PhD Defense by Justin Lawrence
Title: Antarctic Ice Shelves as Ocean World Analogs
Advisor: Dr. Britney E. Schmidt
• Dr. Jennifer Glass, School of Earth & Atmospheric Sciences, Georgia Institute of Technology
• Dr. James Wray, School of Earth & Atmospheric Sciences, Georgia Institute of Technology
• Dr. Alex Robel, School of Earth & Atmospheric Sciences, Georgia Institute of Technology
• Dr. Jeff Bowman, Integrative Oceanography Division, Scripps Institution of Oceanography
The search for life beyond Earth is a primary goal of NASA, and in our solar system ocean worlds such as Jupiter’s moon Europa are among the most promising targets. Europa has a global outer shell of ice which is likely to be tens of km thick – but also a lower mass meaning pressures and temperatures in the upper ocean below the shell may be similar to Earth’s polar oceans. Models for the habitability of Europa’s hydrosphere suggest that exchange of radiolytically-generate oxidants in the ice shell with reduced compounds from the ocean across the sub-ice shell ice-ocean interface, controlled by melting and freezing processes, is important for Europa’s overall habitability. Ahead of the Europa Clipper mission, anticipated to reach the Jovian system by ~2030, we turn to Antarctica’s ice-covered oceans to build our understanding of how sub-ice ecosystems operate, and simultaneously apply these lessons to other oceans worlds in our solar system.
Around the edge of the Antarctic continent, floating extensions of the ice sheet called ice shelves cover 1.5x106 km2 of the coastal ocean, an area the size of Mongolia, in ice hundreds to thousands of meters thick. The largest, Ross Ice Shelf, represents a third of this area alone – but the thick ice poses a significant barrier to exploration. Since the late 1970s only four projects (two of which are included in this thesis) have accessed the ocean below. Sub-ice shelf ocean and ecosystem dynamics are understudied – by analogy this would be like trying to sort out the weather in a region the size of France based on launching one weather balloon a decade.
In this thesis, I have helped to develop and deploy the robotic under-ice vehicle Icefin to study the environments and ecosystems beneath Earth’s ice shelves. Over several field seasons at Ross Ice Shelf, we have contributed a new understanding of interactions between ice shelves, ocean, and seafloor processes. By pairing oceanographic observations with a complementary survey of gradients in bacteria and archaeal diversity under the shelf, we provide additional evidence for the importance of ice-ocean interactions to sub-ice nutrient availability and ecosystem structure. I then applied our novel observant to constrain ice-ocean interaction regimes for other ocean worlds, bounding how these physical processes might modulate the exchange of materials between an ocean and ice shell. Finally, integrating these findings, I outline lessons from ice-ocean interactions and the ecosystems that depend on them to develop science goals for wholistic exploration the hydrospheres of other worlds. Following from these priorities, I present a student-led subsurface instrument suite concept for ocean world life detection. Collectively, these efforts help to advance our understanding of how ice and ocean processes could influence ecosystems beyond Earth, and informs efforts to develop future astrobiology missions seeking to characterize the habitability, and potentially inhabitants, of other ocean worlds in our solar system.