7. Where We Are and
Where We Are Going
As we put the finishing touches to this final chapter, the Cassini mission continues. But we have to draw a line somewhere and take stock even though new findings may be just around the corner—findings as thrilling and intriguing as those from the last Titan flyby we can include, T16 on July 22, 2006.
T16: LAKES AT LAST?
Titan had seemed almost defiant in concealing evidence of present-day surface liquids. The Ontario feature seen optically near the south pole was compelling but not completely persuasive, and the RADAR look at the south pole on T7 had been thwarted. The models that suggested Titan’s high latitudes should be damp were at least consistent with it being somewhat dry—and largely covered in sand dunes—at low latitudes. There was even a certain logic, given the rates of evaporation in summer, to lakes being dominant at the winter pole. And so for many, T16, the first of numerous planned RADAR encounters near Titan’s north pole (Cassini’s trajectory does not, in fact, cover the south polar regions often), was the “last chance” for lakes of ethane and methane. If they weren’t seen in Titan’s arctic, perhaps they wouldn’t be seen at all.
T16 did not disappoint. The RADAR swath, acquired on July 22, 2006, showed a dramatic collection of what had to be lakes at the highest latitudes covered (about 80°). These were shaped like lakes (as was the south polar feature), but there were many more of them. The superior resolution of the radar images showed several to have channels draining into them, although interestingly, most did not. This was consistent with Pascal Rannou’s climate model, in which a steady ethane-rich drizzle, rather than sudden methane thunderstorms, may characterize the high-latitude climate, so that the intense erosion needed to form channels does not often occur. Several of the lakes seemed to have “bathtub rings”—abrupt changes in slope that paralleled the edges of the lake, suggesting that the lake had once been filled higher. None of these factors absolutely meant that they were filled with liquid today—they could be lake beds.
But they were pitch black to the radar—much darker even than the dark spots and Si-Si seen on TA. They were virtually indistinguishable from nothingness, as if these areas had swallowed the radar energy completely. In principle, there were three possible explanations for surfaces behaving this way, although all required some geological process to form the lake-shaped feature in the first place.
First, and least likely, they could be made of something dense, such as ice, sludge, or even metal, that was very smooth. Metal didn’t seem likely, and for ice or sludge to behave this way, it would have to be devoid of internal imperfections that would otherwise give some diffuse reflections. That seemed rather improbable, and could also be dismissed another way. If they were mirrors like this, they would appear like cold space to the radar’s passive radiometer mode. They didn’t; they were strongly emissive.
Second, they could be filled with some low-density absorber like soot, much like the “Stealth” radar-absorbing region on Mars, which is believed to be a thick deposit of fluffy volcanic ash. But this would mean something made the lake-shaped pits, and these were then filled with some fluffy material, which somehow did not get blown around into dunes but stayed perfectly flat.
Or third, by the rule of Occam’s razor, they looked like liquid hydrocarbons filling lakes because that’s what they were.
This blackness required that they were very smooth, that there were no large waves on the surface. Could Titan’s winds of 0.5 m/s, enough to make sand dunes, be enough to kick up waves on such lake surfaces? Some work that had been done on the topic before Cassini’s early results suggested that it might be a waste of time, but the T16 observation gave the question new prominence.
A feature of the Earth sciences is that there are many complex processes that both have important effects on us and are close enough to observe in detail. The generation of ocean waves by the wind is a prime example. From a purely physical and mathematical point of view, the problem is hideous. The drag exerted by a wind on perfectly flat water will cause the surface to become unstable and little ripples will appear. It is the surface tension of water—the “skin” that stops pond-skating insects from getting wet—that pulls these waves back down. But the rippled surface is rougher, and so drags more momentum down from the airflow above and the waves grow. There is thus a complex feedback between the airflow and the water surface, and things get worse as different waves interact: the waves become too steep and break, and so on. The generation of water waves is a problem with which nineteenth- and early twentieth-century mathematicians such as William (Lord) Kelvin, Harold Jeffreys, and Horace Lamb all grappled, with only modest success even for the simplest of cases.
Figure 7.01. Lakes at last. This radar view from the T16 north-polar flyby shows what appears to be a large collection of lakes, varying in size, brightness, and (perhaps) depth of liquid. (NASA/JPL)
So scientists throw up their hands and take the expedient, and perfectly effective, approach of empiricism. It doesn’t matter how the waves grow and interact theoretically—we just put instruments on a lot of ocean buoys. We acquire huge numbers of datapoints of “windspeed x, wave height y” and choose a mathematical curve that goes through the middle of the cloud of points on the graph. Then, given a predicted wind speed, one just applies the equation of the curve and gets a decent estimate of what the wave height will be. (Actually, of course, it is a bit more sophisticated than that: the wave height depends on the width of the stretch of ocean—the “fetch”—over which the wind has had to act, and the wavelength will change with time too. Various empirical relationships exist for all these.)
