apparent changes in brightness at Hotei Regio, seems to have been refuted by many members of the VIMS team. However, Hotei remains unusual and has been the target of follow-up observations. Higher-resolution VIMS data showed finger-like features, suggestive of lava flows. Radar images acquired in 2008 similarly showed features that resembled lava flows. But morphology is always subject to various interpretations. The compositional interpretation of the VIMS spectra remained challenging, with different factions of the VIMS team advocating different materials—one suggesting ammonia, another suggesting carbon dioxide. Either way, though, it is different stuff from the rest of Titan.
Figure A.04. An artist’s impression of the Titan orbiter from the TSSM study. The long antennae are for a ground-penetrating radar to measure the depth of Titan’s lakes and look for subsurface layers.
The discussion about cryovolcanism also became politicized somewhat, since cryovolcanism means water and, especially with Titan’s rich organics, the connections reasonably arise in peoples’ minds between water, prebiotic chemistry, and life. Claims of cryovolcanic activity on Titan would make Titan more astrobiologically appealing as a target for future exploration. This interest became acute in the context of the competition between Titan/Enceladus and Europa to be the destination for the next flagship mission in the outer solar system.
Jeff Moore and Bob Pappalardo, both supporters of Europa exploration, gave a presentation at the fall meeting of the American Geophysical Union in late 2008 advocating the perspective that Titan was geologically inactive or, as their title put it, “Callisto with weather.” They noted that some of the evidence offered in support of volcanism was not totally persuasive. Although this was not unreasonable, there were certainly some people who considered that the talk was conceived specifically to erode the case for Titan as the flagship destination—“teaching the controversy,” as intelligent-design advocates would put it. Still, no scientist is without bias: it is, after all, those biases and prejudices that push a scientist to look beyond what is obvious to everyone else.
While the majority of the community remains unconvinced that there is good evidence for literally “present-day” activity, the fact that the flow-like morphologies are associated specifically with the areas identified by VIMS as compositionally distinct seems to support the idea that this material was emplaced from the interior in the geologically recent past. What that means in terms of the actual age of Hotei and Tui is still unknown. Just because we do not yet have good evidence that Hotei is active at present doesn’t mean it isn’t . . .
TITAN IN THE MEDIA
By 2009, it seemed that Titan had become more widely recognized as a world of interest not only by the scientists working on it but by the public at large and the media. The fact that very Earth-like processes are going on there, on top of the generic appeal of an icy moon drenched in organic chemicals, made it even more relevant. And TV broadcasters like Titan because they can use various dramatic terrestrial landscapes as backdrops for a talking scientist.
RALPH’S LOG, OCTOBER 2009
I had done a handful of TV shoots on location over the years, but in 2009, Titan’s media coverage somehow took off. And so did I, in various aircraft. First, in January, I flew in a hot air balloon with the record-breaking balloonist Julian Nott over the Mojave Desert in California, extolling the virtues of that kind of vehicle for Titan exploration. (There was also a rather surreal demonstration of the decoupling of Titan’s surface rotation from its core involving a large salad bowl, a melon, some water, and dry ice, but the less said about that, the better . . .) Then, in May, I got to buzz around Lake Mead and nearby Las Vegas in a helicopter—with the door open, no less—describing how the landscape resembled that of Titan’s northern lakes. Another TV show set up interviews at the Cassini Project Science Group meeting in London, and followed them up with the filming of myself and Jani Radebaugh on a field trip to linear dunes in the Simpson Desert in Australia. September saw me slipping out early from the DPS conference in Puerto Rico to join a BBC film crew in Alaska—a mere 72 hours from snorkeling among Caribbean angelfish to clambering around in crampons and helmet on the Matanuska Glacier.
LAKES AND ASTRONOMICAL CHANGE
Soon after T39, the first radar coverage of Titan’s deep south showed in late 2007 that there were indeed few lakes in that hemisphere compared with the north (as had been hinted at in the optical data). The question arose of why this should be. In a way, this is a very Martian kind of question, since the dichotomy between Mars’s two very different polar caps is a similar puzzle. Several possibilities sprang to mind. Perhaps this was a seasonal effect. If the lakes were shallow and full of volatile methane, then they could evaporate during the summer and condense in the winter hemisphere. Guiseppe Mitri, an Italian postdoc working at JPL, showed that several meters of methane could in principle evaporate from a Titan lake in a single Earth year. This assumes heat is drawn from the atmosphere, and so works only for lakes that cover a small fraction of the surface, since, as we noted in an earlier chapter, there is only enough sunlight averaged planetwide to evaporate a centimeter or two of methane per year.
Another possibility might be that, like Mars, the northern and southern hemispheres are different at or below ground level. For example, on Mars, the rugged southern hemisphere is generally several kilometers higher in elevation than the northern plains. However, as more and more altimetry and other topographical data came in from Titan over 2008, it became clear that, while the polar regions were indeed both lower in elevation than the equatorial region, they were similar to each other. This doesn’t rule out some difference in geothermal heat flow, or porosity, or some other property, but at first blush there seemed to be no internal explanation for the asymmetric distribution of lakes.
