Titan’s missing river deltas and an Earthly climate connection

“I’ll never forget the moment when I first saw new Cassini data come down from Titan’s surface,” says Samuel Birch. “I was in awe at witnessing this brand new, never-seen-before bit of our solar system.”

Birch explores and models the evolution of the surfaces of planets, moons, and small bodies in the outer solar system, including Saturn’s largest moon, Titan, and the Comet 67P/Churyumov-Gerasimenko — two very different, icy worlds investigated by the spacecraft Cassini and Rosetta. He joins MIT this summer as one of eight recipients of the 2020 Heising-Simons Foundation 51 Pegasi b Fellows bridging planetary science and astronomy, accelerating our understanding of planetary system formation and evolution, and advance new technologies for detecting Earthlike worlds.

Over the years, the Heising-Simons Foundation has generously supported a growing cohort of exoplanet researchers at MIT, including Jason Dittmann, Ian Wong, Ben Rackham, Clara Sousa-Silva, and now Samuel Birch, a research associate from Cornell University. In the coming three years, with support networks, mentorship from MIT Department of Earth, Atmospheric and Planetary Science (EAPS) members like Professor Taylor Perron and Research Scientist Jason Soderblom, and a grant of up to $375,000, Birch will have the space and time to fully explore ideas, deciphering what the surfaces of those objects tell us about their climatological past and potential habitability. He’ll also develop and operate related spacecraft missions and mission concepts that seek to study edges of our solar system.

“I like to think of myself as an explorer of the outer solar system, trying to figure what is shaping the weird landscapes on these icy worlds,” Birch says.

Not quite familiar territory

As scientists learn more about the geophysics of Saturn’s moon Titan, their findings motivate newer and bigger questions that extend to Earth and other planetary bodies, highlighting the need for its continued study. “Titan’s surface is perhaps the most intriguing in our solar system, as there are rivers and seas of liquid methane and sand dunes made of organic plastics — all the result of a dense, nitrogen-dominated atmosphere,” says Birch. With a salty liquid water ocean beneath the surface, and an icy exterior sculpted by rivers, seas, and waves, Titan’s hydrologic cycle is similar to Earth’s. However, when its coastal rivers meet the lakes and sea, they seem to be missing deltas at their ends, Birch says. This may be because deltas like those on Earth do not form (or rarely form) because of differences in materials, dynamics, and coastal conditions. Alternately, their characteristics and representation in Cassini datasets may make them difficult to identify.

To solve this mystery, Birch and MIT researchers will investigate deltaic and river dynamics, using a combination of theoretical, experimental, and numerical modeling, atmospheric simulations, and a re-evaluation of Cassini data for evidence of the resulting landforms. This suite of studies will help them understand what a delta “looks” like and map their distribution, which may unveil a record of Titan’s climate history and reveal how liquid methane has molded its landscapes.

“If we can understand the reasons for the stark differences between Earth and Titan — and with it, the fate of all the mass eroded by Titan’s rivers,” Birch says, “we have the chance to really advance our knowledge of the history of erosion, sea-level, and climate change on Titan.”

Life extensions

This work inherently informs the study of fundamental Earthlike surface processes related to climate and the search for life beyond Earth. Since Titan lacks the complex interplay of diverse physical and chemical processes of Earth’s biosphere — like active tectonics, variable bedrock lithologies, diverse climate zones, vegetation, and (as far as we know) organisms — the moon serves as a natural laboratory for studying the effects of sea-level change on shoreline, river, and delta evolution. Additionally, scientists target deltas because of their high astrobiological potential for harboring life, like those on Mars. Analogous, active environments like Titan’s offer promise for the upcoming Dragonfly mission — when a nuclear-powered, dual-quadcopter will explore the moon, and perhaps these valuable spots.

In the long run, Birch would like to parlay the skills he cultivates here to develop his own research group and continue to participate in missions that address key questions regarding the evolution of planetary surfaces. “I am extremely honored by this opportunity and that the community and the Heising-Simons Foundation value my work … I am fortunate that the mentors I will have at MIT are some of the best in the field,” Birch says, acknowledging the support of his collaborators and advisor, and welcoming the challenge and rewards that the future research will bring. “It is a fantastic opportunity and can’t wait to see what we can all discover on Titan and elsewhere!”

The Heising-Simons Foundation is a family foundation based in Los Altos and San Francisco, California. The foundation works with its many partners to advance sustainable solutions in climate and clean energy, enable groundbreaking research in science, enhance the education of our youngest learners, and support human rights for all people. In addition to Birch, other fellows selected in this year’s cohort will join their host institutions: Elizabeth Bailey at the University of California at Santa Cruz; Ashley Baker and Kimberly Moore at Caltech; Emilie Dunham at the University of California at Los Angeles; Emily First and Eileen Gonzales at Cornell University; and Benjamin Tofflemire at University of Texas at Austin.