No one has confirmed the presence of bacteria or any other life form on another planet. That is the straightforward answer, and it has not changed since the first Mars landers touched down in the 1970s. But the question has never been more scientifically alive. Rovers have found organic molecules and seasonal methane pulses on Mars, lab experiments have shown that Earth microbes can survive years in the vacuum of space, and moons orbiting Jupiter and Saturn have revealed subsurface oceans with the chemical ingredients life needs. The gap between “no evidence of life” and “nowhere for life to exist” has been closing steadily, and the tools being built right now are designed specifically to settle the question.
What Mars Has Revealed So Far
Mars is the planet we have explored most intensively, and it keeps handing researchers tantalizing clues that stop short of proof. NASA’s Curiosity rover measured methane in Gale Crater over a five-year stretch and found a repeating seasonal cycle, with concentrations swinging between roughly 0.24 and 0.65 parts per billion, plus occasional spikes around 7 parts per billion.1PubMed. Mars methane detection and seasonal variations On Earth, most atmospheric methane comes from living organisms, so detecting it on Mars immediately raises the question of a biological source. The seasonal pattern is hard to explain through purely geological or photochemical processes alone, though researchers have not ruled those out.
Meanwhile, the Perseverance rover has been exploring Jezero Crater, a dried-up lake bed thought to have once held standing water. Analyses there have uncovered diverse organic-mineral associations, meaning carbon-containing molecules bound up with the rock in ways that point to complex chemistry.2Nature Communications. Diverse organic molecules on Mars revealed by the first SAM TMAH experiment Organic molecules are not the same as living organisms. They can form through volcanic processes, meteor impacts, and other abiotic reactions. But their presence confirms that the raw building blocks of biology are sitting on the Martian surface, waiting for a more sensitive instrument to determine where they came from.
The Viking Experiments and Why They Still Matter
The first real attempt to detect life on Mars happened in 1976, when NASA’s Viking 1 and Viking 2 landers ran a suite of biology experiments on Martian soil. The Labeled Release experiment added a nutrient broth to soil samples and monitored for gases that living organisms might produce. It got a positive signal. Then the other Viking instruments failed to detect any organic molecules at all, which seemed to contradict the idea that biology was responsible. The consensus at the time was that the results were caused by reactive soil chemistry rather than microbes.
Decades later, scientists revisited that conclusion. The Phoenix lander’s discovery of perchlorate salts in Martian soil offered a new explanation: perchlorate would have broken down any organic molecules inside the Viking instrument’s heated ovens, masking their presence entirely.3Icarus. The Viking biology experiments on Mars revisited Cosmic-ray decomposition of perchlorate into hypochlorite could also explain the reactive chemistry that mimicked a biological signal. A separate analysis argued that the Labeled Release results remain consistent with biology and that extant life on Mars is still a viable explanation that should not have been dismissed so quickly.4PubMed Central. The Case for Extant Life on Mars and Its Possible Detection by the Viking Labeled Release Experiment The Viking debate is not settled, which says something important about how hard it is to prove or disprove life at a distance. The instruments we send need to be far more capable than what was available fifty years ago.
