Jonathan DA Clarke1, David Willson1,2 and John L Keeling3
1 Mars Society Australia
2 NASA Ames Research Center
3 Geological Survey of South Australia, Department of State Development
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Introduction
In planning for future crewed missions to Mars, suitable terrestrial test sites are crucial to development and evaluation of equipment and processes needed for survival and work function. The Moon Plain, northeast of Coober Pedy in the arid region of central South Australia, is potentially one such site. Here the surficial regolith contains abundant hydrated minerals, known to be widely distributed also in the equatorial regions of Mars and identified as a possible source of water in this ice-free region of the planet.
The consumable requirements of a crewed Mars mission impose a major logistic constraint on expedition planning. Mass-wise, the largest consumable has always been propellant, followed by water and oxygen for breathing. Using in situ resource utilisation to reduce the logistic burden of consumables has been a major focus of expedition planning since the early 1970s (Allen and Zubrin 1999).
Figure 1 Approximately 3 m exposure of bedded polyhydrated sulfates in the wall of Endurance Crater at Meridiani Planum on Mars. (Image taken by the NASA Opportunity rover; courtesy of Mars Society Australia).
Water is known to occur on Mars in many forms, including ground and polar ice, as atmospheric water vapour, and in hydrated minerals (Baker 2001). Widespread occurrence of mono- and poly-hydrated sulfate minerals, including those discovered by the Opportunity rover at Terra Meridiani (Fig. 1; Christensen et al. 2004, Rieder et al. 2004), offer a possible water resource at low latitude, low altitude, and relatively flat regions that are likely most suitable for early human Mars missions (Clarke et al. 2015). Hydrated sulfate minerals can contain up to 51% water in their crystal structure. In principle, the development of viable water extraction technology from hydrated sulfate minerals is relatively straight forward, requiring heating of sulfate material to 80–325 °C (Chipera et al. 2006). The relative abundance of hydrated sulfate minerals on Mars indicates that at some localities, processing of comparatively small amounts of surface material (<2 tonnes, t) each sol (Martian day) could provide a stockpile of 14–54 t of water to support four to six astronauts – equivalent to harvesting 26–100 L of water per sol. This water would be used for propellant production (as oxygen and methane), drinking and washing water, and breathing oxygen per sol of water (Clarke et al. 2010).
Mars analogue research
Mars analogue sites are sites on earth that present one or more geological or environmental features similar to those found on Mars and therefore can provide valuable insights into current and past processes on Mars (Léveillé 2014). Such localities are valuable also for the field testing of equipment that may be used on actual missions (West et al. 2010). If water is to be extracted from Martian hydrated sulfates, a suitable analogue field site would greatly facilitate the development of the required technology. The Moon Plain in South Australia is a site that has particular attractions for testing such technology (Clarke et al. 2010).
Moon Plain as a Mars analogue
Figure 2 Stuart Range breakaways, central South Australia. (Courtesy of Mars Society Australia)
The Moon Plain extends over 1,500 km2, 18 km north-northeast of Coober Pedy, in central South Australia (Fig. 2). The locality derives its name from the overall lack of vegetation and a flat hummocky landscape that resembles the surface of the moon. Environmental conditions are mean annual rainfall of 156 mm and average evaporation potential of >2,800 mm per year (Australian Bureau of Meteorology data). Epsomite (MgSO4∙7H2O), along with other hydrated magnesium and calcium sulfates, occurs in the soil profile which may contain up to 20% water by mass. There is limited published (Lock 1988) and some unpublished (Seymour 1983; Scott 1984) data on this deposit, which extends across an area of 510 km2 and includes a localised site with up to 22.5% epsomite (Fig. 3). This is found in the upper 3.5 m of the weathering profile developed on the transitional unit between the Cadna-owie Formation and the Bulldog Shale, of Cretaceous age. Scott (1984) suggested that sulfate-rich groundwater from weathered Bulldog Shale in the breakaways of the Stuart Range percolate through in the permeable, pyritic Cadna-owie Formation. These waters are confined under pressure by the overlying transition unit which, in turn, is kept moist by capillary pressure from below, and the salts are accumulated within the transition unit by the high evaporation potential. The weathering profile is overlain by a thin, silty clay unit, the Benitos Clay (Seymour 1983) of probable aeolian origin, and is mantled by gibbers. The highest grades occur in three deposits spread over 47 km2 that contain 94 million tonnes of in situ material averaging 7.4% Mg2SO4 (Scott 1984). The epsomite occurs at depths of 0–2 m beneath the surface and averages 1.3 m thick.
Figure 3 Location map and geological cross-section of epsomite occurrence on Moon Plain, ~25 km northeast of Coober Pedy. Modified from Scott (1984) and shown over Google Source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community image.
Fractures in the fissile weathered shale contain gypsum, bloedite (Na2Mg(SO4)2∙4H2O), iron sulfates and epsomite. The easily excavatable powdery soils contain abundant montmorillonite, fine quartz sand and minor non-swelling clays, in addition to the sulfates. At the soil surface the epsomite is partly converted to hexahydrite (MgSO4·6H2O).The deposit has formed through acid weathering of the host rock (Lock 1988), a process that may itself be an analogue for past dehydration and weathering processes on the surface of Mars (Rey 2013).
Proposed Mars analogue
Mars Society Australia, an approved Mars Society group focusing on Mars research, education and outreach, has for several years proposed the construction of a feasibility plant capable of extracting water from the regolith at Moon Plain (Clarke et al. 2010). Given the relatively small amounts of water that need to be extracted, the test plant could be of a similar scale to what would be used on Mars. Such a plant would ideally test excavation technologies, water extraction efficiencies and power requirements of a robotic system. A project of this type would be ideal for funding in collaboration through a university mining or robotic engineering program.
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