The problem is that such empirical relationships are essentially worthless in an entirely different environment. Some half-hearted efforts were made to translate the empirical relations for Earth to Titan, by scaling the wave heights by a factor to take into account the different gravity. But that does only half the job. What about the different density of the air, the different density, viscosity, and surface tension of the liquid? And since the whole wave-growth process has many feedbacks, it is very nonlinear, so it wasn’t even obvious that the gravity factor used was the right one. What was needed was an experiment.
RALPH’S LOG, AUGUST 2003
THE MARS WIND TUNNEL,
NASA AMES RESEARCH CENTER
This is just the kind of experiment I like doing. Something messy—the sort of garage enterprise that no one would want to taint an actual lab with. I like something that sounds simple—even silly—but that will tell us something genuinely new and is small enough in scale that I can just go ahead and do it, and not have to write a big proposal to get special equipment.
Visiting the University of California, Santa Cruz (UCSC), I had encouraged a grad student, Erin Kraal, to think about seas and shoreline erosion on Titan. She was, in fact, working on Martian shorelines at the time, with her UCSC advisor Erik Asphaug and Jeff Moore at NASA’s nearby Ames Research Center. She worked on making a mathematical model of how waves of a given height will erode a shoreline to produce cliffs, but the question remained, How big would the waves be? This was as impossible a question for ancient Mars as it was for present-day Titan; some decent guesses could be made at what the wind speeds might be, but how big would that make the waves? Working with Erin kept me thinking about the waves, and opened the way to doing an experiment.
The key was MARSWIT—the Mars wind tunnel, operated by Arizona State University at NASA Ames. I had heard of this facility before—it was used to simulate the formation of wind ripples and sand dunes on Mars. I was familiar with wind tunnels from my undergraduate degree in aerospace systems engineering, and I assumed that MARSWIT was a closed-circuit tunnel that could be pumped down to a low pressure.
Officially, the tunnel is for research funded by the NASA Planetary Geology and Geophysics program. Other users are supposed to pay a fee. But we negotiated that since Erin was doing NASA-funded research, we could perform at least a trial experiment for a couple of days on a no-guarantee basis. We make a scouting visit to MARSWIT—I always find plenty of interesting people to talk with at Ames anyway.
The facility is not what I expected. Rather than a closed loop, the tunnel is open—just a long duct with a fan at one end. And this 20-m-long tunnel sits in a big concrete building (an old test chamber for rocket upper stages); all the air is pumped out of a huge section of the building! Massive steel doors isolate the vacuum chamber from the outside, and the corridors inside. An inch-thick glass window allows a view into the chamber from the control room, and slats would flick into place to limit the destruction if the window failed. The operators, nerds to a man, had taped a picture of Arnold Schwartzenegger’s character in Total Recall, clinging to a pipe as the air is sucked out of his Mars base—just how the control room would look. Who knew Arnold would become the governor of the state in a couple of years!
We set up for a week of experiments, and I arrive at San Jose with some very odd-looking luggage. Not least was a big plastic tray. I have a bag full of electronic parts—ultrasound and infrared sensors to measure the surface height of the liquid.
At first, we just try the sensors out and run the tunnel at room pressure. It makes little waves, which we can see in the data. We tweak the sensors a little (measuring things in the real world, I find nothing ever quite works as expected) and hope we’ll be able to run at low pressure.
The awkward aspect of being a “piggyback” customer is that one is at the mercy of the steam plant’s schedule. It takes an enormous amount of power to pump all the air out of a building, and this comes from a huge steam plant across the street (whose main function is to pump the air out of the arcjet plasma tunnels used to test heat shields elsewhere at Ames). And after a hopeful couple of hours, it emerges that the steam plant is not going to run that day.
Rather than waste the afternoon, we make a dash to a nearby hardware store and buy four gallons of kerosene. Running the wind tunnel with the doors to the building open to get rid of the smell, we find that, indeed, waves in kerosene are larger for the same wind speed than they are in water. That, at least, is a result.
The following day, we get some low-pressure results. I return to Tucson, leaving Erin to run the experiments for a couple of days. She bakes a motivational apple pie for the guys in the steam plant—a master stroke. Somehow the steam plant becomes very responsive to our needs, and in my absence, Erin generates a great set of data, with runs at different speeds at various pressures.
