Q&A: A Career Shaped by Mars and Weird Fluids
Tracy Gregg was headed toward a career as a medical doctor when an encounter with Mars changed her course. Assigned to read about NASA’s Viking missions for an elective undergraduate class, Gregg found herself transfixed by a photograph taken by the Viking 2 lander of a frosty Martian landscape strewn with boulders. Glancing briefly out of the window beside her desk, Gregg’s eye fell on the red planet, which was visible that evening in the night sky. Switching her focus back to the photo and then again up to the sky, Gregg had the weird sensation the she was actually standing on the distant planet with the rocky scene spread out in front of her. Something inside her stirred, and in that moment her dreams of being a doctor ended. She was going to be a planetary scientist.
Today Gregg works at the University at Buffalo in New York where she uses computer models to study how fluid flows shape the surfaces of celestial bodies including Mars, Earth, Venus, and Titan—Saturn’s largest moon. Physics caught up with Gregg at the recent Lunar and Planetary Science Conference (LPSC) in Houston to find out about a “funky” mixture of methane and ethane that flows on Titan and how this gasoline-like liquid carves out rivulets in the moon’s surface. We also discussed the conference and how it’s changed—for better and worse—since she first attended back in 1990.
–Katherine Wright
Tell me about Titan and its weird fluid.
The images we have of Titan’s surface indicate it houses canyons that look a lot like Earth’s narrow, steep-walled “slot” canyons. On Earth, slot canyons form in desert regions—think Arizona and the rest of the American Southwest—after incredible flash flood events that occur every few hundred years and transport huge amounts of sediment. The floods act like liquid sandpaper, carving out these deep structures.
What I’ve been trying to understand is how rivers of liquid ethane and methane can pick up, transport, and deposit sediment to carve out canyons in the water ice on Titan. This weird, funky mixture doesn’t occur on Earth. It’s a lot runnier than water and also a lot less dense.
What have you learned?
The takeaway is that Titan’s fluid has to be traveling three times faster than water to move sediments. But once those sediments are incorporated into the flow, it’s a lot harder for them to settle out—the fluid has to slow down much more than water does to drop its load.
This behavior has implications for how rivers made of the fluid carve Titan’s canyons and how they forge deltas, which develop when a river meets a lake or ocean. It suggests, for example, that it’s harder to form deltas on Titan. Not impossible, just harder. And the deltas that do form should be longer and thinner.
Is there any evidence for such deltas?
It’s unclear. We do see vast smooth plains, which could indicate deltas. But we need more data to be sure.
You’ve been a planetary scientist for nearly three decades. What is it about the research that holds your fascination?
I never know for sure if my ideas are correct. There’s this area on Mars that I’ve been studying for 25 years. I still can’t say with 100% confidence whether it’s formed from runny lava pouring out of a volcanic vent or whether it’s formed from lake sediments. Some weeks I think yes, it’s a big, volcanic plain; other weeks it’s definitely an old lake. I find that uncertainty thrilling. I love that I have to come up with multiple working hypotheses that I then try to disprove. When I’m done, I might be down to two or three theories that I simply can’t rule out without more data.
What would it take to definitely prove or disprove whether that region of Mars is a lava or lake deposit?
Boots on the ground. You need to dig down so you can properly see what’s going on. And if they’re looking for volunteers, I'll go to Mars. I’m compact!
Switching gears, this LPSC is the 29th you’ve attended; what keeps you coming back?
LPSC is the world’s largest planetary geology conference and coming is the best way to cram the most science into the smallest amount of time. The conference also feels like a family reunion—there are hugs and handshakes in the hall and plenty of laughter, tears, and drama that has nothing to do with science. These people have become my friends. My coming back year after year has a lot to do with that.
How has LPSC changed over the years?
The saddest change is the location. The meeting used to be held at the Johnson Space Center in Houston and my first talk was in their gymnasium. LPSC long ago outgrew that space, but what forced us out was 9/11—they could no longer do all the needed background checks to let everybody on site. I feel that change was a big loss. On the positive side, the diversity of attendees has grown. At my first LPSC, I was the only woman speaker in my session. Now it’s common for women to give half of a session’s talks.
Katherine Wright is a Senior Editor for Physics.
Know a physicist with a knack for explaining their research to others? Write to physics@aps.org. All interviews are edited for brevity and clarity.