Posted by Any honest account of where life at Mars could still be hanging on today begins with a subtraction. Not on the surface. The rusty plains rovers photograph, all ochre dust and wind-stripped buttes under a sky the colour of weak tea, are the least survivable real estate on the planet. Anything still metabolising is underground, locked inside salt, or sealed under ice. Probably some combination of the three.
That is a different question from the one most Mars coverage answers. The case that ancient Mars was habitable is genuinely strong. Present-day Mars is the harder problem, and the more interesting one, because it is the only version of the planet where you could catch something in the act of living rather than fossilised in stone.
What the Surface Does to Organic Chemistry
Three hazards make the open surface hostile, and they compound each other. Mean atmospheric pressure sits near 6 millibars, right at the triple point of water, so exposed liquid water boils and freezes almost simultaneously. The air is thin and ozone-poor, letting ultraviolet light reach the ground close to unfiltered. With no global magnetic field and barely any air column overhead, galactic cosmic rays and solar particles hit the regolith directly.
Curiosity has carried a radiation detector called RAD since it landed, and the daily dose it records in Gale Crater runs roughly two orders of magnitude above natural background at sea level on Earth. For astronaut planning that is a health risk. For astrobiology it means something sharper: cosmic rays steadily dismantle the organic molecules any search would try to detect. Alexander Pavlov’s group at NASA Goddard exposed amino acids to simulated Martian surface conditions and watched them break down within tens of millions of years in the top few centimetres. Geologically, that is instant.
Go deeper and the arithmetic flips. Two metres of rock or packed regolith absorbs most of the damaging flux, which is why the European Space Agency built its Rosalind Franklin rover around a two-metre drill. That rover is still waiting on the ground in Europe after the 2022 break with Roscosmos took away its ride.
Which exposes an uncomfortable fact. Every machine humanity has landed has worked the top of the sandwich. Curiosity’s drill bites about five centimetres into rock. Perseverance’s cores are roughly the length of a finger. InSight’s heat probe, the mole, was built to hammer five metres down and stalled near forty centimetres in soil that would not give it friction; the team stopped trying in January 2021. Nobody has reached the depth where the question starts.
Salt Is the Loophole and the Trap
Perchlorates changed this conversation. The Phoenix lander tasted them in northern plains soil in 2008 at under one percent by weight, Curiosity found them in Gale, and the assumption now is that they are more or less everywhere. These salts are ferociously good at depressing the freezing point of water. A calcium perchlorate brine can stay liquid down towards minus 70 Celsius, and such salts also pull water vapour straight out of the thin atmosphere overnight, a process called deliquescence. Brine is the loophole. Liquid water can exist where pure water simply cannot.
Then comes the catch. Biology cares less about liquid than about water activity, the fraction of water molecules actually free to do chemistry. Terrestrial life gives up around 0.6, a limit set by a few stubborn fungi. Cold eutectic perchlorate brines tend to fall below it. They are wet and useless at once.
Worse, the salt may be actively lethal. Jennifer Wadsworth and Charles Cockell at Edinburgh showed in 2017 that magnesium perchlorate under Mars-like ultraviolet light killed Bacillus subtilis within minutes, far faster than UV alone. The chemistry keeping water liquid could be the chemistry sterilising it.
The dark streaks that creep down warm slopes each Martian summer looked like the obvious test case. In 2015 a team led by Lujendra Ojha reported spectral signatures of hydrated salts in these recurring slope lineae, and it was widely read as flowing brine. Later analysis pushed back hard, arguing the streaks behave like dry granular flows, sand avalanching at the angle of repose. The salt is real. The water is arguable.
The Lake Under the South Pole That May Not Be a Lake
In 2018 Roberto Orosei and colleagues published radar data from MARSIS, the low-frequency sounder on Mars Express, showing an unusually bright reflection about a kilometre and a half beneath Ultimi Scopuli in the south polar layered deposits, across a patch some twenty kilometres wide. Bright basal reflections are what subglacial lakes produce on Earth, and the claim landed accordingly. A follow-up in 2021 reported more patches nearby.