There was another, perhaps more subtle, possibility. Because Saturn’s orbit around the Sun is eccentric, summer is shorter but more intense in the south than in the north. However, this has not always been so, nor will it. The orbits of the planets, and the orientation of their rotational poles, are subject to dynamical evolution, in part because of the gravitational perturbations due to Jupiter, among other things. The role of such changes in forcing the Earth’s ice ages was first quantitatively outlined by the Scotsman James Croll in the 1860s, although the cycles are often attributed to the Serbian mathematician Milutin Milankovitch, who just refined the calculations somewhat.
Figure A.05. Polar maps from radar data showing dark regions of liquid. In the north, Ligeia Mare and Kraken Mare, some 500 and 1000 km across, respectively, account for hundreds of times more liquid methane and ethane than all the oil and natural gas on Earth. Yet in the southern hemisphere there are only Ontario Lacus (250 km long) and a handful of small lakes. (Figure courtesy of Alex Hayes, Caltech)
While the total amount of sunlight deposited on Titan’s north and south poles is the same (since the shortness of summer in the south compensates for the stronger sunlight), the nonlinearities of any climate system mean that there is sure to be a net differential effect. For example, hotter regions radiate more heat to space, so the net heat input in the south might be lower than in the north. Another profound factor is the strong dependence of vapor pressure—and thus evaporation rate—on temperature. The hotter southern summers might well have the effect of drying out the south and allowing moisture—methane and ethane—to accumulate in the north, albeit slowly. So Titan’s northern lakes may have been growing for the last tens of thousands of years, but perhaps 50,000 years ago Titan may have had more lakes in the south than the north. And this alternating cycle of liquid deposition may explain why so few craters are found at high latitudes.
CASSINI ENTERS HER TEENS
For years now at Cassini Project Science Group meetings, spacecraft managers Earl Maize, and later Julie Webster, have reported on the health of their distant charge. Cassini—now past the mission for which she was designed—is doing remarkably well. But despite occasional updates to her onboard software, she is aging. The reaction wheels, which had caused such concern back at Jupiter in 2000, continue to be monitored for signs of wear and friction. So far, so good. But the power output of the radioisotope thermoelectric generators (RTGs) inevitably creeps lower with the years, and the hydrazine fuel, though carefully husbanded, nonetheless gets used up and will one day run out.
The history of planetary exploration shows that, if spacecraft do not catastrophically fail for an unplanned reason, they can often last well beyond their design lifetimes, the Mars rovers Spirit and Opportunity, now 2,000-plus days into their “90-day” missions being cases in point. Indeed, it had been expected that Cassini might well keep functioning, and plans for a two-year extended mission (“XM”) were made. At the time of writing, this XM, featuring some twenty-six Titan flybys as well as several spectacular visits to Enceladus, is well underway. And Cassini is still going strong. So little science has been lost to onboard problems that the occasional loss due to arguably more correctable problems on the ground (such as the total loss of T60 due to issues at the Goldstone Deep Space Network station) are particularly frustrating. The only degradation of note on the spacecraft is that the roar of its thrusters began to stutter, perhaps due to a buildup over the years of some contaminant on the catalyst beds in the thrusters. This led managers to switch to a spare set of thrusters, which will have to be watched carefully for the same effect.
And when the XM ends in July 2010, what then? Cassini still works, there is fuel in the tank. Why stop there? This question naturally arose soon after the end of the prime mission—but simply asking for more of the same does not sell well politically. There needs to be a hook, a theme. During these discussions, it was Larry Soderblom, perhaps emboldened by his experience working with the Mars rovers, who boldly suggested “go for the solstice.” After all, observing seasonal change would be a key motivation for continuing the mission. But this would necessitate Cassini operating for yet another seven years!
The mission designers worked their magic again, designing an orbital tour that would address the various science goals at Titan, at the rings, and so on. But this time, the philosophy was not to cram as much science as possible into a short period but to conserve fuel to the maximum extent possible. The question of “disposal” comes up too—the absurdly remote probability of Cassini and its plutonium being slung out of the Saturnian system cannot be left to chance. Thus an “endgame” to the mission was planned, in which Cassini is caused to burn up in Saturn’s atmosphere (as the Galileo spacecraft did at Jupiter). But this would follow an exciting terminal series of low orbits, zipping between the rings and Saturn’s atmosphere, something that would only be dared after all the other mission goals had been achieved.
The costs of running Cassini would have to be brought down, meaning a less-intense pace of flybys. Nonetheless, if Cassini lasts through to northern summer solstice in 2017, it will fly past Titan another fifty-six times. At the time of writing, NASA is expected to approve this “Solstice Mission,” but has not yet formally done so. The planning process, however, cannot wait.
Thus, in mid-2009, Cassini Titan scientists found themselves trying to digest the data from the latest flybys (T49–T54), planning the details of the minute-by-minute turns and observations in XM (now retrospectively renamed the “Equinox Mission”) T64 and T65 in December 2009 and January 2010, and at the same time debating which instrument will be prime at closest approach at 6:21 a.m. on April 22, 2017!
