“Sustaining low-to-moderate temperature hydrothermal circulation under ocean-world gravity“

Speaker: Prof. Andrew Fisher, Earth and Planetary Sciences Department, University of California, Santa Cruz, CA, USA.
Date: Friday, November 1, 2024
Time: 3:30pm – 5:00pm
Location: ISB 102 & Zoom (link)

Controls on sustainability of hydrothermal circulation on ocean worlds are not well understood. We developed three‐dimensional numerical simulations, using a ridge‐flank hydrothermal system on Earth as a reference, to test the influence of ocean world gravity on metrics for hydrothermal fluid and heat transport. Simulations represented ∼4-5 km of a silicate core below an ocean, and explored the influence of: heat input at the domain base; aquifer thickness, depth extent, and permeability; and gravity values appropriate for Earth, Europa, and Enceladus. Calculations illustrate a trade‐off between reduced buoyancy at lower gravity and the concomitant reduction in secondary convection; the latter process saps driving energy and thereby reduces circulation sustainability. Simulations that sustained hydrothermal flow achieved reaction and discharge temperatures ≤150 °C, flow rates ≤2,100 kg/s, and heat output ≤700 MW. Lower gravity tended to increase vent fluid temperatures while reducing mass flow rates and advective heat output. Deeper circulation tended to increase temperatures and flow rates, with a deeper, thin aquifer being more efficient in removing heat from the rocky interior than either a shallow-thin or a deep-thick aquifer. Water‐rock ratios were lower at lower gravity, all else being equal, whereas the time required to circulate the volume of an ocean‐world’s ocean in and out of the seafloor was greater. This may help to explain how small ocean worlds could sustain hydrothermal circulation for a long time despite limited heat sources. We are currently extending these simulations across a broader range of free parameters using a simplified analytical framework, allowing Monte Carlo exploration of conditions consistent with sustaining a hydrothermal siphon on an ocean world, including gravity values greater than that for Earth. Initial results with this approach replicate key numerical results and point to areas for future numerical exploration.

 

About the speaker:

Andrew Fisher is a Distinguished Professor of Earth and Planetary Sciences at the University of California, Santa Cruz (UCSC). Fisher has taught courses introductory Earth Science, hydrology, groundwater, geothermics, and computer modeling, and has conducted studies and published papers with his students and colleagues on hydrothermal systems, marine geothermics, groundwater recharge, surface water – groundwater interactions, water quality, and development of new field tools and methods for measuring hidden flows and processes. Fisher has authored/coauthored >200 papers/reports, served on technical advisory committees for water agencies and municipalities, and leads a regional Recharge Net Metering program to incentivize managed recharge. He holds a BS in geology from Stanford University, and a PhD in marine geology and geophysics from the University of Miami. Fisher is a fellow of the American Geophysical Union, the Geological Society of America (GSA), and the American Association for the Advancement of Science. He has received two Excellence in Teaching Awards at UCSC, and the O. E. Meinzer Award from the Hydrogeology Division of GSA.

 

Recording of the talk: