5. Landing on Titan
Darmstadt, a short distance south of Frankfurt and its massive airport, is an unobtrusive little German town, perhaps an unlikely home to the European Space Operations Center (ESOC). But this is where the data from Huygens were to be received, and in the week or so before the probe’s descent onto Titan on January 14, 2005, the science teams began to gather in anticipation.
RALPH’S LOG, JANUARY 8, 2005
TUCSON, ENTRY MINUS SIX DAYS
I am in my cluttered office in Tucson. I like to think with all the mountain of papers and books here that I am doing my bit for carbon sequestration, fending off global warming by at least a little. There has been enough snow in the last few days to permit skiing on Mt. Lemmon, something I would normally take a day off to do, but somehow the risk of breaking a leg just before the culmination of a career’s work makes me chicken out.
I am packing for Germany and the probe descent. I try to guess what I’ll need. The real analysis of the data will take place over the months and years after it is received, but in the hours and days immediately afterward, there will be the need for quick and dirty answers, and we’ll need background information. Which temperature sensor was closest to the SSP electronics box? What is the density of liquid ethane? E-mails from my colleagues on the SSP team assure me that all the SSP calibration info will be to hand. I try to think of what is not obvious to bring, what we could possibly need that everyone else doesn’t think we’ll need. Half reluctantly I pack the bulky Huygens user manual—someone in ESOC somewhere will have one, but perhaps not in our room, not to hand. Most of the other information is, I am sure, on my notebook computer.
I also fish out my test penetrometer—built at a different time with slightly different cables, but made to the same pattern, and machined out of the same piece of titanium with a piezoelectric disk from the same batch as its cousin, now a billion miles away and coasting quietly inward to Titan. Assuming airport security doesn’t confiscate it, it might make a useful prop for media demonstrations if we get data back from a solid surface.
I realize that I don’t recall where, years ago, I put the diskette of my Huygens crash simulator, to help translate the impact deceleration signature into mechanical properties of the soil. It is software I basically haven’t touched since I wrote it in 1993—in GWBASIC on a DOS PC. Perhaps if we get data on this event, I’ll rewrite it better, and in a more modern programming language. But for now, the old program will have to do. I made sure my current notebook has BASIC on it (not actually a trivial issue) so the code will run, but where is the code?
Rather than turn my office upside down looking for a diskette that I may or may not find and may or may not be readable anyway, I grope under my desk and pull out my old laptop from which I knew the code hadn’t been deleted. This beast—back when laptops really were laptops needing a solid pair of femurs beneath to hold them up, not the handy “notebook” of today—was the machine I wrote the code on as a PhD student. Perhaps in anticipation of this very moment, I had never got around to disposing of it. The battery had long since failed, but it turned on first time, its big British power plug hanging precariously on an adaptor to the U.S. AC outlet. The internal lithium battery that keeps a memory alive has also died years ago, and so the boot-up process is interrupted—the machine needs to be told what kind of hard disk and floppy it has (this is before Windows and Plug and Play). Prompted by a Post-It note above the keyboard, I enter type 42 for the type of hard drive—a whopping 40 MB—and then enter a 4 for a 1.44-MB floppy. Ye gods, was there ever another kind of floppy disk? I vaguely remember something about double density versus high density. Anyway, reminded what its own faculties are, the machine fires up with a beep.
I navigate quickly to my old directory and copy the various versions of the program, imaginatively titled “CRASH1.BAS,” “CRASH2.BAS,” and so on, to a floppy, which groans and grinds but finishes OK.
On my office PC, I copy the files onto a dinky little memory stick the size of my thumb, costing thirty dollars yet possessing the storage capacity of a whole crate of floppies. And I’m ready to leave.
I suddenly realize that, depending on what happens next week, the reasons for hanging on to a lot of these papers, and the old laptop in particular, will be gone. Maybe we’ll land on a solid, and my decade-plus interest in splashdown dynamics will have been for naught. Maybe we’ll get no data at all. One way or another, a major spring cleaning will be in order. Many of my own memories, too, have been formed in this project. Perhaps, when I return in ten days, I will be a different person—reformatted by the anguish of a lost mission.
The old hard disk on the laptop whirrs noisily. Even though I doubt I’ll need it ever again, it seems disrespectful to switch the machine off without parking the disk, which had to be done “manually” on machines this old. Another Post-It reminds me what to do. I type “BYE.”
As the final few days ticked away, ESOC became busier and busier. More and more scientists appeared. Some of the first to arrive were the junior but no-longer-so-young “regulars” who, for all the years of waiting, had reported their instruments’ status in orbit or laboratory analyses, or had helped develop the workarounds to the various problems that had cropped up. These foot soldiers, approaching their finest hour, prepared the ground for the looming battle—getting computers set up, getting the printer working, learning the planned sequence of events. Toward the end of the week, many others elbowed their way into the PISA (the Principal Investigator Support Area, a big open-plan office made temporarily available)—some with key roles defined, some being scientists who had been involved in the original proposals or early development, and some VIPs associated with experiments in some political context, keen not to miss the space spectacle of the year. A few journalists also began to trickle in.
ESA arranged a dinner for the project personnel and their guests. It was a welcome reunion for current team members and former colleagues, many of whom had moved on to other projects or had retired.
THE PLAN FOR HUYGENS
At this stage, there was nothing the mission teams could do on the ground to influence the events that would shortly unfold on distant Titan. The hope was that all the commands loaded on Huygens and Cassini would be executed according to a plan largely shaped years earlier.
Huygens would spring into life when it was a few tens of thousands of kilometers above Titan and traveling at a speed of about 6 km/s. The computers would boot up and instruments would be turned on. The thin atmosphere would start to have a measurable effect on the probe at an altitude of 1,000 km or more, even though the atmosphere is a billion times thinner there than at the surface. Huygens would begin to be slowed down by the drag of this thin air on its blunt, conical heat shield. Scientific measurements would commence. The shiny foil coating that protected the probe from the Sun’s heat when Cassini was at Venus would be torn from the outside of the heat shield as Huygens plunged deeper into the atmosphere. Peak heating would be at an altitude of about 400 km above the surface. At this stage, Huygens would have barely slowed down, and the air in the shock wave in front of the probe would reach a temperature of 1,400 K. Even the back of the probe was covered with insulation to protect it from the glow of the hot air.
Huygens would feel its maximum deceleration at a height of around 250 km, a few seconds after peak heating had occurred. Although moving more slowly than it had been, the probe would be in denser air and feel a deceleration of about 15 g. This was, however, quite modest by planetary probe standards. About a minute after this peak of deceleration, the probe would be down to an altitude of about 170 km and its speed reduced to about 350 m/s, which is about one and one-half times the local speed of sound. The effects of breaking the “sound barrier” apply whether speed is increasing or decreasing; the fairly flat Huygens probe would have begun tumbling as it passed Mach 1, so the deployment of the parachutes would begin here. A mortar on the back of the probe would shoot a small “drogue” parachute through the probe’s back cover. This parachute would both hold Huygens stable and pull off its back cover. As it came off, the back cover would pull out the large main parachute, some 8 m across.
With the deployment of the main chute, the front heat shield would fall away and the probe would decelerate quickly to a steady descent rate of about 50 m/s. A cover that had protected the DISR (descent imager/spectral radiometer) from material coming off the heat shield would fly off as its springs were released. Caps on the inlet pipes of the GCMS (gas chromatograph/mass spectrometer) would be broken off by explosive actuators. Two small arms carrying the electrical field sensors for HASI (the Huygens atmospheric structure instrument) would swing out and lock into position. Small vanes mounted around the edge of the probe would keep it spinning slowly so that DISR’s cameras could pan around. (The line to the parachute had a swivel to prevent it from twisting up.)
The large size of the main parachute had been dictated by the need to pull the probe away from the heat shield safely. But if the probe were to continue to descend under the main parachute, it would take some five to eight hours to reach the ground, by which time the Cassini orbiter would have disappeared out of range before data could be taken from the surface. So, after ten minutes, explosive bolts would be fired to detach the main parachute. A smaller stabilizer parachute would take over and allow the probe to descend more quickly.
