4. Cassini’s First Taste of Titan
With the tension of orbit insertion behind them, the science teams could indulge in their last speculations about Titan and get to work on the first results from Cassini’s initial approach and the T0 flyby. A major conference, COSPAR, took place in Paris at the end of July 2004 where these early findings were presented. Presentations from most of Cassini’s instruments focused on Phoebe, Saturn, and the rings, but the optical remote sensing instruments, VIMS and ISS, had dramatically new perspectives on Titan, too.
To: A GRANDSTAND VIEW
The ISS imager had a grandstand view of Titan’s south polar regions. Not only was the resolution it could achieve rather higher than the best from Keck and HST, but these polar areas were only ever seen slantwise from Earth, at the edge of Titan’s disk, where Cassini was looking more or less straight down on them. It would be a quantum leap forward, a factor of ten better than anything so far, and of an area never seen before.
Distinct bright and dark areas were visible, and details down to a few kilometers across could be seen—rather better than the gloomiest predictions about blurring due to the haze—although the real test of imaging detail would require getting closer than the 300,000 km distance of T0.
But these bright and dark patterns were, much like the HST map a decade before, rather inscrutable. There were some patches, some narrow wiggly lines (could they be rivers?), and some rather straight-edged features, hinting at the possibility of some sort of tectonics.
Figure 4.01. A Cassini ISS image of Titan’s southern hemisphere seen during the T0 flyby (see figure 3.14 for map). Intriguing dark patches and sinuous features are seen, together with brighter areas and a complex of bright clouds around the south pole. (NASA/JPL/Space Science Institute)
And above this still-mysterious surface, the now-familiar complex of south polar clouds was still active. A sequence of images, each a few hours after the previous one, showed the clouds moving in the wind, and evolving as they did so, just like on a series of satellite pictures of Earth.
The VIMS instrument, able to probe at a longer range of wavelengths, added some “color,” at least at low resolution, to our view of Titan. The VIMS team saw, much as Earth-based observers had, bright and dark areas on the surface. Some rather speculative interpretations were made of dark spots as impact craters or “palimpsests.” (A palimpsest is a patch of discolored surface, the scar of an impact crater left when the combination of the softness of the crust and the length of time since the crater formed has allowed the topography of the crater to relax by viscous flow. Several such patches are seen on Ganymede and Callisto.)
Figure 4.02. An individual frame of the ISS T0 observation. Blurry details a kilometer or two across are visible, notably the dark “roadrunner” in the center. Geological interpretation was difficult, although imaging scientists did note the straight edge of the feature at upper left, which is suggestive of some kind of tectonics. (NASA/JPL/Space Science Institute)
Among the familiar patterns of bright and dark on the surface was the hint of some compositional differences; not all bright and dark regions were exactly as bright and dark at all wavelengths. But until the effects of different slant paths through the atmosphere and of different Sun angles could be understood, solid clues to surface composition would remain elusive.
It was easy to see in the VIMS image that Titan was much “bigger” at some wavelengths. While the near-IR light in the methane windows that allowed surface mapping penetrated (obviously) all the way to the surface and defined Titan’s solid limb, at 3.3 microns, methane gas fluorescing in sunlight some 700 km above the surface showed that, in the stratosphere at least, the methane was fairly uniformly mixed. On the nightside, a faint glow from carbon monoxide at 4.7 microns extended some 200 km above the surface.
Figure 4.03. A movie sequence of the south polar clouds. These images were taken a few hours apart, and show that the clouds around the south pole are changing on short time-scales, suggesting they might be cumulus clouds with precipitation (methane rain or hail). (NASA/JPL/Space Science Institute)
The VIMS results also showed clouds that could be tracked from one image to the next, indicating tropospheric winds near the south pole of only 1 or 2 m per second. Furthermore, study of the spectrum in each pixel would allow the altitude of the cloud tops to be measured, and it would later be determined that the clouds were puffing up at several meters per second, not much slower than cumulus cloud tops on Earth.
Titan was too far off for Cassini to use its radar instrument, but the infrared spectrometer CIRS was aimed at Titan to measure the atmospheric temperature distribution. If there were going to be any big surprises in how Titan’s upper atmospheric winds change with season, they should be obvious in the temperatures too.
Within a day or so, Cassini had zoomed out from Saturn, leaving Titan behind for a while as the scientists puzzled over the new results and tried to anticipate what they would find later. Cassini reached its first apoapsis on August 27, nearly 151 Saturn radii from the planet (or over 9 million km). Like a captured comet, Cassini then began its long, lazy arc back toward Saturn. Near apoapsis, it fired its engine for another fifty-one minutes, bringing the periapsis up from close to the rings to some 300,000 km above Saturn. But first, its new course would encounter Titan on October 26, 2004, enabling Titan’s gravity to help bring the orbit period down and to keep Cassini firmly in the Saturnian system far more efficiently than Cassini’s engine ever could.
Meanwhile, on Earth during the Northern Hemisphere summer, it was still the conference season. One engineering-focused conference was the International Planetary Probe Workshop at the end of August, held that year at NASA’s Ames Research Center. Preparations for Huygens were reported, notably the ongoing reevaluation of the heat shield’s expected performance and the environment it would have to deal with; there were worries that the heat shield might only just be adequate. Also, the Huygens Atmospheric Science Instrument teams reported on a “dress rehearsal” of the descent, made with a mock-up of the Huygens probe dropped from a balloon over Sicily. Gaining familiarity with data would be key to assessing the quality of the data from Titan and developing ways to analyze the data efficiently.