When I return the following week, on the way to the DPS meeting, we do another few runs and pack up. We try a very low-pressure run—10 mbar or so, about the highest pressures that exist on Mars today. The thin air fails to make any perceptible ripples, and the cooling by evaporation, the water being close to boiling at these low pressures, is enough to chill it to the point that a sheet of ice forms over the tray. Well, that’s that, then. Waves are pretty hard to make on Mars.
As we analyze the data some weeks later, to turn the stream of numbers from the sensors into a quantitative relationship between wind speed, pressure, and wave height, it becomes clear that there is a very strong sensitivity of wave height to pressure, but not the linear, well-behaved proportionality one might expect: waves basically are completely suppressed below 600 mbar, where the air density is only 60 percent of that at sea level on Earth. Titan, on the other hand, at 1,600 mbar and four times the air density, might therefore be much more effective at generating waves. And that’s in addition to the weak gravity. It’s a neat result, and a fun experiment that my colleagues appreciate.
Figure 7.02. A “kitchen sink” experiment in a unique facility. The Mars wind tunnel (MARSWIT) is housed in a huge airtight building at NASA Ames Research Center (left). It contains a small open-circuit wind tunnel (center), in which an instrumented tray of water and hydrocarbons (right) was installed to study wind-wave generation in extraterrestrial environments.
Probably someone, somewhere, doesn’t appreciate these experiments. A little kerosene spillage occurred in the rental car, which continued to smell strongly thereof. Let me hereby record my apologies to the next person who rented that car at San Jose airport.
THE END OF THE BEGINNING
It is hard to break off from the ongoing story of discovery at Titan, but break off we must. As this book goes to press, Cassini scientists anticipate another busy couple of years of the nominal mission, and a mission extension of at least a couple of years after that. All of this could change. Fate may catch up with Cassini and cause some instrument or crucial system to fail. Even being a productive asset in space is not always enough to avoid budget cuts. Though Cassini’s science operations were shaved to the bone years before (by some reckonings, the science teams are 50 percent underfunded), Cassini is a big item, a tempting target for budget planners who may not appreciate the damage that even what looks like a small, incremental cut can cause to a thinly stretched team.
What we may have to look forward to in the current plan includes the following highlights. With T11, T22, T33, and T38, the radio science team should have a set of data that nails down the internal structure of Titan, setting the context for any volcanic processes and perhaps indicating how thick the ice crust is.
Atmospheric results will accumulate, building up a multidimensional picture of how the haze, gases, temperatures, and wind vary with latitude, altitude, and time (and, for that matter, perhaps longitude too).
Most of Titan’s surface will be mapped somehow—the sub- and anti-Saturn areas seen so often during Cassini flybys, and low latitudes in general, should be quite well covered in the near-infrared and by radar. The southern hemisphere as a whole, in autumn sunlight during the nominal mission, should be fairly well imaged by VIMS and ISS, while the northern hemisphere is bracketed by many radar swaths. With all this in hand, we should have an understanding of the gross distribution of geological features and be able to say, for instance, whether there are any dunes at high latitudes, lakes in summer, volcanoes or craters in particularly old or young provinces. It seems certain there will be a long list of places we want to see again more closely.
By the end of the nominal mission, the RADAR instrument will have mapped about a quarter of Titan’s surface with its high-resolution SAR mode. This should be enough, except in some poorly covered areas such as the southern part of the trailing hemisphere, to get a good inventory of the various terrain types, and to understand what the global patterns of fluvial, aeolian, and volcanic activity might be.
ALL CHANGE
An extension to Cassini’s mission at Titan will be particularly interesting around 2010, the northern spring equinox (in fact, one Titan year after the Voyager 1 encounter in 1980). The reason is that, as the Sun moves north and crosses the equator, the distribution of sunlight on Titan, which drives the Hadley circulation), will reverse. Instead of the high-altitude winds going from south to north, and dragging haze with it, for a couple of Earth years, there may be an unusual (for Titan) Earth-like symmetric circulation pattern with warm air rising at the equator and descending in both hemispheres. Then the more usual “pole to pole” Hadley circulation will resume, this time going from north to south.
Another phenomenon that occurs at the equinox season is shadows. Titan can pass into eclipse behind Saturn. This is almost like a laboratory experiment in which the effects of sunlight on Titan’s upper atmosphere can be isolated. There are so many correlated combinations of orientation of plasma flow and sunlight on Titan that separating the two effects is difficult, but as Titan goes into eclipse, there is a sudden change from Sun to no Sun, while the plasma environment remains the same.