TIME TO GO BACK TO TITAN
Paradoxically, the NASA/ESA decision to prioritize Europa as the flagship destination may mean that one or more smaller missions to Titan happen earlier than if Titan had been chosen for the flagship role. As NASA geared up for its flagship mission, whatever that would turn out to be, it recognized that outer solar system missions would need to be radioisotope-powered. In the late 1990s and 2000s, NASA’s supply of plutonium-238, sourced principally in Russia, began to dwindle. The RTGs used by Cassini and other spacecraft are exceptionally reliable, since they have no moving parts, but are somewhat inefficient in that they convert only about 5 percent of the heat from Pu-238 decay into electricity. So for a given electrical power need, one needs a lot of Pu-238. However, it is possible to use instead a more efficient mechanical generator, such as a small Stirling engine, that would use four times less Pu-238 to generate a given amount of electricity. One RTG design considered for future missions used Pu-238 generating some 2000 W of heat in order to produce 100 W of electricity. In contrast, an advanced Stirling radioisotope generator (ASRG) could produce the same 100 W of electricity using only 500 W–worth of heat from plutonium—a four-fold improvement in efficiency. But a Stirling engine does have a moving piston, and so NASA was wary of building a flagship mission around such a power system without proving it in space first.
So, in 2008, NASA sought studies of small missions that might use ASRGs. The solicitation was called the Discovery and Scout Mission Capability Enhancement (DSMCE), Discovery and Scout being “small” planetary mission classes (400-odd million) that NASA funds every few years. Expanding this slightly for phonetic ease, the results were dubbed the “Das Mice” studies. One of these, led by Cassini radar scientist Ellen Stofan, was the Titan Mare Explorer (TiME). The idea was basically much like the original concept for the Huygens probe, which, after all, was half-expected when it was designed to land in a liquid. But there were some key differences. First, with the heat and power from an ASRG, TiME could last for months or years, rather than being limited by battery life and the cold environment. Second, without an orbiter (Discovery mission funding doesn’t allow enough money for one), TiME would have to communicate directly with Earth. But that would not be a significant problem, since TiME’s main results, from measuring the lake composition and depth to monitoring the weather conditions, do not amount to a large volume of data. An aeroplane or balloon, seeing new vistas every hour, would require a vast downlink capability to do justice to its observation opportunities. TiME would still take a few pictures, of the waves lapping against its hull, and of the polar summer sky, perhaps with clouds puffing up. And perhaps, as the mission went on, of the looming shoreline. Aimed at Ligeia Mare in the north, estimates are that TiME might drift from the center to the shore after several weeks.
Figure A.06. A “scorecard” showing the Titan and other encounters of Cassini during its prime mission, 2004–2008; its extended “equinox” mission through 2010; and the proposed “solstice” mission to 2017, ending with a series of “proximal” orbits skimming over Saturn’s cloud tops.
RALPH’S LOG, NOVEMBER 2009
My work on TiME—albeit just a proposal as I write this afterword in late 2009—is much more detailed than the work on the flagship studies: identifying vendors for all the various bits, laying out plans for integration and testing, the thousand-and-one details to chase if you are serious about building a spacecraft and its instrumentation for an alien world.
Even though the final deadline for the proposal is months away, we submit the present concept—science rationale, spacecraft design, operations plans, budgets and all—to an internal review panel convened at Lockheed Martin’s facility (where Viking, Pathfinder, Stardust, and many other spacecraft had been built) near Denver, Colorado. In a nine-hour Monday review we work through 300-odd presentation slides. Happily, I seem immune to jetlag, having flown in the day before from the Jordanian desert, where I had visited the Waqf-as-Suwwan impact crater, with a central uplift somewhat reminiscent of that on Titan’s crater Ksa. On that Saturday afternoon I had been 400 m below sea level, bobbing in the languid ripples on the Dead Sea and contemplating the effects of density on wave period. Tuesday I take a day off to get some early-season skiing done in the Rockies, to unwind after the exhausting review: -400 m to +3600 m in four days, quite an elevation change!
Some of the TiME experiments are copies or adaptations of the Huygens Surface Science Package. Somehow, despite a nineteen-year career in the space business arcing from ESA in the Netherlands, via the University of Kent in the United Kingdom, to Arizona in 1994 and since 2006 to APL in Maryland, I find myself confronting the same old questions. What are the winds like near the surface? How big are the waves? How long will a sonar echo take to bounce from the bottom of the lake? While I am physically firmly earthbound, and indeed getting to see a lot of fun places on my home planet, mentally I have been on Titan for nearly two decades!
CLOSING THOUGHTS
And so, we leave this afterword to the paperback edition in 2009 much as we left the hardcover in 2006, with the Cassini spacecraft—older now but still working well—continuing to reveal surprises and new mysteries at Titan, and with uncertain but promising prospects for future missions. But Titan is engaging ever-wider circles of scientists, and Cassini’s data are being looked at by more and more people. The adventure continues.