The ACP (aerosol collector/pyrolyzer) experiment would suck atmosphere in through a filter, trapping aerosol particles and cloud particles. It was thought that the tholin haze particles might act as the cores of small crystals or droplets of ethane or hydrogen cyanide lower down. And at 40 km and below, methane condensation would be possible. Between there and the melting level at 14 km, a thin film of methane frost could form on the probe, although not enough to disturb its aerodynamics; all of these strange environmental effects had to be anticipated and their effects evaluated. A small oven would break down the material trapped in the ACP so that its composition could be analyzed by the GCMS.
As Huygens neared the surface, gently swinging under its parachute, it would be descending at about 5 m/s. Its height would be determined by a radar altimeter on board, and the instruments would adapt their operations to maximize the scientific return. At an estimated two minutes from impact, the SSP (surface science package) experiment would prime itself. Its acoustic sounder—like a small sonar—would begin sending out a rapid series of pings. A lamp would illuminate the surface in the last few tens of meters of descent, allowing DISR to take a surface spectrum despite the anticipated gloom below the haze.
Huygens had to be prepared for anything to happen at impact. Before the mission, no one had any idea what the surface would be like. It had not been feasible to design the probe so it would definitely survive, but a wide range of possibilities had been taken into account, including a splashdown in liquid, and there was reckoned to be a good chance that Huygens would keep transmitting data from the surface. After listening out for the probe until it was no longer visible, Cassini would slew around to relay the probe data back to Earth.
If nothing broke on impact, the probe would continue to take data, but it had not been designed to operate in any specific way after it had landed. There could be pictures, but they would be limited to wherever the camera happened to be pointing when it came to rest—even if that was the underside of the parachute! There was a chance that some surface material would be vaporized by the heated inlet of the GCMS so that it could be analyzed. After a landing in a liquid, the acoustic sounder on the probe could possibly have picked up an echo from the bottom and hence determine the depth.
In the end, the collection of data would cease for one reason or another. The batteries would run out, or something would stop working as the probe cooled down. Even if Huygens continued to transmit, Cassini would eventually move out of range of the probe. The data link would fade and then disappear. Several hours after the end of the probe’s mission, Cassini would direct its antenna toward Earth and download the data it had collected from Huygens.
Figure 5.01. The Huygens encounter was a major media event. Project scientist Jean-Pierre Lebreton announces the status of the probe to the assembled hordes of media. In the background is a full-scale model of the Huygens probe (SM2) that had been used in parachute tests, dropped from a helium balloon 40 km above Sweden in 1995. (ESA)
All this, at least, was the plan as articulated in everything from detailed specification documents to computer animations put together by ESA and others for media use. But nothing ever quite goes to plan.
LANDING DAY
By the time the day of reckoning arrived, the media presence had become almost oppressive from the point of view of the scientists. The European Space Agency was pleased with the level of interest, although its public relations staff numbered only a handful, and the intensity of the event threatened to be overwhelming. About eighty busy scientists were being besieged by some two hundred journalists, reporters, and camera operators.
As one might expect in Germany, the logistics were well arranged. While TV trucks clogged the roads on-site, parking passes were provided for the scientists. The activity would be centered on two rooms at ESOC, the PISA and the MCR (Main Control Room)—the usual sort of control room with lots of screens, where the first news would come in and so all the VIPs could appear to be involved.
The PISA was a big open-plan office: indeed, an agenda item of many Huygens meetings in the previous year was on encounter logistics. How many tables would be needed, how many Internet connections? All these details, mundane as they are, need to be worked out in advance. Access to the various areas was controlled by swipe cards, and with few occasional exceptions, the media were, thankfully, excluded from the PISA, so the real work would not be hindered. The number of people involved meant, sadly, that not everyone would fit in the PISA, and the large DISR team was therefore secluded in a Portakabin a few minutes away.
RALPH’S LOG, A.M., JANUARY 14, 2005
It was going to be a long day—most people arrived around 9:00 or 9:30. Although the probe data would not come in for hours, the phone rang continually with requests for media interviews. I checked up on the ground-based observing campaign: snow and high winds on Mauna Kea in Hawaii were making life difficult for the astronomers. Someone checked his watch and observed that even though the light from the event would take another hour to reach the Earth, Huygens had already encountered Titan.
The HST observation was, of course, not happening. But the Keck and Infrared Telescope Facility (IRTF) telescopes in Hawaii—on the other side of the world— were acquiring images and spectra, respectively, of Titan as the probe entered and descended. The Gemini telescope could not open its dome in the high wind. Meanwhile, in California, the Palomar Telescope (the only large telescope that was set up to observe in the near-UV) was unfortunately clouded out.
Around 11:40, a cheer went up. There was news. The Green Bank Radio Telescope in West Virginia had picked up a signal from Huygens. Whatever happened next, the mission had not been lost without a trace, like Beagle 2 a year before. This signal meant that the back cover was off and a transmitter was on. Huygens had survived its fiery entry into the atmosphere. Presumably, the parachute was out and the probe instruments were working. Detecting the signal on the ground had always seemed like a long shot, but there it was, clear as a bell.
Figure 5.02. Left, the Green Bank Radio Telescope in West Virginia, which eavesdropped on the Huygens transmissions to Cassini, and thereby provided the first news that the probe was operating successfully. Right, a dynamic spectrum or “waterfall” chart, like those used by sonar operators aboard submarines. Brightness represents intensity against time (vertical axis) and frequency (horizontal). Random noise appears as spurious dots, but the vertical line denotes a steady signal—that from Huygens. (NRAO)
Excitement started to build in earnest in the afternoon as Cassini began relaying the Huygens data to Earth. But something wasn’t right, and as the Huygens data began to be read back, word began to circulate from the Mission Control Room about a problem with “Channel A.” Huygens had two separate computers, each wired to a separate radio transmitter, A and B. Cassini carried two separate receivers, so that a single failure anywhere in the system would not cause the mission to be lost. The transmitter and receiver on Channel A were equipped with both ordinary oscillators to control the radio frequency and special ultra-stable oscillators (USOs). The purpose of the USOs was to make Channel A exceptionally precise so that the probe’s motion could be tracked by using the Doppler effect. It turned out that the problem was with the Cassini orbiter. Although everything on the probe was working, on the orbiter the receiver had correctly been commanded to use the USO, but the command to switch on the USO had been missing. It was a simple, but catastrophic, mistake. Without the USO on, the receiver could not find the probe’s signal.
The Doppler Wind Experiment team was devastated by the loss of its data. Others wondered what else might have gone wrong, and what the loss of Channel A would mean for their investigations. Some experiments duplicated the data on both channels, to be more certain that all the data would be received. Others took a gamble and made the most of the two channels by sending different data on each. If both worked, they’d get twice the amount of data. (Each channel supported eight kilobits per second, about seven times slower than a turn-of-the-millennium-era dial-up modem. The total return over three hours would fit easily on a CD-ROM.)
Late in the afternoon, the readout of the probe data from the Jet Propulsion Laboratory (JPL) to ESOC and the distribution of the data began. The stream of information, recorded chronologically by Cassini, had to be chopped up and the right packets of information sent to the right teams. This process took an hour or two, and while the experiment data accumulated, only a few pieces of engineering data could be monitored. But these seemed to offer good news—not least that over an hour of data was received after the probe had landed. The landing was quite late: two hours twenty-eight minutes after the parachute was deployed. The nominal descent was supposed to be two hours fifteen minutes, and no more than two hours thirty minutes.
But it was already known, from the continued detection of a radio signal from Huygens by the Parkes radio telescope in Australia (after the Earth had rotated such that Titan had set below Green Bank’s horizon), that the probe had continued to transmit for several hours after impact!