Figure 4.04. Titan seen by the VIMS instrument during T0 at three different wavelengths. Left to right, the wavelengths are 2, 2.8, and 5 microns. The south polar clouds are bright at all wavelengths. At 2.8 microns the surface is mostly dark, with the hazy limb somewhat bright. At 5 microns, where water ice is not reflective, the correlation with the 2-micron (and 0.94-micron) bright and dark areas suggests that ice is not the main “brightening agent” on Titan. (NASA/JPL/University of Arizona)
But as thoughts began to turn toward the Huygens release at the end of the year, another planetary entry was to take place. On September 8, 2004, NASA’s Genesis capsule returned to Earth, bearing samples of the solar wind that it had been collecting in space for three years. But instead of gliding down under a parachute to be caught gently in midair by helicopter, it smashed into the desert. The g-switches used to start the parachute deployment had been mounted the wrong way up and so had not triggered. Although most of the scientific results were salvaged, it was a very visible failure. Coupled with the loss of ESA’s Beagle 2 Mars lander nine months earlier, it was a reminder that entry, descent, and landing are always a challenge. And it made everyone nervous about Huygens.
Figure 4.05. Glow-in-the-dark Titan. This presentation of high-phase (crescent) Titan images from VIMS during T0 shows, at left, a methane window image (seeing down in natural sunlight to the surface, hence the smallest-diameter image). Next is an image at 3.3 microns, where methane fluoresces. Since this glow is stimulated by sunlight, it appears also as a crescent. The third image is at 4.7 microns, where carbon monoxide causes emission in the warm stratosphere. This thermal glow can be seen even on the nightside. The rightmost image is a composite of the other three. (NASA/JPL/University of Arizona)
TITAN IN FRONT OF THE CAMERAS
TA, the first close Titan encounter, would be the first opportunity to try out all of Cassini’s instruments on one of their principal targets. It was therefore a particularly busy encounter, requiring the spacecraft to pirouette and roll like Jackie Chan in a martial arts movie. In the hour or two around closest approach, the bus-sized craft would observe Titan optically, then swing around to sweep its radar beam in a raster pattern “painting” a big rectangular patch on Titan’s surface, then swing through 90 degrees to aim VIMS to some well-lit regions, then swing back to point INMS forward around closest approach while doing radar imaging, tracking Titan through nearly 180 degrees as the spacecraft whipped past at 6 km/s.
As at T0, there was great media interest and JPL was abuzz with reporters and TV trucks. This was the first encounter with a new world. T0 hadn’t really counted for unveiling Titan because it hadn’t been close to Titan and was sufficiently long after SOI that reporters had already filed their space stories. What would Cassini find?
Most of the data was downlinked to the Deep Space Network’s 70 m dish in Madrid, since the preferred station in Goldstone, California, which enjoys more reliable weather, was under maintenance for the latter half of 2004. The order of transmission roughly followed the order of the observations themselves.
On this occasion, the dayside part of the pass happened to be on the inbound leg, as Cassini was approaching Titan, and so the imaging results came down first. (Later in the mission, as Cassini’s orbit rotated around Saturn and so changed its orientation, the opposite would become the case.)
Thus, the early focus was on the orbiter’s camera, which had already demonstrated during the initial approach and at T0 that it could see things on the surface. As images were read out and displayed, essentially in real time, scientists from all the teams gazed in wonder at the features.
The imaging team members themselves struggled to enhance their images, piece mosaics together, and write captions while the media buzzed around. At first, the patterns of bright and dark were challenging to interpret. “I just wish we knew what we’re looking at,” noted imaging team leader Carolyn Porco. But at least some prospects could be ruled out. “No herds of roving dinosaurs, yet.”
Imaging scientist Alfred McEwen made a dramatic color composite. Typically, a color image is the synthesis of three different color channels, represented by red, green, and blue, though the wavelengths in which the three images are taken do not necessarily correspond to these colors in reality. McEwen made the blue channel in the composite an ultraviolet image taken by Cassini, which showed the haze in the upper atmosphere. The green channel brought out the surface features at 940 nm, while the red channel was generated from a methane band image at 889 nm, with an obvious difference between the north and south hemispheres. Even though the composite was in “false color,” it was an attractive and information-rich image, and was reproduced in many magazines and presentations thereafter.
But when scientists looked in more detail at the higher-resolution images, they saw some tantalizing trends. These trends lay somewhere between the most pessimistic and optimistic predictions about blurring due to haze scattering: Cassini’s camera seemed to be able to detect surface features down to a resolution of about 1 km, depending on observing angle and other factors.
A very sharp and irregular boundary could be seen at the western edge of Xanadu between bright and dark terrain. In terms of its shape alone, it looked for all the world like an irregular coastline. That, and the presence of isolated irregular bright areas looking like islands, made Titan strongly reminiscent of Earth. The Aegean was a particularly appealing analogue to the region seen on TA. But were these really coastlines?
Also visible were some highly intriguing dark linear features embedded in bright regions, sometimes branching. They looked as if they might be river channels, but in at least one case, the dark line went all the way across the bright region. (This area was informally referred to as Great Britain, since that’s what it looked like, but its official name became Shikoku.) Such behavior was not to be expected from a river, which flows from the middle of a continent, where it rises, to the sea and not from one sea to another. So the interpretation remained tentative.
Figure 4.06. A contrast-stretched and enhanced ISS mosaic of Titan at 940 nm from TA images. The south polar clouds are visible at the bottom. Xanadu is at right, the dark area in the center is Shangri-La, with the bright feature Shikoku arrowed. The ellipse shows the expected Huygens landing site. (Adapted from NASA/JPL/University of Arizona image)
But it was puzzling. The morphology of the bright and dark regions was similar to a coastline, but if the dark regions were liquid, there should have been a specular glint—a mirrorlike reflection from the smooth surface of a hydrocarbon sea. And none was seen, either by Cassini’s camera or by ground-based telescopes such as the Keck. Scientists struggled to reconcile the observations with their expectations, especially as specular glints had apparently been seen in the 13-cm Arecibo radar observations. The lack of an optical specular glint did not accord with that observation. Among the ideas thrown around were fluffy aerosols floating on the liquid, making it rough at micron wavelengths but smooth at centimeter wavelengths. However, this was clutching at straws. In principle, such aerosols should sink unless they trapped air to remain buoyant because the density of liquid ethane is quite low. Another possibility was that the surface was too rough for an optical glint—perhaps due to wind-driven surface waves. Or, perhaps, there was no liquid after all and we were being fooled by our hopes.
