Department of Astronomy Center for Radiophysics & Space Research

Sam Birch (EAS)

Professor Alexander Hayes
Sam Birch (EAS)

Research Interests: Planetary Geomorphology, Titan, small body geology, Mars hydrology, numerical landscape evolution

I’m a fourth-year graduate student in the Department of Earth and Atmospheric Sciences, working with Alex Hayes in the Astronomy Department. My research focuses on understanding the evolution of the surfaces of planets, moons and small bodies, and deciphering what these surfaces tell us about their past. If we can understand their surface morphologies, by describing and quantifying the topographic shapes of a planet’s surface, we can build process-based models that explain their evolution through time. Beginning with qualitative observations of a surface, we then build numerical models that match those observations. This methodology can apply to just select regions on planets like Mars, to the hydrologically active poles of an icy moon like Titan, to sedimentary worlds like Comet 67P/Churyumov-Gerasimenko. The surfaces of all these bodies are governed by the motions of individual particles, and so understanding the sediment transport pathways on these bodies helps us to understand the evolution of the landscape as a whole.

Titan:

Titan is among the most fascinating worlds in our solar system, as it is the only other place in our solar system that has stable bodies of liquids on its surface and an active hydrological cycle that moves them around. With an atmospheric pressure at the surface of 1.5 bars and a surface temperature of 91-95 K, methane and ethane are both able to condense out of the atmosphere and rain to the surface, where the fluid runoff concentrates, incises channels and transports sediment. Landforms common to Earth are found across Titan, and include lakes and seas, river valleys, fans and deltas, and mountains. Yet under Titan conditions, these familiar landforms have all formed and evolved under vastly different environmental and physical conditions from Earth.

Of particular interest to me are the fluvial and lacustrine features clustered in Titan’s polar regions. Flybys of Titan by the Cassini spacecraft have revealed numerous small lakes and vast seas at the north, while the south shows a markedly different environment with only a few filled lakes, but far more empty lakes, and four large basins. Why this is remains to be seen, but we have begun to address this question through a series of works over the past few years.

Surprising to us, the landscapes of the south and north poles of Titan are quite similar, with only the current inventory of liquid differentiating the two. Most of Titan’s filled and empty lake depressions are located within smooth undulating plains, which themselves are bounded into enclosed (endorheic) basins. These lakes have hundred-meter high raised rims, closed perimeters and flat floors, and with no terrestrial analog, their formation is unique to Titan. Perhaps the origin of the enclosed sedimentary deposits into which they cut should yield some insight.

And yet, as is typical for Titan, our work has revealed more questions than answers. To further model Titan’s landscapes, we have to understand and quantify key questions, such as:

  1. How is sediment generated, transported and deposited, and is dissolution chemistry necessary to form the observed landscape?
  2. What are the pathways for fluids across (and below) the surface?
  3. Over what timescales does the surface evolve and how do those timescales interact to form the current-day surface?
  4. What is the origin the sharp-edged lake depressions and how do they evolve?

While a dedicated future mission should be able to address – at least in part – some of these questions, there is still a lot that can be done today. Cassini has collected a rich dataset that is still yet to be exploited, while numerical modeling has the power to put limits on the rates of different sediment transport processes. I am deeply appreciative to be have been a team member on the Cassini mission, where I could study this Earth-like world in a time when we were still actively collecting data. I am looking forward to seeing what secrets Titan has in store for us!

Comet 67P/Churyumov-Gerasimenko:

Comets are the oldest objects in our solar system, and are the remnant, unprocessed materials from which all the larger planets and moons were constructed. All of our observations, however, only give us access to their current-day surfaces, which for most, have been altered throughout the age of the solar system. Understanding the evolution and the physical/chemical processes responsible is the key to decipher the formation and evolution of the solar system.

We now have the data to do this, as ESA’s Rosetta Orbiter of comet 67P has revolutionized our understanding of these primitive bodies. Surprisingly, 67P appears to be an active sedimentary world, where sediment is produced by jet erosion and deposited in large sedimentary basins. Exposed cliff walls, fields of car-sized boulders, smooth plains and deep circular pits are found across the surface of 67P. While we have made headway as to the processes acting today, a complete understanding of how these features develop and evolve remains elusive. That is the task of our future work, which uses numerical landscape evolution simulations to address the rates of erosion across the nucleus.

Understanding the sedimentary cycle on these bodies is also critical for upcoming missions to comets. NASA’s Decadal Survey laid out a plan to return a sample of a comet as part of its New Frontiers program, and so giving context to a sample return mission is crucial. How deep the sedimentary lag cover is will be crucial for both the survivability of a spacecraft and for understanding what it is your spacecraft samples. To what degree are these sedimentary basins processed, how do they evolve and what amount of volatile materials do they contain, if any, will be key questions to answer as well.

Selected Publications:

A.G. Hayes, S.P.D. Birch, et al, “Constraints on the evolution and connectivity of Titan’s lacustrine basins as revealed by stereo photogrammetry and altimetry,” 2017. Geophysical Research Letters, submitted.

S.P.D. Birch, A.G. Hayes, et al, “Morphologic Evidence that Titan’s Southern Hemisphere Basins are Paleoseas,” 2017. Icarus, submitted.

S.P.D. Birch, Y. Tang, A.G. Hayes, et al, “Geomorphology of Comet 67P/Churyumov-Gerasimenko,” 2017. MNRAS 469, S50-S67.

S.P.D. Birch, A.G. Hayes, et al, “Geomorphologic Mapping of Titan’s polar terrains: Constraining Surface Processes and Landscape Evolution,” 2017. Icarus, 282, 214-236.

S.P.D. Birch, A.G. Hayes, et al, “Alluvial Fan Morphology, Distribution and Formation on Titan,” 2016. Icarus 270, 238-247.

S.P.D. Birch, et al, “Penetration of spherical projectiles into wet granular media,” 2014. Physical Review E 90, 032208.