Now, the atmospheric pressure is so low on Mars that water cannot exist on the surface. Nevertheless, on-orbit satellite and rover studies showed that Mars had been relatively damper and warmer than it currently is. This statement can be proved looking at the dry valleys and fossil deltas which were almost certainly created by running water.
Planetary, but quite localised, detections of hydrated minerals coming from chemical deterioration of rocky materials by water brought other convincing evidence. The silicate minerals deteriorated by water and turned into phyllosilicates (clay minerals) are mostly absent from the Martian surface, apart from many outcrops in the oldest geological units, informing us that the planet spent most of its recent history dry. However, an international team of scientists reported in the International Journal of Solar System Studies (Icarus) that these indicator minerals were detected in the Tyrrhena Terra region, a former highland, which is located between the northern plains of Isidis Planitia and the huge Hellas impact basin.
The hydrated mineral detections were performed by two imaging spectrometers: OMEGA (Observatory for Mineralogy, Water, Ice and Activity) on board ESA's Mars Express and CRISM (Compact Reconnaissance Infrared Spectrometer for Mars) on board NASA's Mars Reconnaissance Orbiter.
OMEGA provided an almost complete coverage of the Martian surface with resolution from hundreds of meters to several kilometers per pixel, whereas CRISM (with a resolution of 15-20 m per pixel) could be directed to provide mineralogical close-ups of specific sites.
The observation satellites disclosed outcrops hosting hydrated silicates on 175 different sites on Tyrrhena Terra, a large majority of which were ejecta, walls, central peaks and ramparts of many craters. The craters in which these minerals were detected were between less than 1 km and 84 km diameter.
Even if the region was divided up by many valley networks, the water-deteriorated rocks were rarely visible between the craters on Tyrrhena Terra. This implied that the chemical deterioration revealed by the craters was not associated to liquid water erosion on the surface, contrary to several other Martian regions.
How did hydrated silicates form on Tyrrhena Terra?
Two main hypothesis were proposed.
The chemical deterioration might be due to the impact processes. An hydrothermal system could have gone up a crater's peak or shores and formed minerals such as serpentine, chlorite and smectites. Nevertheless, this process did not explain why similar hydrated silicates were present on the central peak and near the ejecta.
Moreover, the observations of the crater floors did not show any surface element associated to possible hydrothermal systems such as vents and fractures. Finally, the presence of many hydrated outcrops associated to small craters, some of which were less than 1 km diameter, could not be explained by hydrothermal systems resulting from these lesser energy impacts.
The team concluded that this hypothesis was unlikely to be the only process associated to the rock hydration and that its contribution could only be secondary. Its preferred explanation was that the hydrated minerals formed underground with underground water. The buried rocks hosting hydrated silicates were then exposed and ejected by future impacts. These impacts mechanically ejected the hydrated minerals without producing any significant mineralogical transformation.
The presence of some outcrops hosting phyllosilicates on some knolls of the region, far from the visible craters, also enhanced the hypothesis that not only the near-crater areas were deteriorated but the whole crust.
The phyllosilicates are hydrated mineral formed by the interaction between rocks and liquid water. This important group of minerals includes micas, chlorite, serpentine, talc and clays: primary products of the chemical deterioration of rocks. The minerals formed at different temperatures and seemed to be located at different depths with temperatures up to 300ºC at some kilometers below the surface.
“These minerals are being associated to recent craters formed after the water activity”, Damien Loizeau, research officer at ESA and main author of the study, explained. “They probably represent former hydrated soils which were revealed by crater forming impacts. These impacts revealed the Noachis Terra's crust which had been chemically deteriorated more than 3.7 billion years ago.”
Finally, the hydrated silicates were not detected near or in the small craters (diameter < 5 km) on the flat volcanic plains of Tyrrhena Terra, even if there were a lot on the Noachis Terra's divided up soils. The presence of hydrated silicates was thus proved not to depend on the impact process but on the prior deterioration of the impacted soil.
The presence of phyllosilicates in the largest craters of the flat plains could be explained by the excavation: the lava plain was thin enough to allow the biggest craters to pass through the underlying rocks, buried units of the Noachis Terra. So the craters hosting phyllosilicates in the volcanic plains showed the plain's thickness variations and the excavation resulting from the buried hydrated rocks.
The deeply excavated hydrated silicates
“The wide size range of craters which was studied showed that the hydrated silicates were excavated from the depths from tens of meters to some kilometers”, Damien Loizeau explained. “The rock chemical composition had been so deteriorated that the underground water had to be present during long time periods.”
"The water circulation took place at various kilometers depths within the crust 3.7 billion years ago, even before most of the craters were formed”, Nicolas Mangold, co-author from the Laboratory of Planetology and Geodynamics, University of Nantes, added. “The water produced a large range of chemical changes within the rocks which reflected temperature drops near the surface and high temperatures in depths, without any direct relation with the surface's conditions at the time.”
“The role of the liquid water on Mars was crucial for its habitability and this study, using Mars Express' data, described a very large area where underground water was present during long periods”, Olivier Witasse, scientist on ESA's Mars Express project, said.
Loizeau D. et al., Characterization of hydrated silicate-bearing outcrops in Tyrrhena Terra, Mars: Implications to the alteration history of Mars, Icarus, 219, 476-497.