Figure 4.07. A mosaic of VIMS data showing the Huygens landing site region, the inset area being at higher resolution. The small circle denotes the best a priori estimate of the landing site, at the edge of the bright feature. (NASA/JPL/University of Arizona/US Geological Survey)
It became clear from the TA images that the probe was destined to descend over an interesting area. The target region was at the boundary between bright and dark terrain. With luck we would get to see both close-up and learn what bright and dark really meant.
A DIRTY EXOSPHERE
As Cassini flew above Titan during TA, a small aperture pointed in the forward direction (the “ram direction”). This aperture guided gas molecules into a mass spectrometer—the ion and neutral mass spectrometer (INMS). This instrument counted the molecules, and as Cassini swung down to 1,200 km altitude and back up, it yielded profiles of the abundances of different compounds. In fact, at these very high altitudes, lightweight methane is more abundant than molecular nitrogen, which has a much larger molecular mass.
The INMS had been used during SOI to sniff the “atmosphere” sputtered off the rings, but TA was its real debut, and some software problems in the instrument had only just been ironed out. But this first Titan encounter worked well. Remarkably, the INMS did not just detect a handful of molecules like N2, CH4, C2H6, C2H2 and so on (with molecular masses of 30 or less), but rather yielded a complex mass spectrum of compounds with molecular masses up to 100 and more. The instrument could only go as high as mass 100, but even at 100, there was plenty of material. No one expected such a zoo of large molecules at this altitude.
The INMS data were crucial in evaluating how low Cassini might go in the future. Indeed, it was so much so that these data were labeled “critical” and were downlinked especially early. If the density of the atmosphere at high altitude were greater than expected, then the attitude control system would need to fire its thrusters more frequently to keep on track. In the extreme case, the drag torques, which depend on the orientation of the spacecraft in space, would be too large for the spacecraft to deal with, and flybys would have to be made at higher altitudes than originally planned. This would essentially require a slight redesign to the whole tour.
And at this time, all the worry about the Huygens probe was still present. Although the INMS data pertained to altitudes far higher than those where peak heating would occur, if the INMS results were really surprising, doubts would be raised. Despite the surprising abundance of heavy molecules, the atmosphere overall seemed to be within the range of uncertainties allowed by the models that had been used. But atmospheres can be fickle things, and no one would be comfortable until more data had been obtained; the Mars Global Surveyor spacecraft nearly broke off one of its solar panels when the density of the Martian atmosphere suddenly increased in response to a dust storm.
Figure 4.08. A mass spectrum of Titan’s upper atmosphere recorded by the INMS instrument above 1,100 km altitude. Heavier molecules—remarkably heavier than C6 compounds like benzene—are toward the right and were surprisingly abundant. (NASA/JPL/University of Michigan)
RADAR IMAGE
As Cassini returned to the surface, although the ISS results were impressive, it was no secret that Cassini had a radar for the express purpose of seeing Titan through the haze. On this first close flyby, it would make its first observation of the surface, and that would set the stage for the rest of the mission. Although it seemed highly improbable, nobody could be sure that the 1990 prediction made by a prominent radar astronomer who wasn’t on the team—that Titan was featureless to radar, an uninterpretable mess of subsurface reflections—might not turn out to be true.
The radar team would not get its data until much later, after media prime time. It was ensconced in a design lab run by Steve Wall, the deputy team leader (who, like almost everyone working on Cassini, works on other projects too), where computer projectors could show large images on three of the four walls of the room. This was to become the “radar war room” for a few days a year. Not only was it an ideal venue for studying the new data, but apart from a visit by a 60 Minutes film crew, it was secluded enough to allow the team to be productive.
The RADAR team gathered early in the morning on Wednesday, October 27. Even before the science team in the war room saw a huge image of Titan for the first time, a few engineers in an anonymous building a quarter of a mile away got a sneak peek as they massaged the raw data into an image. By the magic of synthetic aperture radar (SAR) processing, the bits of Titan contributing to the echo could be teased apart, making an image with pixels far smaller than the width of the beam itself. Processing the raw radar data from each of the five overlapping beams required several hundred sophisticated calculations for each burst of radar pulses. It took around an hour on a fast PC to work through the whole image with its thousands of bursts. And unlike most of the other instruments, which had had plenty of opportunity to refine their procedures and methods by looking at Saturn or other targets in the months and years up to this point, this was the first time ever that the mapping mode of the radar was being used.
The very first image looked pretty ratty, striped with dark lanes where the beams overlapped. Because each of the beams hit the surface at a different angle and from a different distance, the signal strengths were different, and so it was difficult to get a uniform brightness across the scene unless all the parameters were set correctly. Some of these parameters depended on the exact trajectory. As it turned out, Cassini actually flew past Titan on this first flyby 26 km lower than was planned for, the difference being as much due to Titan’s position not being known precisely as anything else, but this would not be measured until some hours later. The team tweaked the parameters in the processor and waited again for the sorcery of fast Fourier transforms, pulse decompression, and correlation to work their magic.
“We own the mission,” observed the usually reserved veteran radar astronomer Steve Ostro. Even the first bad SAR processor run had shown that Titan’s surface was rich in features. There was definitely stuff down there, though it was too early to make sense of it. When the next run came through, the dark gaps between the beams were full of detail too.
Figure 4.09. The long swath of radar data acquired during TA in October 2004 showing a bewildering array of unfamiliar features. The swath is about 180 km wide at its narrowest point and is so long it is cut in half to fit on the page. The circular feature Ganesa Macula has had the most attention so far, being a possible cryovolcano. Bright triangular features may be alluvial fans. A bright lobate feature two-thirds along the swath could be a cryolava flow. The dark spots like Si-Si might be lakes, but evidence at this point was not compelling.
