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The Lakes of Titan

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    Ellen Stofan (83) discusses her experiences as a planetary geologist.
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    Radar image obtained by the Cassini spacecraft's radar mapper of lakes (darkest regions) in the polar regions of Titan. The image is approximately 500 km in width. Image from the Jet Propulsion Lab and NASA.
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    Titan with Saturn in the background. Titan has a diameter of about 5,100 km and is cloaked in a thick atmosphere. Image from the Cassini spacecraft's narrow-angle camera. Image from the Jet Propulsion Lab and NASA.

Planetary geologist Ellen Stofan returned to her alma mater in April and talked up a storm—not your usual storm, but an incredibly cold tempest in which liquid methane takes the place of rainwater, which falls on water frozen literally hard as rock.

Methane rainstorms, cryovolcanoes spewing out a water-ammonia mix and debate about ethane glaciers are all just part of the unfolding understanding of the mechanics of the exotic geology of Titan, one of Saturn’s moons. In a presentation to an audience of geology faculty and students—the 1983 graduate spoke of the strange conditions on Titan being revealed by radar examination from the Cassini spacecraft in orbit around Saturn.

Two geology professors emeriti—Stephen Clement and Gerald Johnson—showed up to hear their former student. “It was a great thrill for me to see them there. I hadn’t seen them in years,” Stofan said. Did the presence of her old professors make her nervous? “A little bit,” she laughed. “There’s always the ‘you’re the student and they’re the professor’ sort of thing. But mainly it was just nice to see them there.”

Stofan was the lead author on a study of Titan’s geology published in Nature. She continues to evaluate scans of Titan’s surface being sent back from the Cassini orbiter. The depiction of the surface of the satellite is based on applying earthly geological principles to the data received from Cassini and from other sources, notably the Huygens probe sent down from Cassini to the surface of Titan.

Titan is of special interest to scientists, she said, because it’s a “pre-biotic world,” with a chemistry based on organic compounds that might eventually produce amino acids. Initially, researchers expected to find a heavily cratered surface, most of which were filled with liquid methane, possibly even a global methane/ethane ocean.

“There has been methane detected in the atmosphere and for there to be methane in the atmosphere it has to be replenished from some sort of source on the surface or subsurface,” she explained. When the Huygens lander touched down, it hit what Stofan characterized as a “crème brûlée” surface. “It hit a hard surface, then kind of squished through a hard crust,” she said. “So it was felt that at least the top level of the surface of Titan is relatively dry and underneath there is some kind of soggy stuff. Presumably, what is making it soggy is methane.”

The surface is covered with little pebbles, which she said were made of water ice. “But you have to stop thinking of it as water ice and think about it as rock, because at 90 degrees Kelvin, water ice is so hard that it behaves more or less like rock does,” she said. There was no global ocean, but plenty of lakes including several called “Lake Powells,” because they look so much like dammed reservoirs on Earth, plus at least one “sea” larger than Lake Superior. Much of the surface so far is exhibiting familiar-looking features, including some impact craters, but not nearly as many as expected. “Of course this doesn’t mean that Titan’s surface has not been impact-cratered,” she said, “but something has erased those craters.”

Stofan was brought up in a science-saturated household. Her father worked for NASA and her mother was a science teacher. “And so from about the time I was five or six years old, I knew I wanted to be a scientist,” she said. “Eventually I decided I wanted to be a geologist, partially because my mother took a geology course when she was doing a master’s degree in education. I tagged along on her field trip—and I loved it!”

She made the most of her father’s contacts at NASA, coming to realize that she could keep her head in the sky and her feet on the ground by combining planetary studies and geology into one field.

“I asked a lot of people who were working in the planetary field where I should go for an undergraduate degree—I knew I also would need to get a Ph.D.—and they said go to a good liberal arts school and get a good, well rounded, traditional geological education,” she said. “One of them—he actually was a former professor at Brown—recommended William & Mary.”

Stofan doesn’t always study geology from afar. In fact, she has been known to plan family vacations that just happen to be in the vicinity of active volcanoes. She said she became a geology major “back when the department was in Small Hall” and credits her many and varied undergraduate field experiences as helping her to understand the geology of earth as well as in less homely places.

“We did a lot of field work going up into the Appalachians. For almost every class I took, we would be able to go to some part of Virginia, because William and Mary is in a great setting,” she said in an interview following her presentation in McGlothlin-Street Hall. “If we were doing soft rock, more sedimentary rocks, we’d go out to the shoreline—the Chesapeake.”

Looking at a lot of terrestrial formations proved to be indispensable for a scientist who studies the methane table—or methanifer—of a remote moon and how it interacts with the numerous lakes and seas found on its surface. There also are the contributions of tholins—rocky hunks of carbon-based compounds that hail down on that crème brûlée surface of Titan. If geology is odd, so are the conditions; Stofan’s team gets its information in the form of long skinny strips of the moon’s surface from individual passes by the Cassini radar. It takes a trained eye to decipher the features correctly.

“I had such a valuable experience that a lot of people don’t have. In the field of planetary geology, for instance, a lot of people come from a physics background. They don’t have the geology background, let alone field-based experience,” she said. Planetary experience is helping Stofan understand the secrets of plain old terrestrial geology as well.

“When I go out into the field and work on volcanoes, I realize that what I can see in the remote sensing data for terrestrial volcanoes really only gives you a limited view of how that volcano works,” she said. “You always have to keep that in mind when you go down and look at a volcano. When you are able to go to other planets and look at volcanoes, you can put all that information together. It allows you to say: How does volcanism work as a fundamental process? It’s like a doctor who only has one patient. You might have some great theories of how the human body works, but all of a sudden if you have a hundred people, you realize that gosh, maybe some of the theories were oversimplified.”