Why the Subsurface Is the Best Place to Look
The surface of Mars is punishing. Ultraviolet radiation pours down unfiltered, temperatures swing wildly, and the thin atmosphere provides almost no pressure to keep liquid water stable. If anything is alive on Mars today, it almost certainly lives underground. The same logic applies to most other planetary bodies in the solar system: surfaces tend to be irradiated, desiccated, or both, while subsurface environments can offer shielding, stable temperatures, and liquid water.5PubMed Central. Subsurface Life on Earth as a Key to Unlock Extraterrestrial Mysteries
Earth’s own subsurface gives researchers a working model for what that underground life might look like. Deep marine sediments, crustal aquifers, rock fracture fluids, and permafrost soils all host microbial communities that survive under extreme conditions and chronic energy scarcity. Some of the best-studied examples are subsurface lithoautotrophic microbial ecosystems, communities of microbes that live in igneous rock and feed on chemical energy from reactions between water and basalt rather than sunlight. These are the only known ecosystems on Earth that could, in principle, function on Mars today, given that water, basalt, and inorganic carbon are all thought to be available in the Martian subsurface, possibly at depths of two kilometers or more.6FEMS Microbiology Reviews. Lithoautotrophy in the subsurface
Mars also has lava tubes, long cave-like tunnels formed by ancient volcanic activity. On Earth, researchers have found active microbial communities living in ice deposits inside lava tube caves that serve as high-fidelity Mars analogues. The caves shield organisms from surface radiation and temperature swings, and ice provides a water source. Given those results, researchers have concluded that similar Martian caves containing ice could support active microbial communities.7PubMed. Taxonomic Characterization and Microbial Activity Determination of Cold-Adapted Microbial Communities in Lava Tube Ice Caves from Lava Beds National Monument, a High-Fidelity Mars Analogue Environment Separate work studying an active lava-induced hydrothermal system in Iceland, associated with the 2014–2015 Holuhraun eruption, found that lava-water interactions generate habitable conditions for microbial communities, and that similar interactions on early Mars could have done the same.8PubMed. Microbial Response to Increased Temperatures Within a Lava-Induced Hydrothermal System in Iceland: An Analogue for the Habitability of Volcanic Terrains on Mars
Ocean Worlds Beyond Mars
Mars gets most of the attention, but some of the most promising places to look for extraterrestrial microbes are the icy moons of the outer solar system. Enceladus, a small moon of Saturn, has geysers that spray water ice and vapor into space from a global subsurface ocean. The Cassini spacecraft flew through those plumes and detected organic molecules, molecular hydrogen, and silica nanoparticles, all consistent with hydrothermal vents on the ocean floor. Modeling work has shown that dissolved phosphorus in Enceladus’s ocean could reach concentrations close to or higher than those in modern Earth seawater, which had previously been a major concern since phosphorus is essential for all known biology.9Proceedings of the National Academy of Sciences. Abundant phosphorus expected for possible life in Enceladus’s ocean That finding removed one of the strongest chemical objections to habitability there.
Europa, a moon of Jupiter, is another prime target. It has a subsurface ocean beneath an ice shell, with tidal heating from Jupiter providing the energy to keep the water liquid. Instruments designed for future lander missions, like the compact laser desorption mass spectrometer called ORIGIN, are specifically built to detect extremely low concentrations of amino acids and other biomolecules on icy surfaces.10PubMed Central. ORIGIN: a novel and compact Laser Desorption – Mass Spectrometry system for sensitive in situ detection of amino acids on extraterrestrial surfaces A separate prototype, a submersible capillary electrophoresis analyzer, has been developed as an early step toward sending underwater chemical analyzers to ocean worlds.11PubMed. Submersible Capillary Electrophoresis Analyzer: A Proof-of-Concept Demonstration of an In Situ Instrument for Future Missions to Ocean Worlds These are still prototypes, but they signal where the field is headed: directly sampling alien oceans for signs of biology.
The Venus Phosphine Controversy
In 2020, a team announced the detection of phosphine gas in Venus’s atmosphere, which set off a firestorm because phosphine on a rocky planet has no well-understood non-biological source at the quantities claimed. Venus is not an obvious candidate for life: surface temperatures exceed 450°C, and the atmosphere is dominated by carbon dioxide and sulfuric acid clouds. But the cloud layer between roughly 50 and 60 kilometers altitude has more moderate temperatures and pressures, and some researchers have proposed it as a possible habitable niche for airborne microbial life.
Follow-up analysis cooled the excitement considerably. A reanalysis of the radio telescope data suggested much lower phosphine concentrations than originally reported. And photochemical modeling showed that abiotic reactions in the Venusian atmosphere could produce phosphine at levels up to about 2 parts per billion between 50 and 60 kilometers altitude, which falls within the range of the revised detection estimates of 1 to 4 parts per billion.12Astronomy & Astrophysics. Uncertainty in phosphine photochemistry in the Venus atmosphere prevents a firm biosignature attribution The conclusion from that work was blunt: even a firm detection of several parts per billion of phosphine would not necessarily mean a biological origin, because the uncertainties in Venus’s phosphorus chemistry are too large. Venus remains interesting, but the phosphine story is a cautionary tale about how quickly a tentative detection can be misread as evidence of life.