A 60 Minutes presenter asked Larry Soderblom if we knew enough to be surprised. “Oh, we knew we’d be surprised . . .”, and sensing a certain unflattering probing in the question, added for good measure “so we were right!” Titan’s surface seen at radar high resolution certainly didn’t look much like any planetary surface anyone was familiar with. It didn’t even seem to resemble the parts of Titan that had just been seen optically. At closest approach when the radar was operating, Cassini was sweeping over fairly high latitudes, so the coverage did not overlap areas that had been seen by ISS, either earlier on during the TA encounter or before. It was terra incognita. But even allowing for the different location, there was really nothing in the radar image that could be compared with what was seen optically.
The sharpest part of the image was dominated by a large, dark circular feature, 180 km across, with a bright edge. For an hour or two it seemed to be a crater, but like some Rorschach test that probes subjects’ preconceptions more than their vision, that impression didn’t last. Somehow, magically, the circular feature popped itself forward, becoming a dome instead of a crater. And the picture made more sense. As well as some of the chaotic background looking like small lava flows, the bright wiggly lines inside the dome seemed to fit as small canyons flowing down the sides of the dome, broadening as they went. And it looked strikingly like the volcanic features called pancake domes on Venus. For the moment, its identification as a volcano is tentative, and the structure bears the name “Ganesa Macula.”
Lots of other features were there too: a region of giant bright fingers, looking like the lobes of some great lava flow; dark crescentlike and angular patches; and some bright, striated triangular areas that seemed to connect to bright, sinuous lines. Perhaps these were alluvial fans, sheets of debris deposited at the mouths of canyons by flash floods. But that was an entirely speculative suggestion at the time. We still didn’t really know what we were seeing on Titan.
The RADAR team leader, Charles Elachi, who had a couple of years before become the director of the Jet Propulsion Laboratory, drew particular attention to some irregular dark regions in the image. Some of these formed a vaguely connected archipelago with a couple of angular edges toward the top. “Si-Si the Halloween Cat,” Elachi called it, suggesting maybe they could be lakes of liquid hydrocarbons. The interpretation was a little speculative and the name was clearly not an official one, but Elachi’s sense of the moment—it was three days before Halloween—probably got some Titan results into newspapers that wouldn’t otherwise have carried them.
Another, more obscure, feature of the radar data was also presented at the press conference that week (by the first author of this book, although the bulk of the work had been done by Mike Janssen and others on the team). This was microwave radiometry—using the radar receiver to detect the faint natural radio glow from surfaces rather than radar echoes. If you know something about the temperature of the surface and can measure its microwave brightness (usually expressed as a “brightness temperature”), then you can deduce how good a microwave emitter or reflector it is, which may help to rule out or favor particular kinds of surfaces. Those covered in organics, for example, would be good emitters and would have high brightness temperatures, whereas dense rock or metal (or liquid water, for that matter—not that water was expected in this case) is a good reflector and so would basically reflect the cold of space without adding much emission of its own.
The brightness temperatures were quite high at 70–80 K, suggesting on balance that Titan was quite a strong emitter. Titan was definitely no iceball on the outside: it had to be largely covered in organic material. The change of brightness temperature with view angle also suggested the same interpretation. The full story of Titan’s surface composition would take some time to emerge—indeed, it is still struggling to do so as we write—but Titan was clearly an exotic, organic-rich place even on the basis of those early observations.
The radar also swept a region in a low-resolution “scatterometry” mode, in which it just measures the radar reflectivity in the beam without the clever SAR processing. This showed an impressive difference between Xanadu and the darker region to the west that the imaging team had observed: Xanadu was considerably more radar-bright, perhaps meaning rougher terrain. The scatterometer also hit the probe landing site, in an attempt to guess whether the probe was in for a splash or a crash. But it was known from the optical images that the landing site was in a rather mixed-up area with some bright and dark terrain, so the low-resolution scatterometry and radiometry couldn’t tell us much yet.
A final piece of data was a profile of altitudes from the radar as it looked straight down on Titan. The distance from Cassini to the surface could be measured with an impressive precision of around 50 m, but it seemed that, even over several hundreds of kilometers, there was hardly any relief on Titan, maybe only 100–150 m of elevation change. This meant that the slopes on the scale of tens of kilometers were very shallow—only a fraction of a degree, like the northern plains on Mars, sedimentary basins on Earth, or the flat terrain of Europa.
A HURRIED REPLANNING
In addition to the frenzied activity associated with the flyby itself, some project business also needed to be attended to. In particular, true to the adage that “No battle plan ever survives contact with the enemy,” a new compilation of data on Iapetus had introduced uncertainties about exactly how massive the satellite Iapetus was. Formally, its density was now estimated at 1.25 + 0.17 times that of water, which was barely consistent with the previous estimate of 1.02 + 0.1. Normally, this would not have been a problem, but after the trajectory redesign to accommodate the Huygens probe receiver problem, both the probe and the orbiter would independently fly rather close to Iapetus, arcing out before swinging back in toward Titan for the encounter in January. Because the heat shield design was perceived as close to the limits of what could be tolerated, a change in the angle at which the probe entered the atmosphere could cause problems.
Figure 4.10. Cassini’s third orbit around Saturn, setting up the delivery of the Huygens probe. In contrast to the geometry suggested in the artist’s impressions, Huygens was released while Cassini was flying away from Titan, not toward it. In fact, both probe and orbiter flew rather close to Iapetus before arcing back toward Titan. (NASA/JPL)
And if Iapetus’s mass was uncertain, then the amount by which it would tweak the trajectory, and thus the impact point and entry angle of the probe, would also be uncertain. NASA was not in a mood to tolerate uncertainty, especially after the Genesis failure, so the trajectory had to be changed.
This was bad news and enormously frustrating because it was one of those things that would probably just go away with a bit more data. It was late in the day for mission changes to be made, and the navigation team was working hard. The trajectory could be adjusted to give Iapetus a wider berth, to be sure, but not without a cost. First, the changes would expend precious fuel, and by changing the flyby distance from 55,000 km to 120,000 km, the resolution of the Iapetus images would be reduced by a factor of two. Second, any changes to the trajectory or timing meant that the science teams would have to redesign all the observations in the second and third flybys at the same time as designing the observations for later in the tour and analyzing the data from the first encounters. Third, whether redesigned or not, some observations were highly sensitive to small changes and were wrecked.
