Hydrogeology. Kevin M. Hiscock
on Mars are currently cold and dry, with water ice unstable at the surface except near the poles. Geologically recent, glacier‐like landforms have been identified in the tropics and the mid‐latitudes of Mars and are thought to be the result of obliquity‐driven climate change (Forget et al. 2006). The relatively low volume of the EGL, coupled with widespread indications of chemical and geological landforms shaped by areas of recent groundwater seepage (Malin and Edgett 2000) and extensive palaeohydrological activity (Andrews‐Hanna et al. 2010; Michalski et al. 2013; Salese et al. 2019), has resulted in the search for other extant water resources, as well as evidence of how much water, hydrogen and oxygen was stripped from the Martian atmosphere about 4 Ga.
The most likely reservoir for extensive storage of water on Mars is groundwater and a global Martian aquifer was long been assumed to exist beneath the permafrost at a depth where crustal temperatures maintained by geothermal heating may support liquid water. Depending on latitude, this melting isotherm is tentatively estimated to be located between depths of 5–9 km and to be overlain by a layer of mixed soil and ice (Farrell et al. 2009; Harrison and Grimm 2009). It is thought that topographic and temperature gradients act to create a significant and prolonged difference in hydraulic head between the melt water‐fed, polar groundwater ‘mound’ and the equatorial aquifer and this is assumed to facilitate significant subsurface flow over geological timescales to establish a global equilibrium depth to the melting isotherm (Baker et al. 1991; Clifford 1993).
Martian groundwater research advanced greatly in the 1980s and early 1990s when the currently accepted ideas regarding subterranean dynamics and subsurface structure were hypothesized. Contemporary investigations are examining these assumptions using the imagery and data now collected by the extensive array of Martian orbiters, landers and rovers, notably NASA's Mars Odyssey satellite, launched in 2001, and the ESA Mars Express, in orbit since 2003. As Mars has a very thin atmosphere and no planetary magnetic field, solar cosmic rays reach the planet's surface unimpeded where they interact with nuclei in subsurface layers up to 2 m in depth, producing gamma rays and neutrons of differing kinetic energies that leak from the surface. Instruments on board the Mars Odyssey orbiter can detect this nuclear radiation and use it to calculate the spatial and vertical distribution of soil water and ice in the upper permafrost layer (Plate 1.4) (Mitrofanov et al. 2004; Feldman et al. 2008). The results indicate water ice content ranging from 10 to 55% by mass, depending on latitude, with the highest concentrations in and around the southern sub‐polar region (Mitrofanov et al. 2004).
The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument mounted on the Mars Express satellite analyses the reflection of active, low frequency radio waves to identify aquifers containing liquid water, since these have a significantly different radar signature to the surrounding rock. The initial findings of the MARSIS sensor effectively identified the basal interface of the ice‐rich layered deposits in the South Polar Region with a maximum measured thickness of 3.7 km, with an estimated total volume of 1.6 × 106 km3, equivalent to a global water layer of approximately 11 m thick (Plaut et al. 2007). However, more recent studies using the MARSIS instrument presented a lack of direct evidence for the existence of subsurface water resources on Mars, possibly as a result of the high conductivity of the overlying crustal material (a mix of water ice and rock) resulting in a radar echo below the detectable limit of the MARSIS sensor (Farrell et al. 2009).
Other studies based on groundwater modelling approaches to explain various topographic features on Mars, such as chaotic terrains thought to have formed owing to disruptions of a cryosphere under high aquifer pore pressure, have concluded that a global confined aquifer system, for example as proposed by Risner (1989), is unlikely to exist and, instead, regionally or locally compartmentalized groundwater flow is more probable (Harrison and Grimm 2009).
Interestingly, the discovery of recurrent slope lineae (RSL) may indicate the presence of seasonal brine water flow on the Martian surface (McEwen et al. 2014). From data acquired by the Mars Reconnaissance Orbiter, RSL appear as narrow, dark markings, typically extending downslope on steep slopes from bedrock areas, often associated with small gullies, and indicative of intermittent, flow‐like features (McEwen et al. 2014). It is conjectured that RSL emanate from bedrock outcrops and progressively lengthen during warm seasons and fade during cooler seasons, preferably on equatorial‐ and west‐facing slopes. From structural mapping using observations from the High Resolution Imaging Science Experiment (HiRISE) of RSL source regions along the walls of craters, together with heat flow modelling and comparison with terrestrial analogues, Abotalib and Heggy (2019) considered that the source of RSL could be natural discharge along geological structures from briny aquifers within the cryosphere at depths of 750 m. The conceptual model presented by Abotalib and Heggy (2019) suggested that deep groundwater occasionally surfaces on Mars under present‐day conditions (Fig. 1.7). The presence of RSL suggests that there is potentially abundant liquid water in some near‐surface equatorial regions of Mars (McEwen et al. 2014), a necessary requirement of habitability for any planet.
Fig. 1.7 Schematic diagram of the control of seasonal melting and freezing of shallow subsurface recurrent slope lineae (RSL) activity on Mars in which discharge of deep groundwater under high hydrostatic pressure occurs preferentially along fault‐related ridges and scarps. (a) In winter, the system shuts down when ascending brines freeze within fault pathways in the near‐surface. (b) In summer, the system resumes when the brine temperature rises above freezing point (Abotalib and Heggy 2019).
(Source: Abotalib, A.Z. and Heggy, E. (2019) A deep groundwater origin for recurring slope lineae on Mars. Nature Geoscience 12, 235–241. DOI: 10.1038/s41561‐019‐0327‐5.)
1.5 The water cycle
A useful start in promoting a holistic approach to linking ground and surface waters is to adopt the hydrological cycle as a basic framework. The hydrological cycle, as depicted in Fig. 1.8, can be thought of as the continuous circulation of water near the surface of the Earth from the ocean to the atmosphere and then via precipitation, surface runoff and groundwater flow back to the ocean. Warming of the ocean by solar radiation causes water to be evaporated into the atmosphere and transported by winds to the land masses where the vapour condenses and falls as precipitation. The precipitation is either returned directly to the ocean, intercepted by vegetated surfaces and returned to the atmosphere by evapotranspiration, collected to form surface runoff, or infiltrated into the soil and underlying rocks to form groundwater. The surface runoff and groundwater flow contribute to surface streams and rivers that flow to the ocean, with pools and lakes providing temporary surface storage.
Of the total water in the global cycle, Table 1.1 shows that saline water in the oceans accounts for 97.25%. Land masses and the atmosphere therefore contain 2.75%. Ice caps and glaciers hold 2.05%, groundwater to a depth of 4 km accounts for 0.68%, freshwater lakes 0.01%, soil moisture 0.005% and rivers 0.0001%. About 75% of the water in land areas is locked in glacial ice or is saline (Fig. 1.9). The relative importance of groundwater can be realized when it is considered that, of the remaining quarter of water in land areas, around 98% is stored underground, and so making groundwater the second largest store of freshwater in the global cycle. In addition to the more accessible groundwater involved in the water cycle above a depth of 4 km, estimates of the volume of interstitial water in rock pores at even greater