The False-Positive Problem
One of the hardest challenges in astrobiology is distinguishing the chemical signatures of life from those produced by purely geological processes. On Earth, deep-sea hydrothermal vents can synthesize organic molecules from inorganic ingredients when hot, hydrogen-rich fluids react with dissolved carbon dioxide. Researchers have documented distinct abiotic pathways for creating compounds like formate at the Von Damm hydrothermal field, showing that reduced carbon can be generated without any biology at all.13PubMed Central. Pathways for abiotic organic synthesis at submarine hydrothermal fields If these reactions happen on Earth, they can happen anywhere with similar geology, meaning that finding organic molecules on Mars or Enceladus does not automatically point to life.
This is why the field has moved toward looking for multiple lines of evidence simultaneously rather than relying on any single chemical marker. A single gas, a single organic molecule, or a single reactive soil experiment can always have an alternative explanation. The goal now is to stack the evidence: find organics, find an energy source, find liquid water, and ideally find molecular patterns that only biology produces, like an excess of left-handed amino acids or specific lipid structures. The James Webb Space Telescope is extending this logic to planets beyond our solar system, where detecting biosignature gases through the light filtering through an exoplanet’s atmosphere is, at least in principle, within the telescope’s capabilities.14PubMed Central. Prospects for detecting signs of life on exoplanets in the JWST era
Microbes Can Survive the Trip Through Space
Even if bacteria exist on only one planet, that does not necessarily mean they originated there. The idea that microbes could travel between worlds inside rocks blasted off by asteroid impacts, a concept called lithopanspermia, has moved from fringe speculation to a scientifically testable hypothesis over the past few decades. The question has three parts: can microbes survive being launched off a planet, can they survive the transit through space, and can they survive atmospheric entry at the other end?
For the launch phase, experiments have shown that bacterial spores embedded in rock can survive the extreme pressures of a hypervelocity impact. Spores of Bacillus subtilis in granite survived simulated launch conditions with spall fragments shocked at pressures of 5 to 7 gigapascals, with survival rates on the order of one in a hundred thousand.15PubMed. Bacterial spores in granite survive hypervelocity launch by spallation: implications for lithopanspermia More recently, the radiation-resistant extremophile Deinococcus radiodurans was subjected to pressures up to 3 gigapascals and showed nearly 100 percent survival at 1.4 gigapascals, orders of magnitude higher than other tested microorganisms.16PubMed Central. Extremophile survives the transient pressures associated with impact-induced ejection from Mars
For transit through space, Bacillus subtilis spores were exposed to the space environment for nearly six years on NASA’s Long Duration Exposure Facility. When shielded from solar ultraviolet radiation, up to 80 percent of spores in multilayers survived. UV was by far the most destructive factor, reducing survival by four orders of magnitude or more, but even in completely unprotected samples, thousands of viable spores were recovered.17PubMed. Long-term survival of bacterial spores in space A separate study mounted microorganisms on the exterior of the International Space Station for two years and found that colony counts dropped by about five orders of magnitude overall, but survivors persisted among bacteria, archaea, and fungi.18Scientific Reports. Survival of microorganisms during two-year exposure in outer space near the ISS Spores exposed for 559 days on the ISS’s EXPOSE-E platform showed a broad stress response involving DNA repair, protein repair, and oxidative stress pathways, but they remained viable.19PubMed. Transcriptomic responses of germinating Bacillus subtilis spores exposed to 1.5 years of space and simulated martian conditions on the EXPOSE-E experiment PROTECT
For atmospheric entry, spores of Bacillus subtilis were mounted on the exterior of a sounding rocket, launched into space, and subjected to reentry at 1.2 kilometers per second. Survivors were recovered at rates of 1 to 4 percent from all surfaces except the forward-facing one.20PubMed. Bacillus subtilis spores on artificial meteorites survive hypervelocity atmospheric entry: implications for Lithopanspermia Taken together, these experiments show that every individual leg of an interplanetary journey is survivable for certain microbial spores. Whether the full chain of events, launch, millions of years in transit, and landing, has ever actually delivered living microbes to another world remains unknown. But it is no longer physically implausible.