RALPH’S LOG, P.M., JANUARY 14, 2005
It is a tense wait for the data to arrive. Happily, there is an excellent coffee machine installed in the PISA. I fire up the game Mechwarrior on my laptop and destroy some robots; there didn’t seem to be anything else better to do in between answering calls from reporters. There wasn’t much to say to them—it seemed the probe had worked. The fact that it had apparently kept operating for so long on the surface suggested it had probably landed on something dry. To have anything else to say, we’d have to wait for the actual data to be distributed.
Around 6:00 p.m., I get called away for an interview with ABC Australia. When I return to the PISA, the SSP team members are intently hunched over laptops. Gaaaah! The data have arrived, and I missed the jubilant moment!
Figure 5.03. The “Quick Look” display of the surface science package data—the first view we had of what happened to the probe (impact plots are at upper left). Much of the other data took weeks or months to fully interpret.
“Who’s got the stick? Gimme the f—ing data,” I blurt, desperate curiosity getting the better of politeness. After an agonized minute, I am presented with data that this morning was transmitted from the probe on Titan, a billion miles away.
After descending a rather convoluted directory structure, forced by a last-minute adjustment to the display software, I open the file o518/o518/o518ss/o518_IDL/o518ss_chB. (Stream 518 was the telemetry package from the mission. Later streams would patch up small errors. There had been many previous streams from the in-flight checkouts.)
The penetrometer record looks weird. There’s a suspicious big spike at the beginning. Maybe it broke. In fact, everything looks weird. The speed of sound profile doesn’t look right. The tilt sensor record is all over the place, looking like a seismogram instead of the record of gentle swinging that we had expected.
But the world wants answers, so we have to focus on our assigned tasks. Andrew Ball and I have the job of making sense of the impact measurements, ACC-E and ACC-I. Having the two was a hedge against the unknown. If we splashed down into a liquid or landed on something very soft, ACC-I would catch the event but ACC-E would be useless; if we landed on something hard, ACC-E would come into its own, but all ACC-I would tell us would be how hard the probe resisted as it crumpled on impact. For some range of surface properties in between, both sensors would tell us something—and in fact, that was what had happened.
I look at the ACC-I record. At least it looks like a real impact profile: 15 g—that’s not much, but too hard to be a splashdown. A pity—there goes my bet. Some high school mathematics suggests that the probe came to a halt over a distance of about 15 cm. Knowing the shape of the bottom of the probe allows an estimate of the contact area, and thus the mechanical strength of the surface material. It was soft, like packed snow or sand, or wet clay. I compare the shape of the deceleration curve with simulations I had done with my computer model all those years ago. It doesn’t quite look like the dry sand models. Dry sand, unless it has been packed down, is a little fluffy and so needs to be compressed somewhat before it begins to resist penetration, and the deceleration record seemed to jump up too quickly for that. So a wet clay or packed granular solid seems to make most sense. The HASI team is on the other side of the table. We check with them that the peak impact deceleration they see is about the same as ours—a reassuring Italian shrug confirms 15 g.
We then devote our attention to ACC-E. The big spike at the start was a distraction, and a broad mound on the profile seems to coincide with when the base of the probe would have hit the ground. (ACC-E had been tested numerous times in the laboratory, but not with a 
300 million, 200 kg space probe attached.) Dismissing those two anomalies, we could see that the force profile was pretty flat, about 50 N. So the penetrometer had sensed a force much as if you had pushed your finger into a material with about one-twentieth of your weight, say 5 kg, behind it: packed snow, wet clay, maybe rather stiff molasses.
But what is the spike? Had something broken? We reason that perhaps there could have been some anomaly (like an electrical spark between the ground and probe at impact due to charging by cloud droplets, just like early aviators used to get with rubber-tired planes before it was realized that a conductive skid was better). But otherwise, there was something with more strength or inertia at the beginning of the impact record—a hard crust, or a pebble.
Figure 5.04. The surface science package team’s corner of the PISA, with the first author in center. The level of crowding is evident. This was not an ideal environment for quiet contemplation of the new data.
I am afraid I don’t remember at what point the first DISR pictures appeared, showing the cobbles littering the landing site, looking like a streambed or a beach with the tide out. But at least the first analyses of the impact data were made “in the dark,” with just a squiggly line on our screens, no hint of what the images would show.
We have to prepare two charts of results within the first couple of hours. The project is hungry for results, the media hungry for news. We list the possibilities, and playfully suggest “crème brûlée” as an analogy for the surface. (It may have been Andrew who first spoke the words, one of my favorite desserts, aloud. Even though the findings are under strict embargo, I send an e-mail containing only those two words to Zibi back in Tucson—she’d know what it meant!) We debate whether we should keep it on the chart, but it fits the data, and it seems to conjure up an exotic appeal, so we leave it in. We then have to spend about ten minutes Googling on the Internet to make sure we have got the French accents right.
Figure 5.05. The impact deceleration record (left) and penetrometer signal (right). The 15 g impact lasted less than one-tenth of a second and suggested a somewhat soft surface, whereas the penetrometer suggested a lump or a crust above a soft surface—hence, the “crème brûlée” analogy. The penetrometer gave the most accurate determination of the landing time.
When the SSP principal investigator, John Zarnecki, spoke at the press briefing, the impact results formed the centerpiece of the SSP quick results, since the other data would take longer to interpret. He dutifully read the possibilities, and the media latched onto the food analogy. “Titan team claims just deserts as probe hits moon of crème brûlée” read the headline in Nature.
Though every team was busy making sense of its own data, the highlight results were going to be the DISR pictures. Each individual image was very small by modern standards, about one-twentieth of a megapixel, and highly compressed at that. But they were striking nonetheless. The first images that came up on the DISR team’s screens were the most surprising—the knee-high view of Titan’s surface showing a scene littered with rounded “rocks” on a smooth plain.
This was the first image that was released to the public—sometime after 9:00 p.m. local time. As Emily Lakdawalla of the Planetary Society (who was “embedded” as a reporter at ESOC to cover the event, as well as to present the results of an art competition to portray how Titan might look) observed, “Any geologist worth her salt thinks of one thing and one thing only when she sees round rocks: some river of some liquid has rolled broken chunks around, wearing down their edges, making rounded cobbles.” This was not something anyone had dared predict!
In the second of the three images shown (this one from some height above the surface looking down), the picture of Titan as a river world was reinforced. A bright highland area was dissected by a network of what looked like dark river channels, branching and winding just like rivers on Earth. And they seemed to drain toward a dark, bland region, with the boundary between them fairly sharp, like a coastline. The cosmopolitan group of scientists saw different analogues—some saw the French Riviera, some saw the California coast. But wherever it most resembled, it looked very Earth-like.
A third image was a little inscrutable. This one, from higher altitude still, was much harder to interpret. Not only was the contrast lower so that noise in the image (from the data compression process, for example) made it harder to see detail, but the features themselves were much less familiar. There were many arrow-shaped bright features, some connected together. Even a couple of years later, it isn’t quite clear what these are.
RALPH’S LOG, JANUARY 14–15, 2005
Remarkably, within an hour or so, the efforts of various amateurs around the world, pouncing on the DISR images released on the Web (the dataset was released rather quicker than had been the original plan), started to appear. Some impressive mosaicking efforts were made in the following hours and days; notably, it was only an hour or two before some bright spark had Photoshopped the “Face on Mars” onto one of the Titan images. It got pinned up near the PISA coffee machine, which was running in overdrive, a mountain of used coffee cups building up next to it.
The image placed here in the print version has been intentionally omitted
The image placed here in the print version has been intentionally omitted
The image placed here in the print version has been intentionally omitted
The image placed here in the print version has been intentionally omitted
The image placed here in the print version has been intentionally omitted
The image placed here in the print version has been intentionally omitted
The image placed here in the print version has been intentionally omitted
The image placed here in the print version has been intentionally omitted
The image placed here in the print version has been intentionally omitted
The image placed here in the print version has been intentionally omitted
The image placed here in the print version has been intentionally omitted
Figure 5.06. Among the first images to be released on January 14–15, 2005, was this small mosaic of downward-looking images (left) showing a bright highland dissected by river channels next to a dark, flat lowland. Perhaps the most remarkable image is at right, the unexpected view from the surface, showing a plain studded with rounded cobbles. (ESA/NASA/University of Arizona)
It is near midnight U.K. time—but apparently some half a million people are watching live coverage on TV. On a Friday night, I have to wonder how many of them are sober—but what the hell? I get my fifteen minutes (actually about forty-five seconds) of fame, explaining what the penetrometer did. Wow. It dawns on me that eleven years after I spent three years specifying, designing, and building it, a billion miles from Earth and 180 degrees below zero, the thing had actually worked for the one-twentieth of a second it was supposed to!