It will also be an important time to study the rings, when they are edge-on to the Sun and subtle variations in thickness, and bending waves or other warping will become especially visible. The grazing sunlight on the rings is also important for seeing the spokes that were first observed by Voyager, but have been almost invisible while the Sun has been high above the rings.
NEW TELESCOPES
Although Cassini has the best vantage point for observing the Saturnian system—from within—the view from Earth will nonetheless improve. New developments in telescopes and instruments continue to sharpen the Earth-based view.
HST’s successor, the James Webb Space Telescope (JWST), is set to launch sometime in the next decade. Its mirror, some 6.5 m in diameter, will be over twice the size of Hubble’s, giving it better light-collecting ability and higher resolution in principle, although that may depend on it being given the rather specialized ability to track solar system targets. JWST will make a spectacular series of observations, including images of Titan, from 0.7 microns to 5 microns with its near-infrared camera (NIRCAM), spectra with resolution of 1,000–3,000 over the same wavelength regions with the near-infrared spectrometer, and both images and spectra in the range of 5–27 microns with the mid-infrared instrument (MIRI). The spectral capabilities of the near-infrared spectrometer are much superior to those of Cassini’s VIMS, and JWST’s large mirror will give enough signal to search for the faint traces of organic compounds that have so far eluded identification. JWST, and the ever-growing arsenal of ground-based telescopes—with mirrors 20–30 m in diameter and images sharpened with near-infrared adaptive optics systems—will continue to monitor how Titan’s cloud patterns change with time.
On the ground, the 10-m-class telescopes like Keck and Gemini may give way to arrays of telescopes, almost magically connected to give the resolving power of much larger telescopes, and possibly much, much larger telescopes. These telescopes are still in their early planning stages, but the OWL (Overwhelmingly Large Telescope), many tens of meters across, could yield improvements as significant as the jump from the 1980s 3 m class to the turn-of-the-millennium 10 m class.
At other wavelengths, too, new tools will probe Titan. The VLA radio telescopes will be improved, to form the Expanded Very Large Array, with improved sensitivity and resolution. And there will be improvements, too, at shorter wavelengths, in the millimeter-wave range, where observers in the 1990s profiled a number of compounds, such as HCN and CO, in Titan’s atmosphere. A facility under development operating in this wavelength range is ALMA, the Atacama Large Millimeter Array (oxymoronic as that sounds). This observatory will feature a large number of shiny dishes, spread out over the high, dry Atacama Desert in Chile.
With the high resolution afforded by the array with these telescopes synthesized together—better than Hubble achieves at optical wavelengths—variations in temperatures and gas abundances with latitude and altitude may be monitored and the seasonal fluctuations tracked. All these Earth-based observations will help refine models of Titan’s climate and winds, which will be useful for the next step: flying on Titan.
THE EASIEST PLACE TO FLY
Serious contemplation of Titan exploration began in 1973, at a workshop at NASA Ames. Even then, before we knew any details at all, the idea of flying a balloon on Titan was discussed. After all, Titan had an atmosphere, so why not? The Voyager encounters confirmed that a balloon or airship would indeed be a quite viable concept. In Titan’s thick atmosphere, a balloon could be made rather smaller for a given carrying capacity than it would need to be on Earth.
A case could also be made for heavier-than-air platforms to explore Titan. These would benefit not only from the thick atmosphere but also from the low gravity. A nuclear-powered airplane could explore Titan for years, although accessing the surface would be a challenge. In the last five years, however, great strides have been made in unmanned aerial vehicles (UAVs) on Earth, to the point where they can fly autonomously across the oceans (or, for that matter, fire missiles at people). A more technically ambitious idea than an airplane would be a helicopter. Although more complicated, a helicopter has the obvious advantage that it could land more easily on the surface, to do chemical sampling or even take the seismic pulse of Titan.
But lighter-than-air ideas retain an appeal: they are simple, and somewhat fail-safe in that they should stay floating if they happen to lose power. A favorite option in the early part of the decade was an airship, able to move around more or less at will on Titan, and able to sample surface material by means of some sort of tethered grabber. If launched in 2010 or soon thereafter, it might be directed to Titan’s north pole, where, by the time it arrived in 2017 or so, the season would be midsummer. That meant that the pole would be not only in continuous sunlight but also in basically continuous view of the Earth. With a steerable dish antenna inside the airship envelope, or perhaps using an electronically steered phased-array antenna, it could communicate directly with Earth. Now, at Titan’s great distance, even a steered antenna would be able to pipe data down at one or two kilobits per second, a quarter of the data rate from the Huygens probe to Cassini. And it could do that not for two and one-half hours, but in principle for twenty-four hours a day, week after week.
