Mars Keeps Revealing Water in New Places, But Astrobiology Still Finds No Sign of Life

The south polar ice cap of Mars holds enough frozen water to blanket the entire planet in 11 metres of ocean if it melted. That number comes from ESA's Mars Express radar data, and it is not an estimate, it is a measurement. Beneath that ice, a 2018 study published in Science using MARSIS radar data detected a 20-kilometre-wide liquid water lake sitting roughly 1.5 kilometres below the surface. Then came more signals: possible additional lakes in 2020, also detected by MARSIS. Seasonal dark streaks on Martian slopes, called recurring slope lineae, were once thought to be liquid water flowing in real time. NASA's Mars Reconnaissance Orbiter found them laced with hydrated salts. The picture that has assembled itself over two decades of orbital observation is not a dry, dead rock. It is a planet that had global oceans roughly 3.5 to 4 billion years ago, lost most of its atmosphere to solar wind after its magnetic field collapsed, and has been hiding what remained ever since, in ice, in minerals, in pockets of cold, pressurised brine deep underground.

The subsurface brine beneath Mars's south pole almost certainly stays liquid because of perchlorate salts. Perchlorate is an oxidising compound found in Martian soil at concentrations between 0.5 and 1 percent, first confirmed by NASA's Phoenix lander in 2008. It lowers the freezing point of water dramatically, enough to keep liquid stable at temperatures as low as minus 70 degrees Celsius. That sounds like a loophole for life. It is also a poison. Perchlorate at those concentrations is toxic to most Earth bacteria. It reacts with ultraviolet radiation to produce reactive compounds that destroy organic molecules on contact. The same chemistry that keeps water liquid on Mars actively dismantles the carbon-based structures that life on Earth depends on. Astrobiology researchers have run experiments exposing Earth microbes to simulated Martian soil: the combination of perchlorate, UV radiation, and low pressure kills them within minutes. The water is there. The conditions that keep it liquid are the same conditions that make it lethal.

India's Mars Orbiter Mission, launched by ISRO from Sriharikota in November 2013 and inserted into Martian orbit in September 2014, was designed primarily as a technology demonstrator. It carried five scientific instruments, one of which, the Methane Sensor for Mars, was built to detect atmospheric methane, a gas that on Earth is produced almost entirely by biological or geological processes. Mangalyaan found no definitive methane signal at the sensitivity levels its sensor could achieve. That absence is not a verdict. The instrument's detection threshold was not fine enough to rule out trace amounts. NASA's Curiosity rover, operating on the Martian surface in Gale Crater, has detected methane spikes multiple times since 2012, readings that fluctuate seasonally and remain unexplained. The European Space Agency's Trace Gas Orbiter, part of the ExoMars programme, has been scanning the atmosphere at far higher sensitivity since 2018 and has so far found no methane at all. Two instruments, both credible, producing contradictory results. The methane question on Mars is open. ISRO's contribution was to add one more data point to a puzzle that the most sophisticated instruments in orbit have not yet resolved.

Astrobiology defines habitability narrowly: liquid water, an energy source, and the right chemistry. Mars clears the first requirement in specific locations and at specific depths. The energy source is available, geothermal heat likely keeps that subsurface brine liquid, and the Sun provides radiation at the surface. The chemistry is where Mars fails the test. The surface is saturated with perchlorates. The atmosphere is 95 percent carbon dioxide and too thin to block ultraviolet radiation. The average surface temperature is minus 60 degrees Celsius. Any organism that evolved to survive subsurface brine would need to tolerate near-zero water activity, extreme cold, high salinity, and complete darkness. Earth does have organisms, called polyextremophiles, that survive two or three of these stresses simultaneously. None are known to survive all of them at once. The subsurface lakes, if they are confirmed as stable liquid water bodies and not radar artefacts (a debate that is still active in the planetary science literature), sit in a zone where the pressure, temperature, and chemistry might allow microbial survival. Might. The word carries the full weight of everything we do not yet know.

NASA's Perseverance rover, which landed in Jezero Crater in February 2021, is drilling cores from what was once a river delta, sedimentary rock that on Earth is among the best environments for preserving biosignatures. Those cores are being cached for eventual return to Earth by the Mars Sample Return mission, a joint NASA-ESA programme whose timeline has been delayed and whose budget is under review. The analysis that will matter most cannot happen on Mars. The instruments sensitive enough to detect molecular biosignatures, isotopic ratios, chirality in amino acids, specific lipid structures, require laboratory conditions that no rover can replicate. ESA's ExoMars Rosalind Franklin rover, when it eventually reaches the surface, will drill two metres down, below the layer where UV radiation and perchlorate chemistry do their worst damage. Two metres is where the science gets serious. Mars has given up water at every scale of measurement. What it has not given up is any chemistry that looks like it was made by something alive. The search is not over because the search has not yet reached the right depth, in the right place, with the right tools. That is a different kind of silence than an answer.