The TB encounter was to have been high enough (2,200 km) that Cassini would be well above Titan’s atmosphere, and so it could use its reaction wheels for fine-pointing to make excellent optical images. The new trajectory brought the spacecraft down to 1,200 km, so low that it would have to use its thrusters, which would make for jerkier motion. Though the images would nonetheless be of remarkable precision, the jerkiness would be enough to make mosaic images not match up and introduce some blurring.
The T3 encounter featured a radio occultation, where Titan would pass between Cassini and Earth, and the atmosphere would be profiled by radio signals at several wavelengths. These would be best if the spacecraft were close to Titan at the time, so that the beam would not have spread out much, and if the spacecraft’s position as seen from Earth moved slowly relative to Titan. The new T3, at a height of 1,580 km instead of 1,000 km, was going to be far poorer than had been planned. And so a debate ensued as to whether it made sense to do a bad occultation in T3 or scramble to do something else. Sadly, it was too late to make any major changes to TB.
A trade-off was worked out. The radar team would observe on T3 instead in exchange for giving up their observations on T12, which had a geometry with a good radio occultation. Although there was some initial reluctance to the trade-off, it worked out better for everyone. The radar team was particularly pleased, since T3 in February 2005 would come in what would otherwise have been a long wait between TA and the next radar imaging during T7, almost a year later.
DIGESTING THE DATA, AND A NEW ENCOUNTER
After dumping its data from TA, Cassini flew by Saturn and also started to snap some of the other satellites. It got a long-distance look at Tethys, showing its old, cratered surface with much better clarity than Voyager had done. The second orbit around Saturn was only half as large as the first and so was faster. The second Titan encounter (TB) was on December 13, 2004.
Figure 4.11. Two ISS views (from late 2004 and early 2005) of Titan’s haze, which turned out to have a much more complicated vertical structure than had been anticipated, especially near the north pole (in darkness at top of right panel). (NASA/JPL/Space Science Institute)
TB was more or less over the same region of Titan as TA, but was valuable even so. Seeing the same places at slightly different angles is important for understanding the reflection properties of surfaces. Often, what can look like a change in surface coating is just an effect of different viewing geometry. A puddle can look black or like a mirror. Also, even if the geometry were exactly the same, there isn’t enough time in a flyby to point the instruments everywhere that can be seen, nor is there enough room on Cassini’s data recorders to store all the data that could be taken if there were. So TB, especially since it had been somewhat compromised by the Iapetus trajectory change, was not much of a headline grabber but was rather an opportunity to fill in and build up the accumulating mountain of data. Close-in views of the haze showed it to be rather more complicated than was previously thought. And one big surprise was that the clouds that had been so prominent around the south pole in T0 had essentially disappeared.
Ground-based monitoring by graduate student Emily Schaller and her colleagues at Caltech, using an impressive combination of a fourteen-inch telescope in New Mexico and the 10-m Keck telescopes in Hawaii, showed that Titan’s clouds came and went. Perversely, although there were prominent clouds in Keck images on October 7 and November 4, whose presence was also indicated by deviations in Titan’s lightcurve of several percent measured with the “amateur” telescope, the clouds seemed to disappear almost entirely, with only a 1 percent deviation indicated, during the TA and TB encounters. Titan’s clouds seemed almost to be deliberately hiding from Cassini. In reality, of course, the weather patterns were just changing as Titan’s seasons wore on, and the clouds were just the sputtering remnants of southern summer storms.
Figure 4.12. Ground-based observations help fill the gaps between Cassini flybys. These data, from Emily Schaller at Caltech using a small telescope, show how the cloud-induced brightness declined abruptly after TA. By TB most of the clouds had disappeared.
One observation on TB was important enough to the project for it to be given a special “protected” status. That meant being stored in a special partition on the data recorders such that it would not be overwritten until ground controllers were sure the data had been safely received on the ground. (The data from the Huygens probe, radar observations of the landing site, and some other special observations were accorded similar status.) This observation was the occultation of two stars, Spica and Lambda Scorpii (also known as Shaula, the twenty-first brightest star in the sky), to be observed by the ultraviolet imaging spectrometer (UVIS). These observations would record the abundance of methane and other gases at high altitudes, and would be important in deciding how low Cassini would be able to go in future flybys.
This UVIS observation and the INMS data from TA encountered different bits of the atmosphere (different latitudes, different times of day, and so on) but seemed broadly concordant. But the Titan Atmosphere Working Group (TAMWG)—projects have an impressive proliferation of teams, groups, committees, and so on, which belies the fact that basically the same few scientists are participating in them all—had discovered an unsettling discrepancy. The engineers monitoring the Cassini attitude control system were able to estimate the atmospheric density too, by gauging how much fuel the spacecraft had to use to counteract the torque on its appendages. Their estimate was four times higher than the INMS one, which meant the density might be too high to allow Cassini to make the flybys planned for 950 km up.
Some INMS scientists were inclined to dismiss the indirect engineering measurement; after all, INMS had been designed to precisely measure Titan’s atmospheric density. How could a bunch of engineers armed only with an angular momentum estimator and a thruster duty cycle plot be telling them they were so wrong? Probably someone’s assumptions were wrong somewhere. But in the end, it wasn’t what INMS measured that would determine the flyby altitudes, whether that was the real density or not, but what the attitude system could cope with.
Much discussion and investigation failed to resolve the discrepancy, and some months later, it would be decided that the flyby altitudes had to be increased somewhat, just to be safe. Yet more replanning of observations would be needed.
THE “SNAIL”
The weeks following TA gave the teams time to start making interpretations of their data. One intriguing observation came from the VIMS instrument—in fact, from a rather unpromising moment. Near to Titan, Cassini needs to use its thrusters to turn quickly, while farther away, well above the atmosphere and where turns to track a target like Titan can be slower, it can use reaction wheels. But for the attitude control system to make the transition between these very different systems takes twenty minutes or so, during which time Cassini can’t make any exotic turns, although it can track a specific target. During one of these transitions, the VIMS instrument stared at a random spot on Titan. The spectral image cube it generated—only sixty-four pixels square, but with many hundreds of wavelengths—was impressive.
