Random Walk
Dune the Math
Despite its name, the Arroyo Seco (Spanish for “dry streambed”) meandering past Caltech’s Jet Propulsion Laboratory does actually contain water. But even if it were bone-dry, perpetually so, you wouldn’t need a planetary scientist like JPL’s Serina Diniega (BS ’03) to deduce that at one time something liquid this way came.
Shift now to Mars. Its empty riverbeds mark where water once flowed; its towering sand dunes don’t. Yet, observes Diniega, the dunes’ faces are scarred by deep channels—elongated hourglass-shaped gullies sluicing from crest to base. The wind eventually smoothes everything over; a decade’s worth of orbiter images confirms that. But eerily, new gouges keep appearing.
What’s flowing up there? Groundwater? Snowmelt? Condensation? All physically improbable, Diniega says: though those processes are common on Earth, “Mars is a different planet with its own mysteries.” The most likely culprit, she’s concluded, is carbon dioxide: the layer of dry-ice frost that blankets the planet’s vast dune fields through the Martian winter.
In recent papers in Science andGeology, Diniega and her collaborators postulate that as the spring thaws approach, the warming sands heat the frost layer from beneath, causing its underside to vaporize before its surface does. Trapped, the newly liberated carbon dioxide gas rushes sideways, picking up sand particles as it accelerates, until it reaches a crack—such as the ones typically found along the crests of dunes. There the sand squirts out like feathers from a ruptured mattress; falling back, it triggers miniature landslides that gash the dune’s face.
Diniega’s current focus is on the dynamics of lava flows here on Earth—quite a shift from Martian dunes. Diniega is fascinated by the dynamics of everything from lava flows to river channels. “My interdisciplinary research path has been an interesting one,” she acknowledges. At Caltech she was a math major—unusual for a planetary scientist. “I knew from my undergraduate experiences that I wanted to study how landforms evolve on different planets, and that I wanted to use mathematical techniques to do it. During my first year at the University of Arizona, I attended a math colloquium about ‘coalescence dynamics,’ in which you start with a large number of small things, but end up with a small number of large things—the way droplets of water collect, for example. That same week we discussed sand dunes in my planetary geomorphology class. Small sand dunes move faster than larger sand dunes, as there’s less sand to move, so the small ones catch up to the big ones and sometimes get absorbed. I decided I wanted to know more, and fortunately Karl Glasner, the applied mathematician who’d given the colloquium, was also intrigued. He ended up serving as my PhD advisor.”
So for the native Hawai`ian (whose interests include hula and who once captained Caltech’s fencing team, specializing in saber), the transition from Galois fields to lava fields wasn’t much of a lunge. “I’ve always enjoyed showing how I use math,” she explains, “in studies of more ‘real’ questions about how the world works.” —DZ
This Mars Reconnaissance Orbiter view of a dune field in early spring caught a dust cloud (arrow) kicked up by a mini-avalanche down a dune face some 40 meters tall. The dark streaks are believed to be landslides triggered by sand squirting from the dunes’ crests. The bright patches are carbon-dioxide ice remaining from winter.
