Posted by If anything ever lived on Mars, the last place to look for it is the part we can see. The surface has spent three billion years under ultraviolet light with no ozone layer to soften it, soaked in cosmic rays that a thin atmosphere barely slows, and salted with perchlorates that turn destructive when the sun hits them. Whatever organic chemistry sat exposed up there has been taken apart. Which is why serious discussion of life on Mars keeps moving downward, into rock, ice and the pore spaces where conditions have hardly changed since the planet was young.
What the top few metres do to evidence
Curiosity’s radiation detector has been measuring the dose at Gale crater since 2012, and the numbers are grim for anything unshielded — orders of magnitude above what a surface organism on Earth absorbs. Galactic cosmic rays penetrate rock, breaking molecular bonds as they go, and models suggest amino acids left in the top handful of centimetres would be destroyed on timescales far shorter than the age of the terrain.
Perchlorate salts, which NASA’s Phoenix lander found in the soil in 2008 and which appear to be widespread, make it worse. Under ultraviolet light they generate reactive compounds that oxidize organic material, and there is a reasonable argument that Viking’s 1976 experiments cooked and destroyed the very molecules they were trying to detect.
Two metres of rock changes the arithmetic entirely. That is why the ESA Rosalind Franklin rover, delayed repeatedly and now aimed at the end of this decade, carries a drill designed to reach that depth. Nothing that has landed so far has scratched more than a few centimetres.
The Earth precedent nobody expected
The reason anyone takes a Martian subsurface seriously is what turned up beneath our own feet. In 2008 a team working nearly three kilometres down in the Mponeng gold mine in South Africa described a bacterium living in water that had been isolated from the surface for millions of years. It goes by Desulforudis audaxviator, and it runs almost entirely on hydrogen produced when natural radioactivity in the surrounding rock splits water molecules.
No sunlight. No contact with the biosphere above. An ecosystem of essentially one species, powered by radioactive decay.
That discovery removed the main objection to a Martian deep biosphere. Mars has radioactive elements in its crust, it has water ice and probably brine at depth, and the same radiolysis reactions should run there. Modelling published in 2018 by Jesse Tarnas and John Mustard suggested the Martian subsurface could generate hydrogen at rates comparable to the settings where terrestrial deep life survives.
Liquid water is the harder requirement. Radar sounding has mapped buried ice through the mid-latitudes, and in 2018 the MARSIS instrument on Mars Express returned a bright reflection under the southern polar cap that its team read as brine, a claim other groups have challenged by showing that clays or certain frozen materials can mimic the same signature. Perchlorates would keep such water liquid at temperatures far below freezing, though a cold, salty, oxidizing brine is a punishing habitat even by microbial standards. Frozen ground is a better bet for preservation than for survival: permafrost on Earth holds intact organic material for hundreds of thousands of years.
The methane argument that will not resolve
Methane is where the case for present-day life on Mars gets both its best headline and its most frustrating contradiction.
Curiosity’s tunable laser spectrometer has repeatedly measured a low background of methane inside Gale crater, varying with the seasons, with occasional spikes to several times that level. Methane should not last long in the Martian atmosphere — ultraviolet light breaks it down over a couple of centuries — so anything detected has to be produced or released recently. Serpentinization, where water reacts with olivine-rich rock, makes methane without any biology. So do microbes.
Then the ExoMars Trace Gas Orbiter arrived, carrying instruments far more sensitive than Curiosity’s, and found nothing. Its upper limits sit well below what the rover reports.
Both measurements are probably correct, which is the interesting part. Proposed reconciliations involve methane seeping out near the ground at night and being destroyed before it mixes upward, or highly localized emission the orbiter cannot resolve. Nobody has closed the case, and it has been open for over a decade.
Rocks that have been argued over for thirty years
ALH 84001 is the meteorite that started the modern debate. Found on the Allan Hills ice field in Antarctica in 1984 and later identified as Martian, it was announced in 1996 by David McKay’s team as containing possible traces of ancient microbes — carbonate globules, polycyclic aromatic hydrocarbons, and chains of magnetite crystals resembling those made by magnetotactic bacteria on Earth.
The consensus moved against it. Each feature turned out to have a plausible inorganic route, and a 2022 study argued the organics formed through water-rock reactions. What the episode produced was a much sharper sense of how hard it is to demonstrate biology from morphology and chemistry alone.
Curiosity’s SAM instrument has since found genuine Martian organics in mudstone — chlorinated benzenes and thiophenes in 2018, and more recently long-chain alkanes preserved in a sample drilled from a clay-rich unit. These are real organic molecules on Mars. They are not evidence of life on Mars, since meteorites deliver organics constantly and geochemistry produces them without help.
In July 2024 Perseverance drove up to an arrowhead-shaped rock in Neretva Vallis that its team named Cheyava Falls, and found millimetre-scale pale spots ringed with iron and phosphorus, sitting alongside organic carbon in a mudstone. On Earth, patterns like that are often left by microbes drawing energy from sediment. On Mars they might equally be ordinary chemistry. The rock is cached in a tube on the surface, and settling the question needs a laboratory.
Where to dig
Some places should preserve more than others. Ancient hydrothermal systems, where hot water circulated through rock, provided energy and minerals that trap and mineralize organic material quickly. Silica deposits found by the Spirit rover at Home Plate in Gusev crater have been read as hot-spring precipitates, a class of setting that preserves microbial textures well on Earth.
Salt matters too. Evaporite deposits can trap tiny pockets of brine inside crystals, and on Earth those inclusions preserve organic material remarkably well, though claims of reviving organisms from very old salt remain contested.
Then there are the caves. Orbiters have imaged dozens of skylights and pit entrances on the flanks of the Tharsis volcanoes, almost certainly collapsed sections of lava tubes. A cave interior is shielded from radiation, buffered against temperature swings, and may hold ice. Nobody has flown a mission capable of entering one, though concepts get proposed regularly.
Why the answer keeps receding
Every result so far has been suggestive and none has been decisive, and there is a structural reason for that. Rovers carry a few instruments chosen years before launch, work on power budgets, and cannot repeat a measurement twenty different ways when something odd shows up. Confirming a biosignature takes the kind of instrumentation that fills a building.
That puts the weight on sample return, and the sample return programme has been rescoped and delayed more than once on cost grounds. The tubes are sitting on the surface of Jezero crater in the meantime, which is not a bad place to store them.
What has genuinely changed is the shape of the question. It is no longer whether Mars was ever habitable, since the sediments answered that. It is whether anything took the opportunity, and whether traces of it survive somewhere below the reach of the radiation. SETIworld follows the drilling missions, the disputed detections and the sample return arguments as this one slowly gets settled.