The beauty of communicating directly to Earth was that the mission would not need an orbiter. Instead of a series of orbital insertion maneuvers and separation events, an airship could just be thrown fast at Titan and enter directly.
HOT AIR ON A COLD WORLD
An airship on Titan could use helium for lift, although there would be no reason why one couldn’t use slightly lighter hydrogen instead, since on Titan, where there is no oxygen, the hydrogen couldn’t catch fire. Some more recent ideas involve hot air balloons. It is easier to make these go up or down by modulating the temperature of gas inside the balloon. On Earth this is done by using propane burners. On Titan the waste heat from a radioisotope generator could be used.
Figure 7.03. A rosy future for Titan exploration? An artist’s impression of an autonomous airship exploring a methane vent on Titan. Saturn is visible in the background, and a faint rainbow is cast in the methane steam plume. The disks on the airship envelope form a steerable array antenna to transmit data directly to radio telescopes on Earth. (Mark Robertson-Tessi and Ralph Lorenz)
The thermoelectric devices that convert the heat from a radioisotope power source (RPS) into usable electricity are not very efficient—typically only 5 percent. And so, what is specified as a source of 100 W of electricity is actually putting out about 2 kW of heat. In fact, getting rid of the surplus heat is often a challenge for spacecraft designers, especially when the source is encased deep inside some entry heat shield. Extra fluid loops and radiators often need to be installed. Normally, this heat is just wasted, or perhaps a little is tapped off to help keep a spacecraft warm, but on a Titan balloon or Montgolfière (the name the Montgolfier brothers gave their balloon; both pronunciations seem to get used), the cooling fins of the RPS would be placed at the neck of the balloon, putting enough heat in to keep the air 15 K or more above the ambient temperature. Jack Jones of JPL showed that ballooning this way on Titan is many times more efficient (in terms of lift per watt of heat) than on Earth. Probably the skin of the balloon would need to be double-walled (as some long-duration balloons on Earth are) in order to keep the heat in enough to inflate the balloon as it descended by parachute when released from its entry shield.
A large vent valve would also be placed at the top of the balloon, to allow the warm gas to be dumped to cause a descent. This would allow the balloon to get close to the surface for more detailed inspection of areas of interest, or even to sample surface material with some sort of harpoon.
Julian Nott, a British balloonist who holds seventy-six world records and demonstrated how the ancient Peruvians might have used hot air balloons to direct and observe the Nazca lines—great drawings on the landscape, visible only from above—has embraced the Titan ballooning challenge. At a probe workshop in Pasadena in June 2006, he suggested that one might use a sort of small rocket engine to quickly fill a balloon with hot air as it descended from its entry shell, giving it instant buoyancy, rather than landing on the ground first or waiting for the slow heat of a radioisotope source to warm it up. Whether this will turn out to be an efficient solution will require some detailed study, but that is not the point—ballooning on Titan is capturing the technical imagination of lots of smart people. And Titan as an environment is comfortable enough that there are lots of technical possibilities. Engineers aren’t “walled in” to a single way of doing things.
What might a balloon measure? It would take pictures, of course. It could drift for a year or more, taking sharper pictures than Huygens, and thousands of times more of them, giving us an “airplane window” view of Titan’s varied landscape. A ground-penetrating radar is another idea—one that is able to measure the depth of the lakes, and perhaps the depth of the sands in Belet and Shangri-La, and possibly revealing the structure of buried craters like Guabanito.
And of course, measuring the composition of the organic materials on the surface is a key objective. Just how complex do molecules get when Titan’s tholin is exposed to liquid water for thousands of years?
One can’t land at and sample every spot on the surface, so some way of classifying the chemistry remotely is needed. Near-infrared spectroscopy may offer some clues—Cassini VIMS data are already hinting at some compositional variations. But a rather interesting idea came from work by Rob Hodyss at Caltech in 2003. In studying tholins made in the laboratory, and the compounds formed by the interaction of tholins with water, he discovered that tholins fluoresce. That is, if illuminated by ultraviolet light, they glow. You can see the same effect in a nightclub. Fluorescent materials are added to laundry detergents to give that “whiter than white” appearance, and so white shirts glow impressively under UV lights. The organic compound and antimalarial agent quinine is also fluorescent, so gin and tonics glow a pale green under UV illumination. Interestingly, tholins that have been mixed with water and then frozen into the ice glow a slightly different color from pure, unwashed tholins. And so, a balloon near Titan’s surface equipped with a UV searchlight (remember, all the Sun’s UV has been absorbed by methane and the haze at high altitudes on Titan) could spot tholins exposed to hydrolysis, and hence the most likely sites for the most interesting prebiotic chemicals, by the way they glow under UV illumination.
