As reported recently in Space.com (1), NASA’s Jet Propulsion Laboratory and the University of Texas, Austin have determined underground water deposits in Mars’ Utopia Planitia region larger in volume than Lake Superior (2,3). The deposit, located in the northern midlatitudes, contains 50-85% water ice and is covered by 1-10 m of regolith, shielding it from evaporation and ultimate loss to space. Researchers report an ice volume of 14,300 km3.
It is difficult to understate the importance of this discovery as a resource for human settlement. While ice has been discovered in numerous regions across the planet, most of it is buried along with frozen carbon dioxide at the poles, mixed with large amounts of regolith, or chemically bound to minerals such as calcium sulfate, making extraction difficult (4).
How many humans could such a resource support? One has to look beyond basic hydration and hygiene to include other uses for water in a self-sustaining settlement, including commercial and industrial operations as well as agriculture. While difficult to estimate because no large-scale plans have been fully developed for the Red Planet, we can look to water-constrained states such as California as examples. Raucher and Tchobanoglous (5) estimate that 1 billion gallons/day could meet all municipal needs (e.g., residential, commercial & industrial) to 8 million people in the Los Angeles region. This translates to 125 gallons/day/person, or 473 kg/day/person. However, agricultural use must also be included in any long-term settlement plans; California typically consumes 30 billion gallons/day for agriculture, with a state population of 33.5 million (6); consumption is therefore about 900 gallons/day/person. California is a net exporter of agricultural products, but it also imports nearly as much in terms of economic value (7), so we assume that California’s agriculture approximates a self-sufficient system. While future agriculture on Mars would presumably be much more water-efficient than California’s, we conservatively estimate 50% of this agricultural water demand, or 450 gallons/day/person, resulting in a total water need of 2,175 kg/day/person.
If this water is used only once and discarded, Utopia Planitia could sustain a million-person settlement for 18,000 years. But water can be treated and re-used; even on Earth with our abundant freshwater resources, we are already pushing up against natural supply limits, and are beginning to treat and re-use our water for agricultural, and sometimes municipal, purposes. State-of-the-art water treatment technology promises >98% water recycling using reverse osmosis plus any of a number of secondary treatment technologies (8). At 98% recovery, the Utopia resource could sustain a million-person settlement for nearly 1 million years. Even if the settlement population grew enormously—say to 1 billion people—the resource would still be adequate for 900 years, more than enough time to advance the technology to nearly 100% recovery, essentially sustaining it indefinitely.
One major activity that could permanently deplete water resources on Mars is the production of rocket propellant (H2/O2, CH4/O2 or anything else containing water as a feedstock) that is lost to space when used. Elon Musk’s Interplanetary Transport System would require 1,950 tonnes of CH4/O2 propellant to transport 100 people from Earth orbit to the Mars surface (9), and another 1,950 tonnes to return the spacecraft to Earth (with far fewer people). I calculate that each flight requires about 1,100 tonnes of water (with the balance supplied by carbon dioxide, with some oxygen left over). Musk envisions a mature fleet of 1,000 ships flying to and from Mars every synodic period (2.14 years), which would require an additional 1.4 million kg/day of H2O. While large, it is only 1.4 kg/day/person – a small drain on the demands of the human settlement.
References:
1. Wall, M. Huge Underground Ice Deposit on Mars Is Bigger Than New Mexico, Space.com, 22 November 2016. http://www.space.com/34811-mars-ice-more-water-than-lake-superior.html.
2. NASA Jet Propulsion Laboratory, Mars Ice Deposit Holds as Much Water as Lake Superior, Press release 2016-299, 22 November 2016. http://www.jpl.nasa.gov/news/news.php?feature=6680.
3. C. M. Stuurman et al., SHARAD detection and characterization of subsurface water ice deposits in Utopia Planitia, Mars, Geophysical Research Letters (2016). DOI: 10.1002/2016GL070138.
4. Abbud-Madrid, A., D. W. Beaty, D. Boucher, B. Bussey, R. Davis, L. Gertsch, L. E. Hays, J. Kleinhenz, M. A. Meyer, M. Moats, R. P. Mueller, A. Paz, N. Suzuki, P. van Susante, C. Whetsel, E. A. Zbinden, Mars Water In-Situ Resource Utilization (ISRU) Planning (M-WIP) Study, California Institute of Technology, 22 April 2016. http://mepag.nasa.gov/reports/Mars_Water_ISRU_Study.pptx.
5. Raucher, B., G. Tchobanoglous, The Opportunities and Economics of Direct Potable Reuse, White paper 14-08, WaterReuse, 2014. https://watereuse.org/watereuse-research/the-opportunities-and-economics-of-direct-potable-reuse/.
6. Klein, G., M. Krebs, V. Hall, T. O’Brien, B. B. Blevins, California's Water-Energy Relationship, California Energy Commission, CEC-700-2005-011-SF, November 2005. http://www.energy.ca.gov/2005publications/CEC-700-2005-011/CEC-700-2005-011-SF.PDF.
7. Medina, J. Fast Facts on California's Agricultural Economy, Assembly Committee on Jobs, Economic Development, and the Economy, no date. http://ajed.assembly.ca.gov/sites/ajed.assembly.ca.gov/files/Fast%20Facts%20on%20California's%20Agricultural%20Economy.pdf.
8. Afrasiabi, N., E. Shahbazali, RO brine treatment and disposal methods, Desalination and Water Treatment 35 (2011). DOI: 10.5004/dwt.2011.3128.
9. Musk E., Making Humans a Multiplanetary Species, Keynote talk, International Astronautical Congress, Guadalajara, Mexico, 26 September 2016. http://www.spacex.com/sites/spacex/files/mars_presentation.pdf.