The SSP team had a sweepstake running on the descent time (defined as impact determined by ACC-E relative to T0, the moment at which the parachute mortar had been fired). My guess had been two hours twenty minutes, slightly longer than the nominal expectation. I learn that my chance of winning had, in fact, been tiny—my guess had been closely bracketed by others. It is principal investigator John Zarnecki, who had been my PhD supervisor all those years ago, who guesses impressively close to the real descent duration: two hours twenty-seven minutes (specifically, 8869.7598 s). He denies any inside knowledge; good instincts are just part of the package of skills for a successful PI.
Zarnecki graciously opens his sweepstake prize, a bottle of sixteen-year-old Lagavulin malt whisky. Those of the SSP team still around, plus a few of the TV crew, savor it. What a day!
I get back to the hotel at 2:00 a.m. but am too buzzed to sleep. I work on making a simulated sonar sound from the echo records, which in the end does not get used. But who cares? It worked! The probe had worked! We had got to Titan.
THE DAY AFTER
The probe encounter itself was attended by ESA’s director of the scientific program, David Southwood, and by his NASA counterpart, Al Diaz. Both appeared to find it an emotional experience during the massive press conference on Saturday, January 15, the day after arrival. Diaz’s tearful praise of the teams’ efforts that “culminated in this one moment in history—it’s just incredible” earned him a mocking on Comedy Central’s Today Show. Southwood, who had been the original team leader for Cassini’s magnetometer experiment until he was recruited in May of 2001 to be the ESA’s science director, wistfully recalled a poem of exploration by Keats.
Southwood acknowledged that there had been a problem with the radio transmission from Huygens and that there would be an inquiry. But no one really cared that much; the mission had basically been a success. The hundreds in the audience, journalists and scientists, just wanted to hear some new results—even though it had been only about fourteen hours since the raw data had hit the ground.
Marcello Fulchignoni offered some ear-catching (although scientifically rather meaningless) sounds—one the noise of the wind rushing past the probe as it descended, and the other a modulated techno buzz generated from the radar altimeter signal. The crowd loved it. He also reported the surface conditions at Titan: 93.6 K and 1.46 bar.
Marty Tomasko presented a preliminary mosaic of the images. Although there was no evidence of present-day liquid exposed on the surface, it seemed clear that flowing liquids—and apparently liquids dropped from the sky—had carved channels on Titan’s surface, and rolled materials around. The results of the GCMS instrument also seemed to suggest that methane was abundant at Titan’s surface.
After the adrenalin-filled late-night rush of the encounter itself, and the press conference (and many interviews) the following day, everyone was burned out. It was time to start a more considered analysis of the data.
On Monday or Tuesday, three or four days after Huygens’s successful descent onto Titan the previous Friday, the fuss surrounding the event itself was dying down. The media circus fizzled away, and VIPs found other places they needed to be. ESA was to hold a press conference at its Paris headquarters on Friday, so there was some late juggling of itineraries on the part of the senior figures summoned to appear there. But most team members were heading back to their usual places of work to begin the serious task of interpreting in detail the wealth of data the experiments on Huygens had returned from Titan.
RALPH’S LOG, JANUARY 28, 2005
PILGRIMAGE
A couple of weeks after the encounter, I visited my SSP colleagues at the Open University in England. En route, though, I made a stop at the London Science Museum. Among the many neat exhibits and demonstrations are formidable steam engines of the Industrial Revolution, a replica of Babbage’s Difference Engine, and a selection of aircraft and spacecraft, with the beautiful but petite Black Arrow rocket hanging from the ceiling. For a month or two, there is a special exhibition relating to Huygens—I have to go and see it. And there, in a glass case among all these marvels of technology, is something I made. It is the flight model ACC-E penetrometer (actually mislabeled as an engineering model). This is the unit I had assembled over a decade before, the one that fit together the most perfectly, the one that was supposed to go to Titan. But months after I had put it together, a technician had tightened it the wrong way, cracking the ceramic force transducer. And so, what had flown to Titan was the backup we had made, the flight spare. It is a good lesson for soccer substitutes or acting understudies: one day the call may come to step up into the limelight. Second best as it had been, the flight spare had done well.
Figure 5.07. SSP hardware in the London Science Museum. With the mission now history, hardware is consigned to the museums. These are spare parts of the thermal properties sensor (left) and the penetrometer (right). The penetrometer head is 14 mm in diameter. The unit on display was actually built by the first author as the flight model to go to Titan, but had to be replaced with a spare before launch.
A CASE OF SPIN
After a couple of days, some disturbing and confusing aspects of the descent began to emerge. The tilt sensor data from the SSP looked rather violent, though it would become clear later that the tilts had not been nearly as bad as the readings suggested; it was more a case of sideways buffeting than rocking as such. One of the tilt sensors was arranged radially on the probe, such that the probe’s spin should fling the little slug of liquid outward like water in a washing machine. The tilt readings were the position of this slug, as measured by a pair of electrodes. So in principle, if the probe were doing nothing but descending steady and flat with a uniform spin, the tilt reading would be fairly constant, corresponding directly to the spin rate. Amid all the oscillations and noise, it was hard to see, but by averaging dozens of readings, it seemed at least more consistent with other data.
But at the beginning of the descent, the average tilt value was puzzling. It was in the opposite direction from where it sat for most of the descent. And the tilt was negative—but that made no sense. It wouldn’t matter which way the probe was spinning; the tilt should be positive. The only way to get a negative tilt from spin was if the spin axis were outside the probe. Perhaps the parachute was doing something unexpected, swinging the probe around in a conical motion.
At that point, the DISR team had an even stranger suggestion. As they began to piece together their jigsaw of images, it became apparent to them that, for most of the descent, the probe appeared to be spinning backward, not in the direction it should have been! This suggestion was not greeted with enthusiasm by the industrial team; such an anomaly would need some explanation.
Insight into what was actually going on came sometime later from a piece of housekeeping data that turned out to be far more rewarding than anyone expected: the automatic gain control or AGC loop on the Channel B Huygens receiver on Cassini (Channel A, of course, being out of action). The AGC was a circuit that compensated for the varying signal strength to maintain a more or less constant output in the radio receiver. Thus, by monitoring the state of the AGC, one could get an indirect measurement of the signal strength. This was recorded about eight times per second, and showed a rich variation during the descent. The spatial pattern of radio emission from the probe antennas, which were on the roof of the probe among the parachute boxes and other items that cause all kinds of reflections, was not uniform, but looked like the petals of a flower, with stronger signals in some directions and weaker ones in others. As the probe spun, these petals swept around, and when a strong lobe in the pattern pointed at the Cassini receiver, the signal was strong. The signal strength went up and down like a heartbeat in a repeating but slowly evolving pattern.
Figure 5.08. The “heartbeat” of varying signal strength received by Cassini as the probe rotated under its parachute. Both plots are about two minutes long. During the first (see left), half an hour after parachute deployment, the probe is spinning about 5 rpm, whereas an hour and one-half later (see right), it has slowed to about 2 rpm. The slight differences from one cycle to the next are partly due to swinging and buffeting of the probe.
A week after encounter, a sound file was made out of these data. It was a hypnotic sound, with beats that faded in and out as different swinging modes took over and the spin rate evolved. A graph is not always an appealing mode of presentation, and sounds have their uses, not least for radio news.
Ultimately, analysis of the signal strength changes would show conclusively that the probe had been spinning the “wrong way.” After beginning its descent under the right conditions, it had apparently spun down to nothing and then spun up in the opposite sense. It seemed incredible. Even as this book goes to press, exactly why the probe spun anomalously despite the presence of the spin vanes is not understood. Whatever the explanation, it wasn’t simple. Some detective work later confirmed that the spin vanes were put on the right way!
DISR: HUYGENS’S IMAGES AND SPECTRA
Another surprise was that the high-altitude images were so indistinct. All the models of Titan’s haze had indicated that the haze should “clear out” below an altitude of perhaps 80 km. Titan’s reflectance spectrum does not fit with a model of how light at different wavelengths is scattered and absorbed (most notably in the 619-nm methane band) unless haze is absent at low altitudes. But this belief, held pretty universally, relied on some assumptions about the scattering properties of an individual haze particle that turned out to be unjustified. And these assumptions, Marty Tomasko explained later, were due to computational limitations in performing the modeling.
Many haze models assume that the haze particles are small spheres (“monomers”), but Voyager measurements had shown that a single population of spheres did not fit the data. This led to more sophisticated models, which assumed the haze particles to be aggregates of smaller particles (perhaps spheres). These aggregate particles have some of the optical properties of the monomers and some of spheres close in size to the aggregate as a whole. But calculating the electromagnetic interactions of all these tiny spheres is a formidable computing problem, which increases significantly in complexity according to the number of spheres assumed to make a particle. For this reason, the maximum size of aggregate particles in the pre-Cassini models was restricted.
The Huygens data could, it seemed, be fit only with much larger aggregates, with hundreds of monomers. This increase didn’t make the particles that much bigger: doubling the number of monomers, for example, causes the diameter of the aggregate to increase by only 20 to 40 percent. The change, however, made it possible to accommodate the ground-based observations and yet still have haze extending down to the ground, rather than being washed out in the lower atmosphere.
The DISR team had devoted many person-months of effort to preparing software to be able to process their data immediately when it arrived—not just mosaicking the images together, but analyzing the way sunlight was scattered by the haze. However, while mosaicking could be done by hand (as indeed many amateurs showed by making impressive mosaic products at home), the quantitative analysis of spectra to determine the haze properties required knowledge of the probe’s orientation at the instant the data were acquired.
Most important in this knowledge was the azimuth angle—the rotation of the probe about its central axis (which, by and large, was close to vertical throughout the descent). In theory, this was to be measured with ease and precision by a Sun sensor in the DISR instrument, which generated a pulse when the shadow of the Sun cast by a bar above the sensor head passed across a photodiode. By measuring the interval between these shadow pulses, the DISR instrument could measure the spin rate and interpolate the spin phase, thus giving information on the azimuth.
But several factors conspired to thwart this elegant plan, which made most efficient use of the telemetry bandwidth available by taking images and spectra only at the desired azimuths. First, rapid oscillations of the probe, and what turned out to be its reversed spin direction, misled the onboard algorithm. Also, although the signal from the photodiode was adequate at the beginning of the descent, a subtle combination of the drop in temperature of the sensor, the narrow range of wavelengths it responded to, and the variation of sensitivity with temperature conspired to suppress its signal at lower altitudes. This is the sort of effect that only the most exquisite and expensive testing on the ground would have discovered. But many of these details did not emerge until after some weeks or months of analysis.
However, the random timing of images did give rise to some neat results. Some terrain was imaged several times, from different altitudes. By comparing these images as stereo pairs, Larry Soderblom was able to construct a digital elevation model, showing in detail the surface topography. One such area was the bright highland with its river channels. The gullies were very steep, it turned out, and the bright area stood perhaps 100 m above the dark plain on which the probe landed.
RECOVERING THE DOPPLER WIND EXPERIMENT
The total loss of the Doppler Wind data from Huygens because of the error with Channel A initially seemed like an enormous blow. An attempt to salvage a measurement on Cassini of the Doppler shift on Channel B did not succeed (nor was it expected to), because the drifts and jumps in frequency due to the much less stable quartz oscillator used on that channel made it impossible to attribute the measured changes in frequency to winds with any degree of confidence. But the objectives of the experiment were all achieved after all, thanks to remarkable observations made from Earth.
Almost all the ways of determining wind speeds involved measuring where the probe was at different times and inferring the winds from the probe’s motion. But each of the methods was, in some sense, incomplete.
Figure 5.09. A 3-D view of a topography model generated by stereo comparison of the two DISR images in the inset. The bright area was inferred to be about 100 m above the dark plains. At the scale of the individual dark river channels, the terrain is very rough indeed. (NASA/ESA/University of Arizona/U.S. Geological Survey)
The original Doppler Wind Experiment (DWE) was to measure very precisely the Doppler shift of the radio signal on Channel A of the probe. The experiment relied on a rubidium oscillator—essentially an atomic clock—attached to the probe transmitter. There would be differences in the frequency with which the transmission was received due to various subtle factors, even including the curvature of space-time due to Titan’s gravity. But the dominant factor was the “range-rate”—the component of the relative velocity between transmitter and receiver along the line between them. One factor taken into account when the entry point for Huygens was chosen was to make sure this line was well positioned in a more or less east—west direction, so that the winds, which were expected to be zonal, would produce a strong and easily measured range-rate.
The Doppler Wind team, heartened by the experience of radio astronomers in detecting the Galileo probe signal in 1995, had also lined up radio telescopes on Earth to pick up the Huygens probe signal. In fact, the Huygens signal would be much easier to detect than Galileo’s, even though the latter was only half as far away, because the Huygens transmitter sent a constant carrier tone as well as the data; this constant tone would be much easier to lock onto. The corresponding experiment without such a carrier on Galileo had required a vast postprocessing effort in order to pull the signal out from the background.
Figure 5.10. The profile of wind in Titan’s atmosphere. The rapid high-altitude winds had been expected on the basis of Voyager and ground-based measurements. The rapid wind shear, leading to a layer of almost zero wind at 70 km, was something of a surprise, as was the small reversal in wind direction just near the surface.
Thus, if all went well, there would be two independent measures of the probe Doppler shift. Because the line of sight to Earth was in a slightly different direction from the line of sight to Cassini, the combination of the two would allow the detection of any nonzonal (i.e., north–south) motion as well as the zonal winds.
In the event, of course, the Cassini measurement was lost. But the ground-based data were good enough to recover the bulk of what the onboard experiment was supposed to measure. The ground-based measurement was of lower resolution, the signal being far, far weaker, and there were a few gaps, but most of the wind profile was measured.
There were a couple of surprises. First, there was a layer about 80 km where the zonal winds—blowing at some 100 m/s higher up—dropped almost to zero before increasing again at lower altitudes. This strong wind shear may relate to the amount of sunlight absorbed at different altitudes. Second, in the lowest few kilometers of the atmosphere, the probe had drifted westward, not eastward as during most of the descent (as expected). This surprising reversal of the zonal winds at low altitude was confirmed by the DISR team, which could correlate the position of various surface features in images taken at different times—stereo in reverse—to derive the probe motion relative to them.
Another wind-drift experiment hadn’t worked out. Thought up a couple of years before the encounter by Mike Allison of the Goddard Space Science Institute in New York, it was to have used the DISR Sun sensor to measure the change in the Sun’s position as seen by the probe in order to determine how far the probe was drifting in the wind. It was an elegant idea, but the apparent failure of the Sun sensor to produce much data during descent meant it couldn’t be applied.
Lastly, there was another experiment that was not really introduced until a couple of years before encounter. This was the VLBI tracking experiment. VLBI—Very Long Baseline Interferometry—is the technique of comparing the carefully referenced readings from widely spaced radio telescopes in order to determine the position on the sky of a radio source with great accuracy. The technique was used in 1985 to track the balloons dropped by the Russian spacecraft Vega 1 and Vega 2 into the atmosphere of Venus while en route to Comet Halley. The implementation for Huygens, though, would be much more precise.