The scientific community has much work to do to work out the best measurements to characterize Titan’s surface materials. On Mars, it is easy. Almost any organic molecule would be interesting, since the Red Planet’s oxidizing soils break organics down quickly. Titan is awash with organics, and so ways of measuring the most interesting ones are needed.
Most thinking has, not unnaturally, been directed toward what Titan’s chemistry can tell us about how chemical systems can become progressively more complex and ultimately lead to life. We don’t know how far this process went on Titan. Although nothing is impossible, it seems unlikely that life has evolved on Titan.
It has been argued that Titan’s interior (the liquid water layer) is not in principle uninhabitable, since there could be abundant dissolved nutrients there. So, what if life came from elsewhere to Titan? This question was addressed by dynamicist Brett Gladman of the University of British Columbia in 2006. He noted that rocks ejected from Earth during impacts could make their way to Titan, much like the Martian meteorite ALH84001 that was found in Antarctica came to Earth. And Titan’s thick atmosphere would give them a soft landing. Although the odds are somewhat astronomical, microbes from Earth could have been delivered to Titan’s surface. If they happened to land during Titan’s early years, when the water—ammonia ocean was exposed to the atmosphere, perhaps they might have flourished. You can never say never.
RALPH’S LOG, 2001
TUCSON
Radiocarbon on Titan. It is comparatively rare in modern specialized science, and is often the mark of good science, that an idea can be completely encapsulated in three words. Tim Swindle, a meteoriticist at the University of Arizona, began considering techniques for determining the geological ages of materials on Mars, Europa, and Titan. When he suggested that cosmic rays impinging on Titan’s nitrogen-rich atmosphere would create carbon-14, as they do on Earth, the idea clicked in my head. Of course!
Tim and I, together with Tim Jull, a world expert in radiocarbon measurements using the sensitive technique of accelerator mass spectrometry at the University of Arizona (and, like myself, a Brit by birth—we seem to get everywhere), and Jonathan Lunine, began to work out the details. In the process, I began to wonder how much of a lethal dose I have accumulated crossing the pond while working on Cassini.
The essence of the situation is this. Energetic cosmic rays, flung out from distant supernovae, for example, can do a number of things when they strike matter. One effect is to tear apart molecules, creating ions or free radicals. These radicals are what can attack DNA in living things, causing mutations. Another effect is to dump charge into semiconductors, causing “snow” in digital images or random errors in computer memories. But sometimes, just sometimes, the miracle of nuclear alchemy occurs. Cosmic rays hit a nucleus and transmute one element into another.
An awful lot of the nuclei that a cosmic ray lancing into the Earth from space will hit are those of nitrogen. Nuclei of nitrogen struck in this way tend to turn into carbon-14. In chemical terms, this acts much like any other carbon on Earth, and gets assimilated as carbon dioxide into the tissue of trees and other living things. But carbon-14 is radioactive, decaying with the emission of a beta particle (of a characteristic energy that can be detected by a fairly simple detector if you have enough carbon) with such a probability that half of them do so within about six thousand years. This decay means the fraction of carbon that is radiocarbon in a sample of once-living material like wood decreases predictably with time and makes it possible to do radiocarbon dating.
Cosmic rays produce showers of secondary particles, notably muons. These are the dominant contributor to the background radiation measured at most points on the Earth’s surface. This particle flux increases as one goes higher in the atmosphere. In Tucson, at an altitude of 700 m, a typical count rate for a small Geiger counter is a few counts per minute. Close to sea level, it is typically a factor of two or three smaller. In a commercial airliner on a transatlantic flight, at 10 km altitude, the count is twenty times higher. In ten hours the radiation dose is comparable with that received in a chest X-ray.
There is an altitude range over which the cosmic ray collisions tend to occur. At the very highest altitudes, there are comparatively few atoms in every cubic meter of air for the cosmic rays to hit, and so the rate is low. At the deepest levels in the atmosphere, the mass of gas above has already absorbed most of the cosmic rays. And so somewhere in between, there is a peak production rate. This occurs on Earth at altitudes of a few kilometers. On Titan, with its distended, more massive atmosphere, the production peaks around 70 km, well above the surface. It had been calculated that this cosmic ray absorption peak might produce a detectable increase in the electrical conductivity of the atmosphere around this altitude, as the electrons torn off by cosmic rays float around before recombining with an ion or molecule.
