Geothermal Outside of (the Box) This World
By Dr. Ken Wisian and Jamie C. Beard, Esq.
As geothermal energy proponents with academic interests in all things space, we are eager to find areas where these two disciplines converge. The great news is there are several areas of overlap. The first will be relevant to the development of next generation geothermal energy concepts in the near term, and the second may be a key enabler for extraterrestrial exploration and settlement by humans.
NASA’s effort to land humans on the moon generated a large array of societal benefits, including foundational technologies that make the world what it is today – technologies ranging from integrated circuits and medical sensors, to ubiquitous household products like WD40 (but not Tang as we recently learned). And like the Apollo Mission provided a multitude of spin-off technological advancements that benefited other areas of life and scientific discovery, so too can our Earth-based mission to develop advanced next generation geothermal concepts yield important – even essential and enabling - benefits for the future of space exploration and settlement.
Before launching into the space-centric portion of this inquiry, it’s worth noting that existing NASA developed capabilities and technologies that are currently in use on active missions can and likely will have a significant impact on our ability to drill in some of the most extreme conditions encountered on Earth. Indeed, GEO’s goal of enabling economical directional drilling of next generation ‘closed loop’ geothermal wells at depths of up to 9,144 m and 350C won’t be achieved without advancements in high temperature electronics and telemetry systems, high temperature energy storage devices, and advanced seals, coatings and insulation materials that can reliably perform under this set of extreme conditions. One need not look further than NASA’s current Parker Solar Probe mission for a prime example of potentially game changing technology transfer potential. The Probe will fly through the sun’s atmosphere and encounter extreme radiation and temperatures of 1,377C. Despite these conditions, the probe’s solar shield will maintain the probe’s internal payload near room temperature – a temperature differential of more than 1316C. Simply adapting the insulation capabilities deployed in this mission to protect the electronics systems in directional drill strings could provide a needed step change in ultra-high temperature performance downhole. There are dozens of examples like this, where technology transfer from space exploration into extreme environment drilling may help enable the future of terrestrial geothermal energy. This showcases the interdisciplinary nature of GEO’s inquiry, and makes the case for strong multi-entity and inter-agency collaboration to tackle the future of geothermal drilling. Hey DOE, NASA and DOD – let’s do this.
But now consider the flip-side – where advances in terrestrial geothermal energy production could help enable the future of space exploration. Human travel to and settlement of Mars is the latest buzz, but a sustainable and economical power source on the planet is so far elusive. Solar is obvious, proven and has worked for relatively low power density rovers, but if colonization is the goal, a more plentiful, constant energy source generated using in-situ resources will be needed. With the core of Mars estimated to be in excess of 1000C, geothermal energy could be the solution. Questions remain about the extent of crustal heat flow, and producible geothermal resources are likely to be more sporadic and isolated than on Earth. Still though, this is an area of active inquiry with the NASA Insight Mission hoping to take direct subsurface measurements of Martian heat flow. This Mission is already producing a steady stream of interesting headlines.
Now let’s imagine a further out future, when humans or advanced robot proxies settle at least semi-permanently in the outer solar system. Facilities will need power, but as you travel further from the sun, the practicality of solar energy wains. NASA’s deep space probes use Radioisotope Thermoelectric Generators (RTG) - essentially a small radioactive pile of fissionable material. Besides the obvious problems of radiation and power production declining with time, the mass of a nuclear power source capable of powering a large facility is likely to be impractical to transport across the solar system. Geothermal may again offer the solution.
Several moons of the gas giants are particularly active geologically and/or appear to have subsurface water oceans. These moons remain geologically active due primarily to tidal friction heating of their interiors. These tides, in turn, are caused by the moon’s paths through the very strong gravity fields of their parent planets.
o Io – sulfur laden volcanoes
o Europa – global subsurface ocean
o Ganymede – evidence of some subsurface liquid water
o Titan – complex geology and climate with a liquid hydrocarbon cycle (including “rain” and lakes) similar to the water cycle on Earth, but hundreds of degrees colder
o Enceladus – very small moon, but active cryovolcanoes and a probable subsurface global ocean
Let’s use generously rounded off numbers in a back-of-the-envelope approach to look at feasibility of geothermal energy on moons with subsurface oceans.
Fundamentally, all it takes to generate power is a temperature differential. On Earth, with current technology, electrical power is economically generated using water at a temperature of 100-150°C. On outer solar system moons, with surface temperatures around the order of -200°C and assuming the liquid water is 0°C (possibly higher, but discounting changes in freeing point due to impurities and pressure), you get a minimum temperature difference of 200°C – a value that would make a geothermal power engineer quite comfortable with the prospects.
“But the ice crust of these moons might be 100km thick, we’ll never be able to drill to that depth” you may ask. Perhaps - but consider that one of the highest current priorities in space exploration is to sample the waters on the icy moons of the outer planets for evidence of life. Methods for ‘drilling’ - which in this context is simply melting your way down through the ice - are being developed and could achieve depths of penetration on the order of kilometers. These methods envision using heated probes to melt their way through. Or perhaps this is an area where laser drilling techniques could really excel. Hey Foro, interested in space travel? Note also that the liquids on these moons erupt from the surface in ways similar to volcanos and mid-ocean ridges on Earth (where magma is just below or at the surface), and thus much shallower drilling depths are a possibility on some locations of these moons.
While there are many technical challenges, like how to drill, and with what materials at these frigid cold temperatures, it is clear that the potential for a large energy source that could power human settlement and exploration is present in the outer moons of the solar system. And as it is on Earth, it lies beneath our feet.
Ken Wisian is a research scientist at the Center for Space Research and a retired Major General in the US Air Force. He is a PhD geophysicist, with expertise in geothermal resource exploration and characterization. He is a Co-Investigator of GEO at the University of Texas at Austin.
Jamie Beard is an energy and regulatory attorney, and lifelong proponent of geothermal as the baseload clean energy of the future. She is Executive Director of the Geothermal Entrepreneurship Organization (GEO) at the University of Texas at Austin.