As well as using special radio receivers attached to the radio telescopes and a formidable number-crunching effort afterward to correlate the signals, the technique also required some celestial points of reference. And so, over a year in advance of the mission itself, radio astronomers studied what was then to all appearances a barren patch of sky but would become the place where Titan would happen to appear, as seen from Earth, during the probe mission. In fact, seven months after the probe mission took place, some follow-up observations were made of one of the “Huygens Target Fields.” These improved the accuracy with which a particular radio source (quasar J0744+2120) could be pinpointed relative to other sources in the International Celestial Reference Frame from twenty milliarcseconds to only one milliarcsecond. For comparison, Titan is about eight hundred milliarcseconds across as seen from Earth.
At Titan’s distance from Earth, one milliarcsecond corresponds to about 6 km. The hope (the number crunching is still going on at the time of this writing) is that the position of the probe in the sky may be determined as a function of descent time to within a couple of kilometers. This position is exactly complementary to the information derived by the DWE, and ultimately a combination of the two measurements should yield an accurate three-dimensional trajectory of the probe.
THE RADAR ALTIMETER AND THE PHANTOM RAIN
Many of the experiments had little or no testing in exactly the way they were flown, the exceptions being the HASI experiment, which had wisely arranged several parachute drops from balloons over the years. And thus, the subtleties of the data took considerable time to think through, and phantoms of false conclusions flitted across the discussions before they could be reasoned away.
One of these involved the radar altimeters. They had worked relatively well, apart from a glitch that affected both. This caused them to make a false lock indicating half the real altitude for a while.
Roland Trautner of ESTEC on the HASI team, who had led much of the effort on the radar altimeters (and indeed had been in Brazil conducting a balloon and parachute test on them only six weeks before the actual encounter at Titan), noticed a curious pattern in the returns before they locked onto the surface. They seemed to be indicating a faint echo high above the ground. Cloud droplets would be too small to give a good echo, but raindrops would have the observed effect.
For some weeks the altimeter team thought they were on to something, but it was important to be sure because the probe was the only thing that had actually been to Titan, and whatever findings were reported from the probe would be taken as gospel for years to come. There were also some slight differences between the altitude reported by the altimeters and that inferred from the pressure sensors on Huygens. Roland went through his balloon test data and noted that the altimeters were affected slightly by changes in temperature. So then, some detective work was needed to work out the likely temperature history of the radar altimeter unit on Huygens. As luck would have it, the output from the temperature sensor closest to it had been on Channel A, and so was lost. Nearby sensors were compared with the thermal model and a “best guess” temperature profile was constructed. When that was done with some effort, the revised estimate of the noise levels in the instrument was able to explain what had looked like echoes from rain. It was a pity, but it underscored the need for careful contemplation of results from new instruments in unfamiliar environments before announcing what might be a major finding, or might just be a chimera like this. Ironically, a little over a year later, there were separate indications from other instruments that perhaps the probe had encountered some drizzle during its descent.
RALPH’S LOG, 2005
Just as NASA seems like a monolithic entity to outsiders when it is really a heterogeneous collection of fiefdoms, each with its own culture and style, people talk about “the media” as a whole, but the term hardly captures the range of activities involved in conveying news and information to the public.
There is a fundamental difference between journalists working in news media, and those who work on features or documentaries. To a scientist, interacting with newspeople means the chance of exposure, perhaps only for a couple of seconds, to millions of people. But rarely is there the chance to offer more than a sound bite. News is immediate, and newspeople expect you to drop everything to talk to them. A deadline is a deadline. And even if you’ve spent an hour setting up for talking and filming with them for two minutes, there’s no guarantee that an invasion or earthquake might not suddenly relegate your moment of fame to oblivion. Usually an interview takes the form of a phone call at an inopportune moment, with a reporter who hasn’t researched any of the background to the story but has been told that something is exciting, and thinks you have nothing better to do than explain what it is and why it relates to life on Titan, or nuclear power in space, or whatever other agenda he or she is pursuing.
Documentary filming is generally more satisfying. The producers tend to have done their homework, and have an idea to whom they are talking and why. They might spend a day or two with you, or at least an afternoon. You get the chance to convey a real message (though you might be cut out nonetheless). And sometimes you get to do so in an interesting place.
“What bit of Earth looks like Titan?” I’ve been asked. “Well, who knows?” was the answer for many years. “We think it’s an icy landscape, shaped by similar processes to Earth, perhaps with more impact craters. There should be lakes and seas, perhaps.” At one conference in 2004 I remarked to journalists that Canada and Sweden both have fairly old rocks by Earth standards, and thus have comparatively many impact craters; they also happen to have lots of lakes. To my horror, this train of logic was abbreviated in the newspaper headline to “Saturn moon looks like Sweden,” prompting thoughts of pine trees and blonde maidens. But really the question is as much one of “Which part of Earth looks most like Earth?” I had also advertised my vision of Titan as a southwestern desert analogue, cut by canyons and washes, a landscape shaped by flowing liquid, even though in general the land is dry. (This was, happily, one prediction I did get right.)
And so, back in August 2004, before the first Cassini flyby TA, I had found myself in Moab, in Utah’s Canyonlands, blasting up and down a dirt road in a Jeep Wrangler, kicking up a trail of dust and splashing through streams. “Do it again, but try and get more splash,” the cameraman had requested as he changed the filter on the camera. I readily obliged—it was jolly good fun. The BBC was filming a Horizon program on Cassini (also broadcast, renarrated, and edited in the United States as NOVA).
It was suggested by the producer that the opportunity for me to be filmed on a sky dive to parachute down into this landscape was available, to provide a visual analogue of the Huygens probe. Now, although I flew hang gliders as an undergraduate, the prospect of jumping out of a perfectly good airplane in the expectation that a bundle of cloth on my back would blossom into some descent-arresting structure seemed like an unnecessary leap of faith. In trying to understand the expected descent motions of Huygens, I had also had the opportunity to meet a number of parachute engineers (parachute aerodynamics is a particularly arcane field), and I found it striking that none of them dared try their creations themselves. If I was ever going to give skydiving a try and risk turning myself into a red smear on the desert, I’d do it after Huygens. A local parachutist was found to do the jump, a landlocked surfer dude who was psyched that someone would actually pay him to jump. He had a good time. I stayed unsmeared. Everyone was a winner. In the end, they used only a few seconds of footage from our two days, but it was a fun trip.
Now, post-Huygens, the BBC has persuaded me to be filmed again. This time it is doing a show all about Titan and wants to film terrestrial gullies just like the ones seen by Huygens. I make the familiar drive to Tucson airport, bright and early as usual. But this time, I turn off before reaching the terminal and go instead to a corner of the airfield where a helicopter company has been contracted by the BBC.
In fact, the helicopter and pilot are the same ones used for a calibration flight for DISR and the Huygens radar altimeter some years before. I gesticulate animatedly in the chopper at some vaguely Titan-like channels near Kitt Peak, and shout (in the hope that one or two will be usable) various permutations of remarks about how amazing it is that Earth and Titan’s landscapes look the same, even though the working fluid and surface material are very different. We fly back to Tucson for some shots of me boldly striding to the helicopter, complete with close-ups of putting on my headset and buckling my harness. I am such a stud.
This time, the show uses the footage more extensively (the BBC could hardly fork out all that money on a helicopter and not use it). Viewers get, of course, a completely inaccurate view of how I spend my working day: much as I’d like to be driving Jeeps and buzzing around in helicopters every week, the reality is a little more prosaic. But that doesn’t matter. The purpose of the show is to get some simple ideas across to people who do not have the luxury of exploring planets for a living, and perhaps have barely heard of Titan at all. And for that one needs better visuals than me typing e-mails in my office.
I remember my own teenage years: in 1986, I stayed up late to watch live coverage of ESA’s Giotto probe encountering Halley—the first big European planetary success, featuring many scientists like John Zarnecki and Fritz Neubauer, who later were to go on to work on Cassini. And at age twenty in 1989, I was impressed as I watched a Horizon show “starring” Carolyn Porco and Larry Soderblom reporting on the Voyager encounter with Neptune and Triton. I vividly remember Larry firing off a carbon dioxide fire extinguisher to illustrate Triton’s geysers. These people are now my colleagues, and I find that although Larry doesn’t play with fire extinguishers all the time, he does interesting and important stuff nonetheless. Things have come full circle. Maybe there is some kid out there watching me who now thinks this exploring planets, driving Jeeps and helicopters business is a rather appealing one, and who will become a scientist or an engineer as a result.
