The Water Reservoir of Mars

Much of the research currently focussed on the hydrological resources of Mars is being conducted using the modelling approaches established by the pioneering research studies based on the first successful imaging missions to the planet in the latter half of the last century. Martian groundwater research was advanced greatly in the 1980s and early 1990s when the currently accepted ideas regarding the subterranean dynamics and subsurface structure of the planet were initially hypothesised. Contemporary investigations are examining and testing these assumptions using the wealth of imagery and data now collected by the extensive array of Martian orbiters, landers and rovers previously and currently in operation on or around the planet, particularly 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 cosmic rays from the Solar wind reach the surface unimpeded where they interact with the nuclei in subsurface layer up to 2 m in depth, producing gamma rays and neutrons of differing kinetic energies that leak from the surface. These can then be passively detected by instruments on board the Mars Odyssey orbiter and used to calculate the spatial and vertical distribution of soil water and ice in the upper permafrost layer. The fluxes of high energy neutrons (‘fast’ neutrons) and thermal and epithermal neutrons vary relative to subsurface water content. The results showed water ice content ranging from 53% to 11.2% of soil by weight, depending on latitude, with the highest concentrations in and around the North Polar Region. Knowledge of water content in the upper layers of the subsurface is useful for determining potential outgassing and atmospheric transport.

While the Mars Odyssey orbiter searched for water in the upper few meters of the subsurface, the Mars Express satellite searched for groundwater at depths of up to 5 km using the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument. The sensor analyses the reflection of active, low frequency radio waves to identify aquifers containing liquid water as these will have a significantly different radar signature to the surrounding rock (figure 1).

Figure 1: The principal of operation of the MARSIS instrument onboard the Mars Express orbiter (ESA, 2011)

The initial findings were promising as the MARSIS sensor effectively identified the basal interface, ice density, quantity and thickness of the South Polar Region layered deposits to a depth of 3.7 km, allowing improved volume estimations of the polar reservoir to be calculated.

The most recent studies however presented a lack of direct evidence for the existence of sub-surface and groundwater resources on Mars using the MARSIS instrument. It is thought that the absence of any direct detection of subterranean water could be a result of the high conductivity of the overlain crustal material (a mix of water ice and rock), resulting in the radar echo being under the MARSIS instruments detectable limits. It was emphasised that groundwater reservoirs could still exist and that the lack of evidence supporting this hypothesis was due to the technical shortcomings of the sensor. This article in particular received widespread attention in the technical and popular science media as the null result could mean that important assumptions regarding the storage and role of water on Mars may have to be re-evaluated.

Other current studies concentrating on groundwater modelling approaches to explain various topographical features on Mars have come to the conclusion that a global, confined aquifer system is unlikely to exist and regionally or locally compartmentalised groundwater flow is more probable. This finding is likely to have important implications for the future development of groundwater flow models and the interpretation of remotely sensed data.

References:
ESA (2011). Mars Express. [Online]. Available at: http://sci.esa.int/science-e/www/area/index.cfm?fareaid=9 (Accessed on 6th February 2011)

Mitrofanov, I.G. (2005) Global Distribution of Subsurface Water Measured by Mars Odyssey in Tokano, T. (ed.) (2005) Water on Mars and Life. Berlin: Springer Advances in Biogeophysics and Astrobiology (pp. 99 – 128)

Plaut, J.J., Picardi, G., Safaeinili, A., Ivanov, A. B., Milkovich, S.M., Cicchetti, A., Kofman, W., Mouginot, J., Farrell, W.M., Phillips, R.J., Clifford, S.M., Figeri, A., Oroseq, R., Federico, C., Williams, I.P., Gurnett, D.A., Nielsen, E., Hagfors, T., Heggy, E., Stofan, E.R., Plettemeier, D., Watters, T.R., Leuschen. C.J. and Edenhofer, P. (2007). Subsurface Radar Sounding of the South Polar Layered Deposits of Mars. Nature. 316 (5821) pp. 92 – 95.

Farrell, W.M., Plaut, J.J., Cummer, S.A., Gurnett, D.A., Picardi, G., Watters, T.R., and Safaeinili, A. (2009). Is the Martian water table hidden from radar view? Geophysical Research Letters. 36 L15206

Harrison, K.P. and Grimm, R.E. (2009). Regionally compartmented groundwater flow on Mars. Journal of Geophysical Research. 114E04004

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