The Arizona team calculated that the production rate of radiocarbon should be a little (a factor of four or so) higher than the terrestrial rate, which is about two atoms per square centimeter per second. Because an atom of radiocarbon will decay with a half-life of about six thousand years, it follows that the quantity of radiocarbon would build up until there are so many that they decay as quickly as they are formed. This happens around five hundred billion atoms per square centimeter.
To balance this production of several per second per square centimeter, there must correspondingly be several decays per square centimeter of surface. But this can mean a lot of things—several decays spread through a kilometer-deep layer of liquid and sludge hydrocarbons on the surface, or dispersed through the tens of kilometers of the troposphere. In these cases, the handful of disintegrations per second would be hard to find and count.
But what if the radiocarbon, being formed in the lower part of the haze layer, tended to attach itself to the haze particles? Even though the haze looks thick, in reality it corresponds to a layer only a few microns thick. If most of the radiocarbon were to be incorporated into the haze, then “fresh” haze, falling from the atmosphere in a thousand years or so—well under the half-life of radiocarbon—would have a fairly high proportion of radiocarbon in it, perhaps one part per billion. This would be enough to make the material itself quite radioactive.
Although this stuff wouldn’t be radioactive to the extent that refined radioisotopes are, it would be hundreds of times more radioactive than the most radioactive organic material on Earth (Brazil nuts; the nut trees take up a lot of calcium from the soil, and in so doing also incorporate a fair amount of radium, such that Brazil nuts give off about one decay per second per gram). It would be something easy to detect with a simple radiation counter.
Perhaps some day in the future, a robot airship traversing Titan’s landscape may use a radiation counter to measure whether some deposit of organic gunk in the banks of a streambed is some ancient seam cut into by the stream, or was left behind when a methane storm just a thousand years ago washed material down from a wide area and concentrated it here for our convenience.
SURFING THE TIDE
Simulations by Tetsuya Tokano and Ralph Lorenz in 2005 showed how a balloon on Titan would wander over a wide region and not just stick to a line of latitude, as was thought in the late 1990s. In fact, just how much the balloon would drift north or south depends in a rather interesting way on where it is. The tides cause a pressure bulge to sweep around Titan, causing winds with a substantial north—south component that act at a point on Titan for a couple of Earth days, a fraction of a Titan orbital period, before the tidal bulge sweeps past. How far the balloon is displaced depends on how long it spends in the tidal bulge. At the lowest altitudes where the zonal wind is weakest, the balloon almost sits in place and the bulge sweeps over it briefly, giving a small displacement. Conversely, at high altitude, the fast zonal winds carry the balloon through the bulge quickly, again yielding a small displacement. But at some intermediate altitude, the zonal wind carries the balloon along at just the same speed as the tidal bulge, and so the balloon stays in it, drifting north or south all the time. It’s just like surfing—match your speed with the wave and it will sustain a large motion, whereas a buoy bobbing in one place or a speedboat lancing through a wave will barely feel any effects.
One interesting feature of the wind field computed in the models is that, in some seasons, the zonal (west—east) flow is reversed at low altitude—as indeed was observed by the Huygens probe. This means that a balloon that can control its altitude can first reconnoiter a site by drifting overhead and taking pictures, which are sent back to Earth. At the right altitude (a few kilometers), the balloon does not drift too far during the time taken to transmit them, plus a few hours for analysis on the ground. If the area looks to be of special appeal to scientists, the balloon could be commanded to descend to low altitude, where it would backtrack and sample a designated site.
Of course, many details need to be worked out—for example, how best to mesh the limited onboard intelligence of the balloon (which can respond immediately) with the more formidable intellect on the ground, which is two hours light time away. And at present, the tidal wind model is just a theory. But as the models are progressively tuned to include observations of clouds on Titan, and the orientation of the sand dunes, their predictive capability will improve.
WHERE TO?
Whether all this happens in 2020 or 2025 or 2030 is anyone’s guess. Usually, short-term progress on grand projects like this is much slower than one expects, but in the long term, advances can be surprising.