SWEATING RESULTS OUT OF THE DATA
In time, more and more scientific results emerged from the Huygens data. The temperature profile of an atmosphere is one of its most basic properties. The lower atmosphere profile (see chapter 2) was fairly well-known at one latitude, at least from the Voyager radio occultation, and the profile measured directly by Huygens under its parachute agreed very well with the (indirect) Voyager measurement. But at altitudes where the air was too thin for Voyager to measure, above about 180 km—all the way up to the ~ 1,000 km altitudes sampled by Cassini during its flybys, Huygens was able to measure the profile by recording the deceleration during entry—the air drag relates to the density. This analysis revealed remarkable fluctuations in temperature—similar to those seen in the stellar occultations in previous years. Titan’s atmosphere was rich in structure—perhaps due to gravity waves or tidal effects. It was tempting to associate these fluctuations with the large variations in haze density seen in some Cassini images. But did the temperatures cause haze variations, or vice versa?
At the bottom of the atmosphere, the GCMS team had an interesting revelation. The methane abundance measured in their instrument increased toward the surface, as one might expect it would by analogy with the water vapor profile in Earth’s atmosphere. Evidently there was a source or reservoir of methane at or near the surface somewhere. But much more exciting than that, the methane reading took a dramatic jump after impact, suggesting that there was a reservoir or source of methane—perhaps as a liquid or as clathrate ice.
Figure 5.11. The temperature profile of Titan’s atmosphere. Below 150 km the temperature was measured directly with thermometers. Above 150 km, the temperature is inferred from the deceleration of the probe—compare with the predicted profile in figure 2.07. The variable structures above 500 km are probably due to gravity waves. Compare also with images of Titan’s haze in figure 4.11.
Even some of the simplest sensors can give interesting and important results. For example, the internal temperatures of the probe generally declined during the descent as ever-thicker cold air swept past and through it. Once on the ground, the cooling slowed down—and some components even began to warm up. It was possible to estimate the wind speed on the surface by how much windchill occurred. It seemed that the wind in the lowest meter or so of the atmosphere had to have been less than 0.25 m/s.
One heated component was the inlet pipe for the gas chromatograph/mass spectrometer. This was heated, albeit indirectly, to prevent droplets from blocking the pipe during descent. When switched on, the heater warmed up to about 90°C in the thin air of the stratosphere, but gradually cooled in the colder, denser air lower down. Then at impact, it warmed up again to 80°C, since cold air was no longer flowing through it. By carefully constructing a model of the heat flow from the heater to the inlet, it was possible to deduce that the ground was indeed damp with liquid methane (just as damp sand at the beach feels colder than dry sand); this meant that the methane did not come from some clathrate. But it wasn’t just methane that was sweated out of the ground at the landing site. An unanticipated host of compounds, including ethane, carbon dioxide, and possibly benzene, was present.
Another constraint on the surface winds emerged. Careful study by Erich Karkoschka at the University of Arizona showed that the amount of light received by DISR’s upward-looking photometer dipped slightly at impact. The dip was consistent with the parachute blocking off part of the bright sky as it fell to the ground. Karkoschka could even place limits on the wind speed.
Then there was the question of what exactly had happened at landing. In the impact acceleration record, there was some indication that the probe had bounced or slid somewhat for a second or two after impact. However, it didn’t move far. Some of the optical data from the DISR camera showed that the scene in front of that part of the camera had not changed substantially. An interesting result emerged from the AGC analysis of the radio link. After the rapid modulation of the signal strength as the probe spun and swung its way down to the surface, the probe orientation remained fixed on the ground, and so the signal strength should have varied only gradually when Cassini set on the western horizon as seen from the probe. But in fact, the signal strength underwent several large dips that initially defied explanation.
What was happening was that, because the probe was sitting on smooth ground, the radio signal could also be reflected from the ground up to Cassini. At some angles, the direct signal and the reflected one added together, but at others the two waves were out of phase and canceled out, leading to large drops in the received signal strength. It was a textbook case of what communications engineers call multipath interference. (It is the same effect that leads to huge changes in cell phone signal if you only move slightly.) By fitting the detailed pattern of dips in the signal history, engineer Miguel Perez at ESTEC was able to deduce that the probe antenna was 75 cm above the surface or, in other words, that the probe was just sitting on the surface. Thus, it had skidded out of its hole or, rather, much of the 12 cm of penetration had taken place above ground, by driving some cobbles into the soft sand. This had left the probe resting on top without having itself penetrated deeply. It was even possible to show that, in the direction westward of the probe, where there were no good images, the roughness of the landscape was similar to that seen toward the south where DISR was pointing after impact.
Figure 5.12. The GCMS measured the composition of the atmosphere and, fortunately, the surface too. These are mass spectra; heavier molecules are to the right. Compare this with figure 4.08. The abundance of CO2 was somewhat surprising. The argon abundance showed that Titan has been geologically active, and the detection of benzene showed that the surface is very rich in organic material. (Data courtesy of NASA GSFC/ESA)
Not all of the results made sense together—a symptom of Huygens being more heavily instrumented than Galileo or the Pioneer Venus probes. The SSP tilt sensors and a HASI accelerometer both seemed to indicate the probe was tilted by several degrees relative to the local vertical. But the horizon on the DISR images was pretty horizontal. It seemed unlikely that the ground would be sloping so steeply. Were some of the sensors wrong—and if they were, why did they agree with other factors? Maybe the probe had bent out of shape, changing the relative alignment. In fact, it would be impossible to know.
LANDING SITE CORRELATION
The Huygens landing site location determined by the DTWG (Descent Trajectory Working Group) remained just a set of numbers for over eight months. Observations of the region by Cassini’s camera and the VIMS instrument did not show details of a scale that could be correlated with the DISR images. However, the ninth close flyby of Titan by Cassini, T8, on October 26, 2005, was to pass relatively close to the landing region, and included radar observations near its closest approach.
Although radar observations of the landing site were literally a long shot, during the planning stages a couple of years previously, it had been considered worth a try. Nominally, the radar was to work in its imaging mode only when closer than 4,000 km, but in principle, imaging could be done from farther away using at least the center beam, which had a stronger signal. Imaging the landing site this way would require a good estimate of where the probe was, since using only the central beam would give a very narrow view, like looking through a straw.
However, the first few flybys showed that Titan’s surface was more reflective than had initially been feared. An echo strong enough for synthesizing images could be made from farther away than initially planned, and by setting the instrument parameters correctly, all five radar beams could be used to make a usefully wide image. It would still not be at such high resolution as the “real” opportunity to image the landing site on T41 in 2008, but it was worth a try while the Huygens results were still fresh.
RALPH’S LOG, OCTOBER 29, 2005
THE BREAKFAST CLUB
4:35 a.m.—my watch alarm goes off. The start of a long day—and a Saturday at that. As we (the radar team) planned how and when to work on the new data from T8, it transpired that to get prompt results quickly enough for the various nine-to-five public relations bureaucrats to approve press releases in time for inclusion in Thursday newspaper science supplements, we would have to work on a Saturday. It seemed somehow unfair, so I joked to the team that it seemed like some sort of “detention” at school for bad behavior—reminiscent of the movie The Breakfast Club.
Zibi gets up too, and before I am dressed has already logged in to check the latest Cassini ISS images. Over her shoulder I see some nice crescents—Titan images that I know must have been taken as Cassini receded from Titan. “That’s a good sign”—if the images at the end of the Titan sequence are on the ground, then chances are the radar data came down OK. I gulp some orange juice and clamp a slice of toast between my teeth as I head out the door.
Twenty minutes later, I park my Jeep at the airport. Ten minutes after that, I am at the gate, well in time for my 6:00 a.m. flight. Tucson is a conveniently small airport—easy to escape to or from. Soon after 9:00 a.m. I am in Burbank, California, and drive to JPL. Since it’s not a weekday, the traffic is light.
