A recent study by researchers at the Atmospheric Science Group at Luleå University of Technology (LTU), Sweden, provides a dramatic evidence of transient liquid water -- a key element in the search for life outside our planet -- and its role in the daily interaction between surface salts and the Martian atmosphere. This has profound astrobiological implications since life, as we know it, depends on water.
As early as the 2nd millennium BCE, Egyptian astronomers began recording observations of Mars and its motion. With the invention of the telescope in the early 1600s, we saw a glimpse of possible Earth-like conditions of the surface of the ‘red planet’. Seasonally changing dark patches on the Martian surface gave the impression of stretches of vegetation, though we soon realised that the conditions are too harsh, too cold and dry, to support life as we know it.
Data collected by NASA’s ConTeXt (CTX) imager and the High Resolution Imaging Science Experiment (HiRISE) camera, both on board the Mars Reconnaissance Orbiter (MRO), has proven the existence of water, locked in ice at the polar caps, with a fraction as vapour in the atmosphere.
Geographical studies on Mars show that about 3.8 billion years ago, when the temperature of the planet was warmer, liquid water flowed widely across the planet, shaping its surface as observed in riverbeds, deltas, teardrop-shaped islands, etc. The present frigid and parched landscape appears to be unfavourable for microbial life, except perhaps, in the subsurface of the dark streaks on slopes documented in the equatorial regions that gain and lose heat quickly.
Until recently, the predominant hypothesis for the creation of these slope features was the phenomenon of ‘dry mass movement’ wherein dust avalanches and rockfalls are set off by local disturbances, perhaps quakes or meteor impacts. Alternatively, the lines on the slope could have been created by the flow of wet debris when ice melts. The research team, led by Dr. Anshuman Bhardwaj, a glaciologist by training, argues that the existing models fail when studying analogous terrain on Earth.
“With the available remote sensing data and climate simulations, we are very confident of the mechanisms which we have proposed. The reason why we have high confidence is that the results are not based only on the visual observations and measurements, but are also backed by very significant geo-statistical test results at global scale, for the first time”, comments Dr. Bhardwaj. “Furthermore, the fact that the analogous slope streak features which we find on the Earth are all related to wet flows is a very strong suggestive of a similar mechanism on Mars”, he adds.
With the unprecedented clarity of Mars’ surface imaging, the researchers observed an absence of debris deposits at the base of the slope streaks with undisturbed topography. This study’s compilation of data overturns a previously reported threshold slope gradient of ~20° (steep and more prone to rockfalls and avalanches) to a gentle to nearly flat inclination no higher than 10°. Considering the required momentum and inertia of debris for the formation of the streaks in the reduced gravity of Mars, the study claims that these manifestations are most likely not the product of a large movement of material, either dry or wet, but a symptom of localised diurnal water cycles occurring between salts in the slopes and the water vapor in the air.
Data from the Gamma Ray Spectrometer (GRS) aboard the 2001 Mars Odyssey, a robotic spacecraft orbiting Mars developed by NASA, characterised the presence of salts on Mars. A crucial correlation shown by the researchers is between the slope areas, a high water vapour column and spatial clustering of chlorine (Cl) and iron (Fe). The higher concentrations of these elements correspond to higher probability of formation of salt species which can form brines through a process called deliquescence even at lower temperature conditions present on Mars. In these regions, there are little seasonal variations in temperature. Instead, the temperature rises at least once a Martian day to well above the freezing temperature of the probable brines suggesting that the surface temporarily becomes wet. Flow patterns of slope streaks cover long distances and show fluid characteristics such as branching at an obstacle, and converging again. The ability to climb obstacles as high as 3.5m suggests high adsorption and capillary action through the fine dust in lower Martian gravity.
As stated in the paper, “The possible water activity within the slope streaks covering a large part of the equatorial latitudes is highly significant with respect to the planetary protection policies for future robotic or manned missions to prevent the biological contamination of both Earth and Mars.” According to a press release by LTU, findings from this study will inform the design of the same research group’s first Swedish instrument called HABIT to be sent to Mars aboard ExoMars 2020, a joint mission to search for evidence of life on Mars by the European Space Agency (ESA) and the Russian space agency, Roscosmos. With more detailed imaging and geomorphological studies, we take a further step towards understanding the habitable potential of our most promising neighbour.