Consider Mars exploration. When the Cassini project began in 1990, nothing had been sent to Mars since 1975 except two Russian Phobos spacecraft (the only interplanetary spacecraft more massive than Cassini), which failed before they returned much data. A couple of years later, Mars Observer exploded on arrival in orbit. It was an agonizing wait for Mars scientists as missions in subsequent years slowly started to recover the situation. And yet as of this writing, in 2006, there are no less than four spacecraft orbiting Mars (Mars Global Surveyor, Mars Odyssey, and the recently arrived Mars Reconnaissance Orbiter slowly aerobraking into its mapping orbit, plus the European Mars Express) and two rovers, Spirit and Opportunity, trundling around on the surface. So a decade and one-half is a long time in the space business generally, even if it is only the typical length of a project to the outer solar system.
In the meantime, Cassini’s torrent of data will sustain an extended campaign of scientific study for well over a decade after it ends. New generations of graduate students will trawl through the data, applying new methods and approaches. What happens next will depend on the relative importance of different branches of science, and on the politics of space exploration. A technologically risk-averse approach would favor an orbiter, which would address primarily geological and geophysical objectives—global mapping, global topography, gravity, and magnetic fields—as well as monitor the weather in the lower and perhaps upper atmosphere. We know how to do orbiters.
If, on the other hand, astrobiology and organic chemistry are perceived as being important enough, the priority would shift toward in situ exploration, and a balloon able to sample surface material. Such a mission would benefit from an orbiter, but in principle could be accomplished without one.
But there is competition in the outer solar system. Many scientists favor Europa as a scientific target. It is arguably a more relevant astrobiological target, with a water ocean (and perhaps hydrothermal vents like the Earth) closer to the surface than Titan’s. However, even that ocean is inaccessible as far as today’s technology goes, and airless organic-poor Europa is a body with narrower scientific appeal than Titan.
Another lobby favors a Neptune orbiter. Although all agree that Neptune and its exotic geysering moon Triton, as well as its smaller moons and its rings, deserve systematic exploration much like Cassini has done for Saturn, the harsh reality is that reaching Neptune to enter orbit around it takes decades. The dilemma is that, the faster one flies the 30 AU to Neptune, the more effort one needs—as rocket power or in the form of an aerocapture heat shield—to stop when one gets there. Voyager reached Neptune in about twelve years but didn’t stop. It spent only a meager few weeks close enough to do better than Hubble.
Titan, however, is not getting any less fascinating with time as Cassini’s findings draw interest from sedimentologists to study its dunes, from chemists to study its atmosphere and haze, from meteorologists to study its clouds, oceanographers and limnologists to study its lakes. And for engineers, too, Titan has particular appeal. Although NASA is often seen as a scientific enterprise, in many respects it is more a “mission” organization, oriented around organizing and building spacecraft. A Titan mission, involving aerobraking into orbit and depositing a balloon and maybe a lander too, offers a wide range of interesting but tractable technical challenges.
Cassini’s discoveries at Saturn have also thrown a new player into the ring: Enceladus. If liquid water is the key to finding life, then Enceladus seems a much more realistic prospect for actually touching the stuff than Europa. Although a Europa orbiter is difficult and expensive because of the rocket power needed and the intense radiation at Jupiter, a lander is tougher still. Yet Enceladus is cooperatively hosing its water (and organics with it) out into space, where it can easily be caught and analyzed.
What the next few years will tell us, and the effects they will have on the direction of future exploration, is impossible to guess. Maybe a Europa mission will go first after all, or maybe a diversion of NASA’s resources into sending people to the Moon will stall outer solar system exploration altogether for a while. But sooner or later, we will go back to Titan.
TITAN: A NEW WORLD
The tale of unveiling Titan in the last year or two has been one of science in action, with some ideas tested by data and found wanting, and in other cases guesses being smugly confirmed. Still others are ideas that we want to be true, but the data so far are not good enough to be sure. It is work in progress.
Whatever happens in the future, the last few years have seen a revolution as far as Titan is concerned. Just as Mars has changed since the late 1960s from a world of mystery, with scientists struggling to interpret a few patterns of bright and dark seen through a telescope, into a world whose landscape and climate we are familiar with, and on which our machines (and perhaps one day ourselves) can explore, so Titan has been transformed into a known world—a world strangely like our own. Like Mars’s dust storms, frost cycle, and northern lowlands, Titan’s polar hood, methane monsoons, and equatorial sand seas are entering our consciousness as features of the universe around us. And like the Sea of Tranquility on the Moon (the landing site of Apollo 11) or Meridiani Planum on Mars (the landing site of the Opportunity rover), so ShangriLa, Belet, and Xanadu will become names that instantly evoke pictures— the same pictures for all of us, since we all share the same robot proxy— of a far-off landscape remarkably similar to the ones we live in but nevertheless exotically different.