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 depths range from 53 × 106 km3 (Ambroggi 1977) to 320 × 106 km3 (Garrels et al. 1975).
Fig. 1.8 The hydrological cycle. The global water cycle has three major pathways: precipitation, evaporation and water vapour transport. Vapour transport from sea to land is returned as runoff (surface water and groundwater flow). Numbers in ( ) represent inventories (in 106 km3) for each reservoir. Fluxes in [ ] are in 106 km3 a−1 (Berner and Berner 1987).
(Source: Berner, E.K. and Berner, R.A. (1987) The Global Water Cycle: Geochemistry and Environment. Prentice‐Hall, Inc., Englewood Cliffs, New Jersey. © 1987, Pearson Education.)
Within the water cycle, and in order to conserve total water, evaporation must balance precipitation for the Earth as a whole. The average global precipitation rate, which is equal to the evaporation rate, is 496 000 km3 a−1. However, as Fig. 1.8 shows, for any one portion of the Earth, evaporation and precipitation generally do not balance. The differences comprise water transported from the oceans to the continents as atmospheric water vapour and water returned to the oceans as river runoff and a small amount (∼6%) of direct groundwater discharge to the oceans (Zektser and Loaiciga 1993).
Table 1.1 Inventory of water at or near the Earth's surface (Berner and Berner 1987).
(Source: Berner, E.K. and Berner, R.A. (1987) The Global Water Cycle: Geochemistry and Environment. Prentice‐Hall, Inc., Englewood Cliffs, New Jersey. © 1987, Pearson Education.)
| Reservoir | Volume (×106 km3) | Percentage of total |
|---|---|---|
| Oceans | 1370 | 97.25 |
| Ice caps and glaciers | 29 | 2.05 |
| Deep groundwater (750–4000 m) | 5.3 | 0.38 |
| Shallow groundwater (<750 m) | 4.2 | 0.30 |
| Lakes | 0.125 | 0.01 |
| Soil moisture | 0.065 | 0.005 |
| Atmospherea | 0.013 | 0.001 |
| Rivers | 0.0017 | 0.0001 |
| Biosphere | 0.0006 | 0.00004 |
| Total | 1408.7 | 100 |
Note:
a As liquid equivalent of water vapour.