As I walk into the CSMAD lab at JPL (the Center for Space Mission Analysis and Design, better known to us as the radar “war room”), I see a radar image—just a small part, the highpriority landing site region. It doesn’t look like much, and there will be more later, so I adjourn to a discussion on scatterometry and radiometry modeling, catching up on some issues with measuring the noise floor in the instrument when the attenuator settings are changed. Later, celestial mechanic Nathan Strange updates us on some of the planning activities for the extended mission. He shows us a very creative presentation of how Cassini’s orbit around Saturn maps into each Titan flyby geometry. But the elegant astrodynamics is swiftly abandoned when the news arrives that the full radar image has been processed—and it’s a corker! But all we can do today is select a few choice pieces of the long image and draft press releases on how we see dunes and mountains. The data look great, but there is too much to digest and appreciate in a few short hours. Even though the real work of interpreting the data will take many weeks and hundreds of e-mails, the occasional face-to-face get-togethers are important for quickly exchanging ideas, especially when the team is confronted with exciting and puzzling new data. A few of us grab some dinner, and I head back to the airport. I arrive home close to midnight after my five-hundred-mile commute.
This radar sweep covered more terrain in one go than any of the previous passes—almost 2 percent of Titan’s surface. And yet again, Titan looked different—no impact craters, and hardly any fluvial channels. But there were many of the dark stripes dubbed “cat scratches,” like those seen in T3 in February (see chapter 6). However, many of these new ones were more than just dark stripes—they had bright highlights showing that they have positive relief. And they extended over a massive area, appearing to flow around mountains like the raked grooves in a Zen rock garden.
At the first viewing of the images, a number of vaguely circular features immediately excited the geologists on the team. They looked rather like volcanic calderas. And chains of mountains forming a strange chevron pattern were visible—a signature of tectonic stresses perhaps. Randy Kirk and Larry Soderblom of the U.S. Geological Survey, together with Lisa MacFarlane of the DISR team at the University of Arizona, tried to find a correlation between the radar image, which is most sensitive to slope and roughness, and the DISR images, which were sensitive to the brightness of whatever the surface is coated with. Some things looked as if they matched up, but it was much more challenging than expected: one match seemed as good as another. Matching the high-resolution optical map from DISR with a lower resolution regional optical map from ISS or VIMS and matching that with the regional-scale RADAR map seemed to help, but even then, there were a couple of equally persuasive matches.
A week or so later, however, after more considered study, a correlation emerged. The key was two dark streaks in the radar image that seemed to match up with two dark streaks in a few side-looking DISR images. The DISR team, shouldered with the responsibility of defining the real Titan and defending their conclusions to scientific peer review, were careful not to overinterpret horizontal stripes in data from the side-looking imager (SLI), which could easily be artifacts. Some amateurs, however, unburdened with such rigor, mosaicked the SLI images anyway, and their work showed a couple of dark stripes in the distance. Inspired with this clue, the SLI images were given a more thorough inspection, and Larry Soderblom nailed it.
Around five of the SLI images could be matched up with the dark radar streaks, and the high-resolution downward-looking frames taken at the same instant could be matched up with the rest of the DISR mosaic, making a geometrically accurate correlation between that and the radar image, which in turn was precisely tied to the orbiter’s known trajectory.
Some other features in the mosaic matched up, but there were also some in the DISR mosaic that appeared not to have strong contrast in the radar image. The dark streaks were particularly intriguing, however, because these features were seen elsewhere in the radar images.
RETROSPECTIVE ON HUYGENS
Even at the same time as analyzing the data and reconciling various discrepancies such as the apparently contradictory information from the pressure sensors and the altimeter, the instrument teams had one major duty, that of archiving the data. No one can anticipate all the useful analyses that can be made of data from planetary missions, and so it is important that it be placed in a public archive where anyone can access it. The same is done for Hubble Space Telescope images.
Figure 5.13. A portion (about 80 km top to bottom) of the orbiter RADAR observation from October 2005. At right the mosaic of DISR images is superposed. The two dark sand dunes proved to be the key to identifying the landing site, shown with a small cross to the lower right of center. Otherwise, the small-scale optical and radar appearance of Titan’s surface are surprisingly different. (Data: NASA/ESA/ASI/JPL/University of Arizona/USGS)
However, unlike HST images, which come from the same few instruments under much the same conditions, year after year, the results from a three-hour, one-off probe mission have all kinds of exceptions and complicating factors. These too have to be documented as best as possible—lest someone unfamiliar with the overall aspects of the project read too much into some rogue datapoint. Such documents are archived as plain text files, since it is not certain that any particular word processing programs will be available ten or fifty years from now.
One of the most remarkable products to come out of the DISR analysis was the painstaking work of Erich Karkoschka. Stitching together all the images and measuring their contrasts, he synthesized a movie of Titan as seen from the descending probe, complete with the sky color changing and the surface contrast improving at lower altitude. With such a product (he also made a geeky version with beeps and flashing lights to show when and where images were taken), it was easy to forget about the probe’s strange spin and the missing data. Probably few people in the future will bother to delve back into the raw data because the top-level results—wind profiles, not Doppler histories; neat mosaics of images, not the raw, blurry thumbnails—are the Huygens legacy. But getting to those from the raw bytes that crossed a billion miles on January 14, 2005, was the product of an intensive year and one-half (and a particularly frenzied couple of days) of effort by a few dozen scientists.
The Huygens results set the stage for Cassini’s subsequent findings, of Titan as a world of truly Earth-like complexity. The picture of the “shoreline” with river valleys, and the pebble-strewn view of the landing site, would define this world for ever after.
Figure 5.14. Synthesized views of Titan from the probe looking west (left) and north (right), seen from three different altitudes. The high-altitude views are blurred by the haze, but the two straight, dark sand dunes are visible north of the highland. At lower altitudes, progressively smaller features become visible. (ESA/NASA/University of Arizona)
But the mission was perhaps as important an engineering success as it was a scientific one. Huygens was, of course, a major coup for Europe, whose planetary science endeavors were ramping up (Mars Express in orbit, Venus Express under development, Rosetta on its way). Europe could put the Beagle 2 disappointment behind it and look forward to future missions, and indeed Huygens’s success probably played a significant role in ESA’s committing to future Mars landed missions through its new Aurora program. In the weeks and months after the encounter, SSP principal investigator John Zarnecki was visited by U.K. prime minister Tony Blair, and several French Huygens scientists were invited to the Elysée Palace to meet French president Jacques Chirac.
Psychologically, Huygens made Titan a real place—a place to which we might soon return. Before Huygens, Titan was a “here be dragons” sort of unknown place where one wouldn’t dare venture. Now one could just show the Huygens pictures and the destination was clear. It was not much of an intellectual leap to imagine replacing Huygens’s battery with a long-lived power source and its parachute with a balloon; instead of exploring Titan for only a couple of hours, one could do it for years.
As engineers began to consider what new and wonderful machines might be sent to explore Titan in the future (see chapter 7), the phrase “Huygens results” would become a totem, some heaven-sent stone tablets of laws on the Titan environment, to which one had to defer.
Some results were truly surprising. The low abundance of organic gases in the lower atmosphere (such as ethane), compared with predictions, indicated that some loss process (or some detail of the chemical formation process) was not understood. And the presence of haze all the way down to the ground was not anticipated at all. Some other results confirmed expectations from models or Voyager data, but models can often be wrong. Only by making a direct measurement—by physically going there with an in situ instrument—could we be sure about the temperature profile, or the argon abundance and its implications for the origins of Titan’s atmosphere.
Three decades of planetary science had previously had to make do with guesses and models, developed with much effort, that will now soon be forgotten. Those models had been necessary, indeed, to design the Huygens probe in the first place. But in future textbooks on planets—and despite being a satellite, Titan will doubtless deservedly appear in many (whatever the official definition of a planet)—the Huygens results will be taken as gospel, and quoted without much further comment as if they had always been known.