Look in your newspaper this Saturday, and you may see a paragraph about Saturn’s moon Titan and a giant storm that moved across the surface last May and what that means. With luck they’ll even print it with a tiny little picture of Titan to catch your eye. Your response, if you have one, will likely be “huh.” It’s OK. I’m not offended. It’s hard to distill the richness of a full scientific paper into a paragraph. And it’s even harder, still, to distill the richness of a decade of scientific inquiry into a short scientific paper. But if you’re curious about what that little paragraph means, and how it came to be in your newspaper, and what we’ve been doing for the past decade, read on. It’s a long story, but that’s somewhat of the point.
I became interested in Titan ten years ago, almost as a matter of convenience. It was an excellent solar system target for the then-new technique of Adaptive Optics, which attempts to undo some of the effects of the smearing of starlight caused by Earth’s atmosphere. Titan was a great target because it is just small enough to be completely smeared by the atmosphere, but big enough that, if you could unsmear it, you would still have a nice view. Just as importantly, no one had ever had a nice view of the surface of Titan before because the satellite it covered in a thick layer of smog which mostly doesn’t let light penetrate to the surface. When the Voyager spacecrafts flew by, they took pictures of Titan which look like a big orange billiard ball. I should have said, though, that visible light doesn’t penetrate to the surface. On the earth, red light penetrates smog better than blue light (hence the nice red sunsets on a smoggy day in Los Angeles). The same happens on Titan. Red penetrates better than blue, but infrared penetrates better still. In fact, if you go far enough into the infrared, you can take a picture of Titan and almost not notice any smog there at all. Conveniently, the new technique of Adaptive Optics works best in the infrared. Hence Titan became a natural target to try out the new techniques on. Antonin Bouchez, then a relatively new graduate student at Caltech, signed on to do this project as part of his Ph.D. thesis.
Our first goals were to obtain maps of the then-almost-totally-unknown surface of Titan. And what a strange looking surface it turned out to be! We speculated endlessly about what all of those dark and bright spots on the surface might be (for the most part it is fair to say that we – and everyone else – had no idea whatsoever until we got better images from Cassini a few years later). And then, in late 2001, we found a cloud sitting at the south pole of Titan.
A cloud!
It doesn’t sound like such a big deal, except that it had long been predicted that Titan was incapable of having clouds. Occasionally there was speculation that clouds of methane might be present, but that, if so, they would be tightly confined to the equator. And yet there it indisputably was: a cloud at the south pole.
Antonin and I were so astounded by this that we put Sarah Horst, then an undergraduate at Caltech, at work looking through a tiny 14-inch telescope on the roof of the astronomy building at Caltech. We had developed some special telescope filters which would – we hoped – be capable of penetrating the haze deck and seeing if Titan got a little brighter due to a cloud or two. We wouldn’t be able to tell anything else, but that would be enough to go back to the giant Keck telescope and say “Look now! There will be a cloud!”
It worked. Just a month after our first cloud detection Sarah saw something that looked just like what we expected a cloud to look like. We called people at the Keck telescope and begged them to snap a picture, and there it was. A much bigger splotch, still near the south pole.
I’m an astronomer, not a meteorologist. I had to spend six months learning about how clouds worked, trying to understand precisely why people thought they wouldn’t occur on Titan, and figuring out what was wrong. On a long summertime flight across the country where we continuously skirted afternoon thunderstorms, it all came together: no one had ever previously bothered to consider the effect of Titan’s surface heating. Like Arizona on a summer afternoon, Titan’s surface can heat up and eventually drive convective clouds over it. On Titan, though, it doesn’t happen in the afternoon. It happens in the summertime, when the south pole spends something like 10 years in continuous sunlight.
It was a compelling story, and, I think true. But, even better, it made some fairly clear predictions. The clouds were at the south pole when we discovered them only because it was very close to southern summer solstice. Titan (and Saturn) takes 30 years to go around the sun, so its seasons are quite long. But if you had the patience to watch, you should see the clouds move from the south pole to the north pole over the next 15 years before coming back 15 years later.
Antonin eventually got his Ph.D. and moved on to take a job working with the technical team continuing the development of Adaptive Optics at the Keck Observatory. It was the perfect place to be. Whenever there was a spare moment or two at the telescope, he would swing it over towards Titan and snap a picture. The clouds were nonstop. Sometimes there were just a few tiny specks, but occasionally there would be a huge outburst. It was a thrilling show to watch.