Figure 4.13. What a difference a micron makes. Both of these VIMS images are about 100 km across. The one at left is at 0.94 microns, the same as the ISS camera, and the right frame is at 2 microns, where the haze is much thinner and reveals much more surface detail. Some suggestion of flow features is seen at right, and the spiral was interpreted by some to be a cryovolcanic structure. (NASA/JPL/University of Arizona)
First of all, it showed how much clearer the atmosphere was at a wavelength of two microns rather than at one micron for showing small-scale features. A number of irregular features looked remarkably like river valleys. And another feature looked like some sort of coiled spiral, which was nicknamed “The Snail” (and later officially named Tortola Facula). It was interpreted by some members of the VIMS team as a volcanic structure. More skeptical members of the planetary community believed that the interpretation was rather speculative, and a more dismissive nickname, “The Cat Poo,” was advanced. Whatever it was, everyone agreed it was an interesting structure.
CHRISTMAS EVE: PROBE RELEASE
It was going to be a busy New Year, but a dedicated few on the project attended a major event over Christmas too: the release of the Huygens probe on Christmas Eve (U.S. time) 2004. The command to release the probe was sent. At the specified moment, a pulse of electricity fired the pyrotechnics that released the probe. Three large springs, held compressed for the eight years since the probe was mated onto the orbiter, pushed the probe away. Huygens was guided by rollers along three spiral tracks, with the intention of giving it a speed of about 30 cm/s and a rotation rate of 7 rpm. As the probe separated, its umbilical stretched taut and yanked out the special low-force connector that was its only remaining link with Cassini. The probe was on its own.
On the ground, controllers could verify that the separation event had gone as planned. The magnetometer on Cassini saw a faint, spin-modulated signal begin and decay as the probe spun off into the distance. Also, just as the springs spun up the probe, they kicked back on the orbiter, giving the massive spacecraft a small spin impulse in the other direction. The situation was complicated slightly by all the fuel sloshing around in Cassini’s now half-empty tanks. After a few minutes, the attitude control system kicked in and stopped the rotation induced by the separation. The reaction on the orbiter was perfectly consistent with what had been expected. So far, so good.
Figure 4.14. Have you seen this space probe? Cassini’s ISS snapped this picture showing the Huygens probe against the background of stars on December 26, 2004, two days after the probe separated from Cassini. The blowup (right) shows some shadow detail on the probe, 2.7 m across but some 52 km away. (NASA/JPL)
Huygens was now well and truly on its own. It was set to coast, asleep and gently spinning, for twenty-two days, during which time Titan would sweep around Saturn one and one-half times before probe and moon would meet at the appointed time on January 14, 2005. Before then, it was going to get colder than at any other time during its mission. All its systems were shut down, except for three quartz clocks, set to wake it up a few hours before it entered the top of Titan’s atmosphere. To stop it from getting too cold for crucial components like the batteries to survive, it was kept warm with a couple of dozen pellets of plutonium, which supplied just enough heat to enable Huygens to be safely resuscitated. In preparation for Huygens’s descent, Cassini would reorient itself and make another burn of its engines. This burn would slow Cassini down so that it would arrive near to Titan several hours after Huygens and could relay the radio signals from Huygens.
Within a few hours of the separation, Cassini took a mosaic of images using its wide-angle camera to locate the probe. One of the images showed Huygens as a small circle. Once the probe was found, which had to be done quickly in order to update the command sequence on the spacecraft for taking the subsequent images, a new mosaic was made using the narrow-angle camera. Huygens was by now many tens of kilometers away. It was an impressive feat, and there had been many memos and discussions over the years about whether implementing such an observation was worthwhile and how it should be done. Huygens was apparently pointing in the right direction, and reassuringly, there was only one Huygens rather than a constellation of bits. (In August 2002, the Space-watch telescope run by the University of Arizona had observed three fragments moving in space at roughly the speed that the Contour spacecraft should have been traveling. Apparently, it had broken up or exploded during the firing of its solid rocket motor to depart from Earth.) The position of Huygens on the sky would also add confidence to the navigation solution of where the probe would arrive at Titan. Reassuring as the picture of Huygens was, a little circle in the blackness of space, many couldn’t help thinking that it was just such a picture that had been the last anyone saw of the Beagle 2 Mars probe almost exactly a year before.
A WALNUT ON NEW YEAR’S DAY: THE IAPETUS FLYBY
When Huygens was released, both Cassini and the probe were still arcing away from Saturn before they swung back toward Titan and the next Saturn periapsis. They arced out almost as far as the orbit of Iapetus, and as luck would have it, Iapetus happened to be in the vicinity (which led to the TB/T3 redesign discussed earlier). As the Huygens probe drifted silently past Iapetus, its three clocks ticking off the seconds until its moment of truth at Titan, the Cassini orbiter was busy.
The camera team even made some shots in the dark—literally. Some regions of Iapetus were in the line of sight from Cassini, but were on the nightside. For a normal camera that might be problematic, but Cassini’s rock-steady pointing performance meant that the camera could dwell for a long exposure while tracking Iapetus—long enough that the hidden area could be seen by light reflected from Saturn. Just as the night side of the moon can sometimes be visible as Earthshine, so too was Iapetus brought out of the shadows. The image taken was a striking one, showing not only otherwise-hidden features on Iapetus, but also long star trails, just like a long-exposure photograph of the Earth’s sky.
Figure 4.15. Iapetus. Left, an ISS image of Iapetus’s nightside, lit by faint Saturnshine. The trailed stars in the background indicate both the relative motion of the orbiter and Iapetus during the eighty-two-second exposure and how well it has been compensated by the spacecraft pointing. Right, a more conventional sunlit image taken December 31, 2004, some 172,000 km away, showing the terminator at top and the bright/dark boundary just below. An amazing surprise was the 13 km—high band running around Iapetus’s equator, visible at the bottom of the image. (NASA/JPL/Space Science Institute)
The new images laid out the puzzling distribution of dark material that gives Iapetus its yin-yang appearance, like a tennis ball with one of the two “halves” painted black. Moreover, the color filters on Cassini’s camera made it much easier to discriminate which areas on Iapetus were dark because they were covered in dark brown material, and which looked dark because they were in shadow.
