How liquid water shaped the landscapes of Mars and made the planet habitable Liquid water is thought to have been abundant on the surface of Mars early in its history, but today it exists only in the form of ice. The team behind the MarsFirstWater project are investigating the characteristics of water on early Mars, research which holds important implications for future space missions to the planet, as Professor Alberto Fairén explains. The presence of liquid water is essential to life on Earth, and it has played a central role in the evolution of the planetary surface. Beyond our own planet, water is thought to have been present on several different objects in the Solar system at certain points in history, including Mars. “There is ample evidence that liquid water was abundant on the surface of early Mars, both in the geomorphology of ancient terrains, such as lake beds, deltas and stratified sequences, and also in the presence of minerals that can only be formed in the presence of abundant and persistent liquid water,” explains Alberto Fairén, a Research Professor at the Centro de Astrobiologia
(CAB), an institute supported by the Spanish National Research Council (CSIC). While a reservoir of liquid water has recently been found kilometers deep in the crust of Mars, surface water exists today only in the form of ice, in the polar caps, permafrost, and some isolated underground ice deposits. “Liquid water is not stable on the surface of Mars today,” continues Professor Fairén.
MarsFirstWater project As Principal Investigator of the ERC-backed MarsFirstWater project, following up on the earlier icyMARS initiative, Professor Fairén is now working to build a deeper picture of the
water that existed on the Martian surface during the early part of its geological history, using data from both past and current space missions, as well as terrestrial analogs. This work starts from the hypothesis that icerich permafrost characterised most of the Martian subsurface and the subsurfacesurface interface during the Noachian period, which is thought to have begun roughly 4 billion years ago. “We have identified key thermal, mechanical and chemical conditions that characterised ice-rich permafrost associated with hydrological processes on early Mars,” says Professor Fairén. “We are using simulation experiments combined with
A cold hydrological cycle on early Mars. Illustration adapted from a digital terrain model of Valles Marineris which was created from 20 individual HRSC orbits, and the colour data were generated from 12 orbit swaths. Credit: ESA/DLR/FU Berlin (G. Neukum)
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numerical modelling to investigate the role of fluid dynamics on the morphology of the resulting features, with a particular emphasis on the effect of early fluid composition. We have also provided the first identification of rythmites on Mars.” This shows that impact events from asteroids and meteorites, which occurred regularly in the Noachian period, were a major source of liquid water on early Mars. Professor Fairén and his colleagues have also documented the history of a specific aqueous episode on early Mars. “This provides the first evidence of powerful storms, torrential rains, megafloods and strong waves in a Martian palaeolake at the Gale crater,” he outlines. The Gale crater is one of the locations on Mars where liquid water is thought to have been present in the past, and NASAs Curiosity rover is currently located there, looking for evidence of past environmental conditions and whether they could have supported life. “Life requires water, so we are sure that biosignatures on Mars will be hidden in places where water was present in the past,” continues Professor Fairén. “We’re interested in molecular biomarkers, natural products that can be assigned to particular biosynthetic origins.” The most useful molecular biomarkers are organic compounds that are abundant in terrestrial microorganisms and have a high level of taxonomic specificity, meaning that they originate from a limited number of well-defined sources. These compounds may also be fairly well preserved, as they are resistant to geochemical changes and become concentrated upon sediment diagenesis. “This process is controlled by microbial activity and/ or chemical reactions that are catalysed by mineral surfaces,” explains Professor Fairén. Microbial lipids, in particular alkanoic acids, can help researchers reconstruct past conditions on Mars, and so are of great interest to Professor Fairén. “Alkanoic acids are optimal proxies for the reconstruction of paleoenvironmental scenarios, given their abundance (bacteria and eukaryotes contain 1 to 10 percent alkanoic acids), high taxonomic specificity and resistance against diagenesis,” he says. These attributes make lipid fraction studies a useful means of elucidating the sources and diagenesis of organic matter, and Professor Fairén is using these methods to investigate paleoenvironmental microbiology. A further strand of research involves analysing data on ancient rocks found on Mars, and comparing them with sediments from several extreme terrestrial environments in which the conditions are thought to resemble those on early Mars. “The terrestrial analogs are
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The MarsFirstWater Team at the Centro de Astrobiologia (CAB). Credit: AGF.
places on Earth that are characterised by one or a few environmental, mineralogical, geomorphological, or geochemical conditions that are similar to those observed on present or past Mars,” explains Professor Fairén. “Our investigations are being conducted in several analogs, as there is no single analog on Earth that perfectly reflects the conditions on early Mars. It is likely that early Mars had a diversity of environments in terms of pH, redox conditions, geochemistry, temperature, and so on.”
Terrestrial analogs The project team is conducting research in five terrestrial environments; the High Arctic, Antarctica, the Atacama desert in Chile, Rio Tinto and the Tirez lagoon in Spain, from which Professor Fairén hopes to learn more about the conditions on early Mars. The Tirez lagoon is a suitable analog for investigating the changing paleoenvironmental conditions on Mars over the time between the Late Hesperian and the Early Amazonian (around 3 billion years ago) periods, building on data from space missions. “A number of the paleolakes that have been identified on Mars were characterised by episodic inundation by shallow surface waters with varying salinity, evaporation, and full desiccation over time. This process occurred repeatedly until the final disappearance of most surface water after the wet-to-dry transition at the end of the Hesperian,” outlines Professor Fairén. “Similar conditions can be tested over time at the Tirez lagoon, including ecological successions.” This lagoon saw both wet and dry periods over the course of around 20 years, before it reached a state of complete desiccation in 2015. However, this does not mean that the lagoon is completely uninhabitable. “We have demonstrated that the lagoon was habitable both before and after its complete desiccation, despite the repeated seasonal dryness,” says Professor Fairén. Researchers are using information and insights from these terrestrial field analogs in combination
Fieldwork in the Atacama Desert and the Canadian Arctic. Credit: AGF.
with spacecraft mission-derived datasets to test certain hypotheses on the nature of water on early Mars. “We’re using a range of techniques, including paleogeomorphological reconstructions, computer modelling (geological, geochemical and microbiological models) and laboratory studies,” continues Professor Fairén. “The derived results are producing hard constraints on the physical evolution, geochemical alteration and habitability of surface and near-surface aqueous environments on early Mars.” The project team is working to build a more comprehensive understanding of the inventory of water during the first billion years of Mars’
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