The Contamination Problem Works Both Ways
The hardiness of Earth microbes creates a serious practical problem. Every spacecraft we send to another planet carries hitchhikers. Even in NASA’s ultraclean Spacecraft Assembly Facilities, certain microbes persist and even thrive. Spores of Bacillus pumilus have been repeatedly isolated from these clean rooms, and some strains show extreme resistance to UV radiation, exceeding any previously observed Bacillus species.21PubMed. Extreme spore UV resistance of Bacillus pumilus isolates obtained from an ultraclean Spacecraft Assembly Facility Other isolates from the same facilities have shown resistance to hydrogen peroxide sterilization, raising concerns about contaminating life-detection missions.22PubMed. Recurrent isolation of hydrogen peroxide-resistant spores of Bacillus pumilus from a spacecraft assembly facility
Simulation experiments have shown that even common human skin microbes like Staphylococcus xylosus, which are not especially tough organisms, can survive desiccation, freezing, Martian vacuum, and short-term UV exposure well enough to remain metabolically active. About 70 percent were killed by desiccation alone, but freezing, vacuum, and brief UV exposure only had minor additional effects.23PubMed. Assessment of the Forward Contamination Risk of Mars by Clean Room Isolates from Space-Craft Assembly Facilities through Aeolian Transport – a Model Study If we find microbial life on Mars, we need to be very sure it did not ride there on one of our own spacecraft.
The concern runs the other direction too. When NASA and the European Space Agency eventually bring Martian rock samples back to Earth, those samples must be treated as potential biohazards. A joint team has developed a three-step protocol for assessing whether returned samples contain microorganisms, whether those organisms could be alive, and whether they pose any risk. The samples would be held in a high-containment facility until safety assessments are complete.24PubMed Central. Mars sample return campaign: biological risk and a proposed sample safety assessment protocol The Earth Return Orbiter mission’s design includes redundant containment to ensure no potentially hazardous Mars particles escape during transit or landing, in compliance with the United Nations Outer Space Treaty.25Journal of Space Safety Engineering. The planetary protection strategy of Mars Sample Return’s Earth Return Orbiter mission The level of planning being invested in backward contamination protection is itself a measure of how seriously scientists take the possibility that Mars is not sterile.
Would Alien Bacteria Even Look Like Earth Bacteria?
All life on Earth uses the same basic molecular toolkit: DNA and RNA for storing and reading genetic information, a standard set of twenty amino acids for building proteins, and a shared genetic code that translates between the two. If life arose independently on another world, there is no reason it would use the same chemistry. Researchers in the field of xenobiology are already exploring what alternative biochemistries might look like. One active line of work involves designing xeno nucleic acids, synthetic molecules that can store genetic information the way DNA does but with a completely different chemical backbone.26PubMed Central. Xenobiology: a new form of life as the ultimate biosafety tool Genetic code engineering has gone further, redesigning genes and proteins with non-standard building blocks to create functional biological systems that share no molecular vocabulary with natural life.27PubMed Central. Xenomicrobiology: a roadmap for genetic code engineering
This work matters for astrobiology because it expands what we consider worth detecting. If we send instruments calibrated only for DNA, RNA, and terrestrial amino acids, we might miss an entirely different form of biology sitting right in front of us. Researchers have argued that alternative chemical scaffolds could support a completely new biological world, potentially as functional and versatile as Earth’s existing biosphere.28PubMed. Anticipating alien cells with alternative genetic codes: away from the alanine world! The practical implication is that future life-detection instruments need to look for general signs of chemical complexity and organization, not just the specific molecules that Earth life happens to use. The absence of DNA in a sample does not mean the absence of life.