One early result from the images was that it could be seen that the dark material had been dusted onto Iapetus’s surface—either sprinkled in from elsewhere in the Saturnian system or, conceivably, sprayed out from some volcano on Iapetus (although none were identified). It had not, as the much poorer Voyager images had allowed, flowed on the surface.
The dark material aside (whatever it is—VIMS data are providing some clues), it was clear that Iapetus’s surface was old. It had many large impact craters, the scars of a heavy battering early in its history. The largest of these was almost one-third of the diameter of the body itself. This is, in some ways, not surprising. Many worlds, Mimas and Tethys among them, have such a crater. The reasoning is as follows. There are many small craters and few large ones, and large ones tend to be easy to spot. But no crater can be much larger than one-third of the planetary diameter, or the world would have been blown apart and we wouldn’t be able to observe it. So, statistically, one doesn’t find more than one or two “third-of-a-world” craters, because they belong to the class of planet-busting impacts. One of the only indications of surface changes on Iapetus was a rather impressive landslide deposit, where the walls of one of the craters had slumped.
But one feature of Iapetus really stuck out—literally. A prominent ridge ran around its equator, as if the world had been made in two halves and the glue had oozed out at the seam. Iapetus looked like a walnut. Theoretical work in the following year would determine that Iapetus’s strange shape had rather important implications about when it—and thus the Saturnian system as a whole—formed.
ENTRY OBSERVATION CAMPAIGN
The end of the year marked not only the Sun reaching its highest southern latitudes on Earth, but also the closest approach to Earth Saturn had made for many months, an ideal opportunity to observe it. And this time there was an added motivation: Huygens’s imminent arrival meant there would be ground truth to any observations. Knowing what the conditions were at the landing site from the probe measurements would solve many of the unknowns in interpreting the ground-based results.
Conversely, ground observations would give context to the Huygens results. The experience of encountering Jupiter with the Galileo probe in 1995 underscored this point. The probe had indicated surprisingly dry conditions, which were difficult to reconcile with previous measurements and theories of how much water should be present on Jupiter. The answer was that the probe had just happened to descend in a region of downwelling atmosphere, which had been dried out. On Earth, such downwelling masses of dry air are a characteristic of the Hadley circulation: warm air rises near the equator, its water falls out as tropical thunderstorms, and the dry air descends at latitudes around thirty degrees from the equator, which is why most deserts on Earth are at these latitudes. On turbulent, fast-rotating Jupiter, there is not the same largescale circulation, and these downwellings occur in much more localized swirls and bands. It turns out that such downwelling regions occupy only about 1 percent of Jupiter’s area, and so the probe was very unlucky (or lucky, depending on your point of view) to encounter one. But no one would ever have known that the probe’s environs were special, had it not been for the rather unglamorous observations from the ground. In particular, Glenn Orton at the Jet Propulsion Laboratory had monitored Jupiter’s appearance at five microns and saw that the probe had entered a region particularly bright at this wavelength—“a five-micron hot spot.” The reason it was bright is not that it was warm, but rather that the downwelling dry current was fairly free of clouds, and so the atmosphere was clear. This allowed the five-micron glow of Jupiter’s hot interior, normally blocked by clouds, to blaze through, giving the hot appearance. It was an important lesson. Sending a single probe to a planet is a risky proposition both scientifically as well as in the engineering sense, and simultaneous observations from the ground can be critical in interpreting a probe’s data.
And so there was a concerted effort to observe Titan at and around the time of the probe encounter. Telescopes on Hawaii and in Chile attempted to measure Titan’s winds by the Doppler effect on narrow spectral lines. The adaptive optics imagers were busy searching for cloud activity. With Cassini data coming in, nobody cared about their surface maps anymore, exciting as they had been a year before.
It might be thought that all this activity was superfluous, given that Cassini was right there at Saturn. However, even Cassini’s spaceworthy complement of instruments launched in 1997 couldn’t match all the heavy and capable instruments that had been fine-tuned on Earth for nearly a decade since, such as the spectrometers for measuring winds. And more important, Cassini had to spend a lot of its time pointing its high-gain antenna either at Titan (to receive the probe signal) or at the Earth, to relay the data to the ground. So during these times, optical observations of Titan were impossible. An added complication was that activity, and especially data recording, was restricted during the “critical sequence” to support the probe data reception and downlink. So Cassini’s metaphorical hands were tied, just to be safe.
One other observation was to be attempted. It was a bit of a long shot—namely, to observe the “meteor trail” made by Huygens in Titan’s atmosphere. After all, Huygens was a pretty good-sized meteor. Also, there is a peculiar property of atmospheres with carbon and nitrogen (C and N) in them: when heated or shocked, they emit a brilliant violet (“CN”) glow. In fact, there was concern, among all the other phantoms of risk being chased by the various reviews of the mission, that this violet light might penetrate the heat shield. Instead of being absorbed at the surface of the shield, like the direct aerodynamic heating, where it would be safely insulated from the probe structure, this violent violet radiation might warm up the bondline where the adhesive held the ceramic tiles onto the structure. If that were to happen and the adhesive were weakened, tiles might fall off and the shield would burn through. The loss of the Columbia space shuttle in early 2003 due to a hole in its heat-resistant but brittle wing structure had made the entry aerothermodynamics community particularly alert to such things. A special test was arranged at NASA’s Ames Research Center with a powerful xenon lamp to check that it was opaque to violet light. The test posed some administrative problems: French industry was understandably reluctant to have foreign competitors analyzing its proprietary materials, used in ballistic missiles. But in the end, the test confirmed as expected (the stuff was brown after all, so one could tell by looking that it probably absorbed violet light pretty well) that there should be no risk.
