Mystery on the moon

Graduate student Erin Flowers

Focus on astrophysical sciences

Mystery on the moon

Erin Flowers explores evidence for liquid (and life) on Titan

Photo by Sameer A. Khan/Fotobuddy

By Alice McBride

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Even on Titan, the largest moon of Saturn, it’s the little things that count. Hydrogen is the smallest molecule in the universe, and in Titan’s atmosphere, it’s doing something strange — collecting in surprising quantities where scientists least expect it, like near the moon’s surface.

“Hydrogen is very light, and it should just kind of float away out of the atmosphere,” said Erin Flowers, a graduate student in the Department of Astrophysical Sciences who is studying the chemistry and physics of Titan’s atmosphere. “It shouldn’t really collect or pool.” In an effort to understand this anomaly, Flowers is tracing the source of the mysterious accumulation.

Her search has taken her to another of the solar system’s more diminutive players: micrometeoroids. These little chunks of matter range in size from a centimeter across, roughly the size of a chickpea, all the way down to a single micron, just one-sixtieth the size of a grain of table salt.

Most micrometeoroids around Titan are icy particles generated by collisions in the cometstrewn Kuiper belt, beyond the orbit of Neptune. When these particles enter Titan’s atmosphere, they introduce new material, like hydrogen, to the mix.

Flowers is using atmospheric data — collected on and around Titan itself during NASA and the European Space Agency’s Cassini-Huygens mission — combined with computer modeling to understand how micrometeoroids affect the chemical composition of Titan’s atmosphere. “I’m looking at how those particles move, are destroyed, release their chemical components, and mix in the atmosphere,” Flowers said.

A grand scale

Titan is big, bigger than the planet Mercury, and half again as large as Earth’s moon. But it’s not size that makes Titan loom large in the study of our solar system.

Of all the bodies in the solar system, Titan’s atmosphere is the most similar to ours. On Earth, water exists in all three states of matter: liquid, solid and gas. Titan also has a three-state atmosphere, but instead of water, it has methane, a simple molecule of carbon and hydrogen that we use on Earth for fuel. Titan’s landscape includes lakes and even seas of liquid methane. “This is incredible,” Flowers said. “There are no other bodies in the solar system that have standing liquid on their surfaces.”

Titan’s similarities to Earth make Titan a fascinating target for research, said Chris Chyba, professor of astrophysical sciences and international affairs, and Flowers’ adviser. At its most speculative, research on Titan could reveal a previously unknown type of life, one based on methane instead of water. Less speculatively, it could help unravel the riddle of life here on Earth. “The kind of chemistry happening in Titan’s atmosphere is just the kind of chemistry that, at least in one popular model for the origin of life on Earth, must have happened on Earth 4 billion years ago,” Chyba said.

Grounding roots

Flowers can’t remember when she was first bitten by the space bug. She spent much of her childhood coveting telescopes and asking permission to stay up late to look at stars and watch meteor showers. And in high school, it took only one lab period in her physics class for her general liking for science and math to click together into something more. “I was able to describe the motion of something with math, and it played out in real life,” Flowers said. “I just thought that was the coolest thing.”

Add that to her starstruck childhood, and it’s no surprise Flowers chose to become an astrophysicist. “I’ve honestly always wanted to do astrophysics,” Flowers said. “Once I had a word for it, I was like, that’s what I want to do.”

Flowers combines an in-depth understanding of physics (to characterize the movements of meteoroids) with chemistry (to model atmospheric interactions), while juggling data from multiple spacecraft missions.

Her work is currently undergoing peer review, and Flowers and Chyba think the findings are illuminating. “It looks as though the micrometeoroids might be an important part of explaining these peculiarities of Titan’s atmosphere,” Chyba said.

Reaching skyward

Flowers credits a supportive community — including her fellow graduate students, her adviser Chyba, and a far-flung network of family and friends — as a critical element of her success. And she makes time to pass that support forward through community outreach, including as a math and science teacher at Princeton’s Prison Teaching Initiative, through which inmates can pursue an associate’s degree. Flowers also recently received a University-wide teaching award for exceptional contributions to undergraduate instruction. “She is terrific at making connections,” Chyba said.

Moving forward, Flowers plans to look at the surface of Titan to investigate how methane, once released from Titan’s lakes and oceans, interacts with other elements of the atmosphere to result in pockets of hydrogen. Said Flowers, “There’s still a lot of space for new discovery in this field.”