Mars might have been cold and dry with a transient presence of water at the surface some four billion years ago — the early Noachian period. But it is becoming increasingly clear that the environment below the surface was surely warmer and wetter, with liquid water present at varying depths during the Noachian period. The presence of clay minerals on the floor of many craters clearly indicates that they had formed as a result of long-term interaction of liquid water with the parent rock. Even the presence of massive ridges was noticed earlier, but the likely cause that led to their formation was not known. A paper published recently in Geophysical Research Letters, which studied the over 4,000 ridges in the Nili Fossae and Nilosyrtis highlands, postulates a likely cause: The ridges could be mineral deposits that filled the subsurface fractures and faults caused by massive impacts on the surface. The ridges are found in association with clay-containing bedrock. Hence it is postulated that the hydrous clay present in the rock could have played an important role in supplying fluids to cement the fractures. Another study in Nature Geoscience not only supports the idea of subsurface water but suggests an alkaline nature for the water. This is based on the presence of magnesium-iron bearing clay and carbonates found in the McLaughlin Crater.
The Martian surface faces hostile conditions that are quite inimical to life. However, there is a greater possibility of finding some signs of life on Mars if we remain focussed on exploring the subsurface sedimentary rocks that today lie exposed in many craters. Carbonates in particular are a perfect medium not only to provide an ideal habitat for life, but also to preserve fossil traces indicative of life. Moreover, carbonate minerals can reveal the temperature and chemistry of the depositional environment. NASA’s Mars rover Curiosity, which is all set to drill at four locations in the coming days, may soon provide the answer to the most sought after question — did Mars ever harbour life? For now, the rover has unequivocally proved that Mars had a wet depositional environment in the past. It found the amount of water molecules bound to sand grains in the soil sample was much “higher than anticipated.” If the discovery of gypsum at several places in the past meant water on Mars, the latest find of veins and sedimentary rocks — layered rocks and sandstone — by Curiosity vastly strengthens the possibility of wet depositional environments in the past. After all, Mars’s reduced gravity translates to “more subsurface porosity to a greater depth.” This allows water to accumulate to a greater thickness than seen in Earth.