RALPH’S LOG, 2001–3
Would we be able to see it? Would the probe’s fiery entry be observable from Earth? I can’t remember how many times I’ve been asked this question, but it is more than once, and out of habit I would dismiss it. The probe is tiny, and Titan is so very far away. Not a chance.
I started to think about the problem in more detail at a Huygens meeting in ESTEC in late 2001. I remember when I was a young trainee engineer at ESTEC ten years before, in some idle moment (I think inspired by Hubble Space Telescope images showing a storm on Saturn) Jean-Pierre Lebreton suggested I should look into what would happen if an asteroid or comet fell into Saturn. At the time, I knew nothing about space impacts and next to nothing about planetary atmospheres. I did know that such a thing sounded pretty improbable, and I had lots of other things to do, so didn’t explore the idea very much. Naturally I was kicking myself three years later when a whole fusillade of cometary fragments plunged spectacularly into Jupiter.
And so, when Jean-Pierre mused about detecting the Huygens entry, I gave it a little more thought. The strange chemistry of the Titan atmosphere is such that there could be some strong optical emission from the shockwave—emission in a narrow bandwidth that might make the event detectable. After all, aurorae are faint and difficult to see except on dark nights on Earth, and yet we can see aurorae on Jupiter and Saturn with the Hubble Space Telescope. Maybe Huygens would be visible too.
When one gets the idea that something is detectable, it is a potential discovery, and in some circumstances it might be advantageous to keep it quiet. However, I’m not really an observational astronomer. Others would be better placed to get telescope time to observe the event, and might think of better ways to observe it than I would—better techniques, better filters and instruments. And apart from anything else, on the morning of January 15, 2005, I was expecting and hoping to be busy, awaiting the data transmitted back from the probe itself. So it made sense just to advertise the possibility in the hope of inspiring someone to do it. And a publication is a publication after all.
I chose the journal Astronomy and Geophysics. They had a reasonably “popular” bent, a short publication cycle, and most important, published papers with attractive color figures. This avenue would be ideal—a nice marriage of astronomy and physics, and an attractive forum to build interest in Huygens and Titan. A journal article could easily be distributed to the observing community, giving them the scientific and technical case to justify their proposals. The article was printed in 2002, together with an eye-catching artist’s impression of Huygens with nice violet CN emission around its heat shield by my friend James Garry. One of his favorite aphorisms is that a technically flawed but well-presented aerospace concept is ultimately doomed, whereas a technically perfect but badly presented concept is immediately doomed. I don’t know how much the picture helped, but it certainly didn’t do any harm.
There was only one way to find out for certain whether Huygens’s entry could be seen from Earth, and that was to conduct the experiment. The good news was that those allocating time on the HST were persuaded it was a good idea. The observations would provide some useful additional data about Titan too.
The bad news was that the HST observation would, in practice, never take place because of a technical failure with the instrument that was to have been used.
RALPH’S LOG, 2004
HST CYCLE 13
January. When it came time to actually propose the details of an observation, a year before the encounter, I teamed with Keith Noll, a veteran planetary astronomer who, based at the Space Telescope Science Institute, would know how to get our rather awkward and special observation implemented. Mark Lemmon, with whom I’d worked on most of the Hubble observations of Titan over the years, would be a vital ally. And Jean-Pierre belonged on the team, acting as conduit for the final tracking information to target the observation. Plus, with Hubble being a joint NASA-ESA project, having a European coinvestigator wouldn’t hurt politically.
The proposal was pretty slick, I thought. We’d ask for one and only one HST orbit, since these things are like gold dust. Plus, having served on the solar system science panel for HST, I knew that the end of the allocation process was a bit of a knapsack problem—after the front-running handful of proposals had been selected, there might be one or two orbits left over, with the next-highest-ranking proposals needing maybe four or six. So a proposal needing only one orbit might sneak in, just because of the statistics of small numbers. Our proposal might fit into what was left, but a higher-ranking but bigger proposal wouldn’t. Risking one orbit on the uncertain chance of detecting the entry was probably all that prudence would allow anyway.
We designed our observation to give good Titan science anyway, even if the entry wasn’t detected. We’d be able to measure the north-to-south variation in Titan’s haze and detect the polar hood (if it was still there), and the spectrum of Titan at the probe descent region we would obtain from above would be a useful dataset to compare with the probe’s observations from below.
Happily, our proposal was selected. Of course, this just means more work—the detailed timeline of the HST orbit would have to be choreographed, which meant installing and learning HST’s sequencing software. This constructed a timeline that minute-by-minute accounted for all the steps of zeroing in on Titan, taking the sequence of spectra and allowing for the precious seconds spent reading the data out of the instrument.
Summer: Bad News from Orbit. The rumors reached me before the official e-mail did. In August 2004, a five-volt power supply in the space telescope imaging spectrograph (STIS) had failed in orbit. “Switch to backup” is the usual response of a starship commander to such a problem. However, this already was the backup—the prime five-volt supply had failed long before. So STIS was dead.
And every observation proposing to use STIS was dead too—including mine. The first and only successful HST proposal I’d ever written (and judging from the steady decline of the telescope and the hiatus in shuttle flights after the Columbia accident, quite probably the last), and it was torpedoed by some little circuit burning out.
Having been on an HST panel, I knew how many excellent proposals using other instruments had been rejected, and so there was no danger of HST itself being unused. There was to be a process of reevaluating observations. If a program’s goals could be met with another instrument like the advanced camera for surveys, then it would use that. But otherwise, the observation was lost and the time would be allocated to another proposal.
We looked into it, but naturally ours was impossible with another instrument. We relied on the spectrometer to pick out the faint filigree spectral signature of CN violet emission. A camera with a broad filter just wouldn’t do it. All that effort wasted, and perhaps the best chance of detecting the entry gone. That’s show business.
With the probe released to its own fate, the Huygens science teams had a week or so of respite before attention would shift from sunny California to chilly Darmstadt, Germany, home of ESA’s European Space Operations Centre (ESOC).