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Author for correspondence: Charles S. Cockell, E-mail: [email protected]
Introduction
The exploration of the deep subsurface of other planetary bodies is motivated by potentially high scientific returns. Particularly on bodies such as the Moon or Mars, where impact gardening has disrupted and perturbed surface environments, the deep subsurface can provide access to relatively unaltered materials. On Mars, the deep subsurface is recognized to be a location that may have hosted habitable conditions in its past and has a high possibility of hosting such conditions today (Boston et al., 1992; Hofmann, 2008). For example, these locations have the potential to provide access to materials influenced by groundwater via upwelling events. As observed on Mars in regions such as the Burns Formation, groundwater had the ability to move through permeable sediment rock pathways to record ancient water–rock and water–mineral interactions (Clark et al., 2005; McLennan et al., 2005; Andrews-Hanna et al., 2010). Should these features have been host to organics or biogenic features in early Martian history when the climate was more hospitable (Ehlmann et al., 2011) and Earth-like, it would simultaneously provide protection and preservation of targets of astrobiological interest for future missions. Thus, the subsurface of Mars is a promising location to test the hypothesis of past life on Mars and the existence and persistence of habitable conditions on that planet.
In terms of human exploration, subsurface environments provide potential refugia from harsh surface conditions including Solar particle events and micrometeorite impacts. Although a permanent troglodyte existence may be unappealing to denizens of the Earth, in extraterrestrial environments such locations provide safe havens on the Moon, Mars and even asteroids, particularly if deep caverns are used that have already been formed by natural processes.
Access to the subsurface of other planetary bodies can be achieved by investigating naturally uplifted crater materials (Michalski and Niles, 2010), indirectly through radar sounding (Picardi et al., 2005; Watters et al., 2006), or by drilling in robotic and human missions (Smith and McKay, 2005). However, it is now understood that natural access to the subsurface is also provided by features such as volcanic and impact-produced caves and lava tubes (e.g. Cushing et al., 2007; Williams et al., 2010). These features provide compelling...