Emily Schaller entered graduate school at Caltech at just about that time, and she decided to do her thesis on watching and understanding these developing clouds on Titan. The first year was exciting, indeed. She saw a monster cloud system cover the south pole of Titan and remain for more than a month (disappearing just as one of the first close Cassini flybys went in to take pictures; Cassini saw a few wispy little clouds but missed almost all of the action). Henry Roe, a recently graduated Ph.D. from the University of California at Berkeley who had been using the Adaptive Optics on the Gemini telescope to study Titan, moved down to Caltech to work with us, and the odd discoveries about the clouds poured in. They appeared to finally move north from the pole; they appeared tied to one spot at 40 degrees south latitude for a while; they untied themselves; bright clouds in one spot seemed to foretell bright clouds in another. It was clear that we were amateurs here. We enlisted the help of Tapio Schneider, a professor of environmental engineering at Caltech and one of the world’s experts on atmospheric circulations, to help us make sense of what was going on. Things were finally falling into place.
In one final piece of exceedingly clever astronomy, Emily Schaller replaced our clunky nightly observations with a 14-inch Celestron, originally begun by Sarah Horst, with a sleek set of nightly observations from NASA’s Infrared Telescope Facility on top of Mauna Kea. The IRTF would take a quick spectrum of Titan every night possible, and Emily could quickly look at the rainbow of infrared light to tell precisely how many clouds were there. And when they looked good, she could tell Henry Roe, who would get the Gemini telescope to examine them.
And then the clouds stopped.
For years and years Emily would look at her data in the morning and walk across the hall to my office to mournfully say “no clouds again last night.” Seeing no clouds is scientifically interesting, and she dutifully wrote papers and indeed an entire chapter of her Ph.D. thesis demonstrating and trying to explain this years-long lack of clouds. But, really, I understood. Explaining a lack of something is not nearly as satisfying as actually getting to see something happen. As her advisor, I would have been happy to fly to Titan to perform a little cloud-seeding, but no one had yet figured out exactly what chemicals or incantations might do the trick.
On April 14th last year, Emily walked across the Caltech campus to finally turn in her thesis. Then she did what she did most mornings: she walked to her office, downloaded the data from the night before, and checked to see if Titan had clouds. That morning, I suspect, she came close to falling out of her chair. She was likely exhausted from those final stretches of thesis writing, and I am sure that the first time she plotted her data she did what I always do when I see something astounding: she assumed she had made a mistake. She probably re-downloaded the data, double-checked the coordinates, and shook herself a little more awake. But it was no mistake. Titan suddenly had the largest cloud system seen in years. She likes to say it was Titan throwing a graduation party for her. But I know better: I think Titan likes to hide its secrets as long as possible, and knew it was finally safe to let go.
The scientific paper that Emily wrote along with Henry, Tapio, and I that appears in Nature describes the big cloud outburst and its scientific implications. And the implications are pretty fascinating. This big cloud outburst – the biggest ever seen – began in the tropics of Titan, where it has been speculated that clouds, if they ever form, should be weak wispy things. The tropics are where, of course, the Huygens spacecraft that landed on Titan took dramatic images of things that look like stream beds and shorelines and carved channels. How could those be at the equator if there are never clouds and never rain? People asked.
This discovery doesn’t actually answer that question, because we don’t know why there was a huge outburst of clouds in the tropics of Titan. But it does perhaps answer that lingering question: How could those be at the equator if there is never rain? Because there is rain.
Now, however, I am going to allow myself to speculate a bit more than we were comfortable speculating in the scientific paper. I am going to ask: Why? Why were there clouds in the tropics? Why did they appear suddenly at one spot? What is going on?
What I think is going on (again, I warn you, rampant speculation follows…) is that Titan occasionally burps methane, and I think Emily found one of the burps. For many years scientists have wondered where all of the methane in Titan’s atmosphere comes from, and, I think, here is the answer. The surface occasionally releases methane. Call it what you want. Methane geysers? Cryovolcanoe? Titanian cows? Whatever happens, the methane gets injected into the atmosphere and, at that location, instantly forms a huge methane cloud. Massive rainout ensues downwind. The stream channels, the shorelines, and everything else in the otherwise desert-seeming regions are carved in massive storms.
Evidence? Evidence? Where’s the evidence? You scream. Fair enough. I am giving you a snapshot of how science is done, and, at this point, this is the hypothesis stage. Or hunch. Or speculation. This hunch is the type that then guides what we go off to try to observe next. What will we see? Will the spot Emily found burp again? That would be pretty striking confirmation. Will other spots blow? (I should mention that we do indeed think we saw a different spot burp a few years ago).
At this point we have observed Titan well for about 7 years, from the winter southern solstice until the northern spring equinox, which actually just occurred last week, the terrestrial equivalent of late December to late March. What will the rest of the year reveal? We’re still watching, waiting. Maybe in 23 years, when we’ve finally seen an entire season, we’ll call it a day.