The objections arrived fast and they are not trivial. SHARAD, the sharper-eyed radar on Mars Reconnaissance Orbiter, has never seen the feature, and a campaign that rolled the whole spacecraft to boost its signal still came up empty. Thermal models struggle badly: keeping brine liquid at that depth demands extreme salt concentrations plus a local heat source nobody has identified. Modelling by Dan Lalich and others reproduced the bright reflection using constructive interference between thin alternating layers of ice and dust, no liquid required. Saline ice and smectite clays can mimic the signal too.
So it sits unresolved, which fairly describes most claims about life at Mars in the present tense. The value of the argument is less the lake than the address. The base of a polar cap is shielded by kilometres of ice, thermally stable, and in contact with rock. Even a briny film down there beats anything available in daylight.
Caves You Could Lose a City In
Mars has caves, and they are absurd. The Odyssey orbiter’s THEMIS camera picked out a cluster of dark pits on the flanks of Arsia Mons in 2007, informally the seven sisters, almost certainly skylights into collapsed lava tubes. Low gravity lets such tubes grow enormous: comparative work by Francesco Sauro’s group put Martian ones one to two orders of magnitude larger in volume than terrestrial tubes, with spans measured in hundreds of metres.
Inside, conditions stop being Martian in the worst sense. Temperature swings flatten out, radiation drops away, cold traps in the floor can hold water ice indefinitely, and any tube connected to the deeper crust gives volcanic gases a path upward. The properties that make a lava tube a plausible microbial refuge also make it the leading candidate for sheltering human crews, a rare case of science and engineering pointing at the same rock.
No spacecraft has been inside one. That is a mobility problem rather than a scientific one.
Rock That Makes Its Own Fuel
The strongest reason to keep taking present-day Mars seriously comes from mines on Earth. In 2008 Dylan Chivian and colleagues described an ecosystem 2.8 kilometres beneath South Africa’s Mponeng gold mine made up essentially of one organism, Desulforudis audaxviator, running on hydrogen and sulfate produced by radioactive decay in the surrounding rock. Barbara Sherwood Lollar’s team has sampled water at Kidd Creek in Ontario isolated from the surface for over a billion years that still carries the chemistry to support microbes.
Neither ecosystem needs sunlight, an atmosphere or a surface. They need water in cracks and rock that decays. Mars has the second ingredient in quantity, and Jesse Tarnas and co-authors modelled radiolysis in the Martian crust in 2018 and concluded it could plausibly sustain cell densities comparable to Earth’s deep biosphere.
Whether the water is there is the open part. Analysis of InSight’s seismic data published in 2024 by Vashan Wright and colleagues argued that the mid-crust, roughly 11 to 20 kilometres down, could hold liquid water in fractured rock. That is a model-dependent inference from one seismometer at one spot, and it is unconfirmed. It is also exactly the sort of address worth arguing about.
The Microbes We Bring With Us
There is a way to ruin all of this, and it needs no malice. The Viking landers were baked whole at 111.7 Celsius for thirty hours before launch, and nothing since has been sterilised that aggressively. Later spacecraft rely on cleanroom assembly and spore counts, which is a statistical argument rather than a guarantee. Cleanrooms breed their own specialists: a bacterium named Tersicoccus phoenicis was first isolated in the Florida facility where Phoenix was assembled, then found again in ESA’s cleanroom in French Guiana. It survives the regime designed to eliminate it.
Under COSPAR planetary protection rules, places where temperature might exceed minus 25 Celsius and water activity might exceed 0.5 count as special regions, and spacecraft that cannot prove their cleanliness are steered away. Curiosity has driven under exactly that restriction since its drill bits were taken out of their sterile container before landing.
The awkward arithmetic waits at the end. A single human body carries something like ten trillion microbial cells, and no suit, airlock or protocol contains all of them. The first crewed landing effectively closes the pristine chapter of this investigation. Carl Sagan put it bluntly enough that it still gets quoted: if there is life on Mars, then Mars belongs to the Martians, even if the Martians are only microbes.
None of this adds up to a discovery. It adds up to a shortlist of addresses, ranked by how much rock, ice or salt sits between a hypothetical organism and the sky. The search for life at Mars has quietly become a search for shielding, and the instruments that would settle it are drills, ground-penetrating radar and whatever eventually crawls into a lava tube.
If that shortlist interests you more than the headlines do, SETIworld follows these arguments properly, from Martian brines and subsurface radar to biosignature fights on worlds far past our own. Come read along, and bring your objections.
