Investigating Underground Water on Mars

For decades, the search for water on Mars was primarily focused on visible surface features. Satellite imagery revealed winding channels, dry river valleys, and alluvial fans, providing compelling morphological evidence that liquid water once flowed across the Martian terrain. However, because Mars has an extremely thin atmosphere and cold surface temperatures, liquid water cannot persist today in exposed environments without either freezing or vaporising into the arid air. Consequently, planetary scientists have shifted their attention downwards into the planet’s crust. Investigating the Martian subsurface has opened new debates regarding how much water remains locked below ground, what physical state it assumes, and what this implies for the geological history and potential habitability of the planet.

To detect deep water reservoirs without landing drills, scientists have relied extensively on orbital radar sounders that emit low-frequency electromagnetic pulses capable of penetrating kilometres into the crust. Dr Elena Vance has been a leading figure in interpreting anomalous radar reflections beneath the South Polar Layered Deposits. Her team observed bright basal reflectors that she interpreted as pockets of liquid brine, insulated beneath thick caps of carbon dioxide and water ice. Vance argued that dissolved perchlorate salts could depress the freezing point of water sufficiently to keep it liquid at temperatures well below standard freezing levels, sustained perhaps by lingering geothermal heat. However, Vance has conceded that discerning whether these radar returns stem from authentic water or other dielectric anomalies remains a methodological challenge.

This interpretation has faced considerable scepticism from other researchers who suggest that alternative materials can produce identical radar signals. Dr Tariq Mansour conducted extensive laboratory simulations and numerical modelling to evaluate how different geological substrates respond to radar frequencies under cryogenic conditions. Mansour demonstrated that certain clay minerals, smectites, and iron-bearing volcanic basalt rocks produce dielectric properties indistinguishable from liquid water when subjected to extreme Martian temperatures. According to Mansour, assuming liquid brine exists requires an improbable amount of residual volcanic heat in modern Mars, whereas chilled, mineral-rich strata naturally explain the orbital data without requiring extraordinary thermal anomalies. His work highlighted the danger of relying on single-instrument observations to deduce subterranean conditions.

Taking a completely different methodological path, Dr Ingrid Lindholm turned to the Martian atmosphere to reconstruct the fate of missing subterranean water. By analysing the ratio of heavy hydrogen, known as deuterium, to standard hydrogen in atmospheric water vapour, Lindholm sought to determine how much moisture had escaped into space over billions of years. Since lighter hydrogen escapes more readily than deuterium, the atmospheric ratio serves as a proxy for total historical water loss. Lindholm concluded that atmospheric escape alone could not account for the vast quantities of ancient water indicated by Martian geology. Her findings suggest that the majority of the planet's ancient hydrological inventory did not simply vanish into space, but instead migrated downwards and remains sequestered in deep crustal hydration.

Further insights into subterranean water chemistry have emerged from mineralogical surveys of impact craters. Dr Callum Brody focused on spectroscopic data of deeply excavated impact sites, where high-energy meteorite impacts have unearthed ancient crust from several kilometres beneath the surface. Brody identified widespread deposits of phyllosilicates, or hydrous clay minerals, that could only have formed in sustained alkaline and neutral water environments early in the planet's history. Brody asserted that these subterranean clays indicate that water-rock interactions were not brief, catastrophic flood events, but rather persistent, long-duration hydrochemical systems. His analysis demonstrated that ancient subterranean environments were far less acidic and substantially more hospitable to prebiotic chemical reactions than the modern Martian surface suggests.

While deep crustal investigations reveal ancient history, mid-latitude features offer insights into more recent, accessible ice reservoirs. Dr Siobhan Gallagher analysed lobate debris aprons—curious landforms surrounding isolated massifs and crater walls across the mid-latitudes. Through kinematic modelling and topographical surveys, Gallagher confirmed that these features are actually debris-covered glaciers. Gallagher showed that thick mantles of rocky regolith act as thermal insulation, preventing the underlying ice from sublimating into the low-pressure atmosphere. She concluded that these shallow, buried ice sheets contain exceptionally pure ice rather than rocky rubble, representing a critical, readily accessible reservoir that expanded during periods when Mars experienced higher axial tilt and different climatic regimes.

The contrasting findings of these researchers illustrate the complexity of Mars’s hidden hydrology. Where some see vast sheets of subsurface ice protected by dust, others trace deep chemical alteration or debate the presence of subglacial lakes. Integrating these diverse lines of evidence is essential for resolving the planet's climatic evolution. As orbital instruments continue to collect data and future missions prepare ground-penetrating radar arrays and deep-drilling platforms, scientists are moving closer to a comprehensive model. Resolving whether liquid water currently exists beneath the ice caps or if the subterranean inventory is entirely frozen or chemically bound within rocks will profoundly influence future exploration and our understanding of planetary habitability.