Life on Mars

thumb|Mars in true color.

The possibility of life on Mars is a subject of interest in astrobiology due to the planet's proximity and similarities to Earth. To date, no conclusive evidence of past or present life has been found on Mars. Cumulative evidence suggests that during the ancient Noachian time period, the surface environment of Mars had liquid water and may have been habitable for microorganisms, but habitable conditions do not necessarily indicate life. Scientific investigations for potential life on Mars began in the 19th century and continue today with telescopes and robotic probes searching for water, chemical biosignatures in the soil and rocks at the planet's surface, and biomarker gases in the atmosphere.

Mars is of particular interest for the study of the origins of life because of its similarity to the early Earth. This is especially true since Mars has a cold climate and lacks plate tectonics or continental drift, so it has remained almost unchanged since the end of the Hesperian period. At least two-thirds of Mars's surface is more than 3.5 billion years old, and it could have been habitable 4.48 billion years ago, 500 million years before the earliest known Earth lifeforms; Mars may thus hold the best record of the prebiotic conditions leading to life, even if life does not or has never existed there.

Following the confirmation of the past existence of surface liquid water, the Curiosity, Perseverance and Opportunity rovers started searching for evidence of past life, including a past biosphere based on autotrophic, chemotrophic, or chemolithoautotrophic microorganisms, as well as ancient water, including fluvio-lacustrine environments (plains related to ancient rivers or lakes) that may have been habitable. The search for evidence of habitability, fossils, and organic compounds on Mars is now a primary objective for space agencies. The discovery of organic compounds inside sedimentary rocks and of boron on Mars are of interest as they are precursors for prebiotic chemistry. Such findings, along with previous discoveries that liquid water was clearly present on ancient Mars, further supports the possible early habitability of Gale Crater on Mars. Currently, the surface of Mars is bathed with ionizing radiation, and Martian soil is rich in perchlorates toxic to microorganisms. Therefore, the consensus is that if life exists—or existed—on Mars, it could be found or is best preserved in the subsurface, away from present-day harsh surface processes.

In June 2018, NASA announced the detection of seasonal variation of methane levels on Mars. Methane could be produced by microorganisms or by geological means. The European ExoMars Trace Gas Orbiter started mapping the atmospheric methane in April 2018, and the 2022 ExoMars rover Rosalind Franklin was planned to drill and analyze subsurface samples before the programme's indefinite suspension, while the NASA Mars 2020 rover Perseverance, having landed successfully, will cache dozens of drill samples for their potential transport to Earth laboratories in the late 2020s or 2030s. As of 8 February 2021, an updated status of studies considering the possible detection of lifeforms on Venus (via phosphine) and Mars (via methane) was reported. In October 2024, NASA announced that it may be possible for photosynthesis to occur within dusty water ice exposed in the mid-latitude regions of Mars. On 10 September 2025, NASA reported Perseverance found a possible biosignature in a Jezero Crater rock the previous year, hinting at ancient microbial activity.

In 1854, William Whewell, a fellow of Trinity College, Cambridge, theorized that Mars had seas, land and possibly life forms. Speculation about life on Mars exploded in the late 19th century, following telescopic observation by some observers of apparent Martian canals—which were later found to be optical illusions. Despite this, in 1895, American astronomer Percival Lowell published his book Mars, followed by Mars and its Canals in 1906, proposing that the canals were the work of a long-gone civilization. This idea led British writer H. G. Wells to write The War of the Worlds in 1897, telling of an invasion by aliens from Mars who were fleeing the planet's desiccation.

The 1907 book Is Mars Habitable? by British naturalist Alfred Russel Wallace was a reply to, and refutation of, Lowell's Mars and Its Canals. Wallace's book concluded that Mars "is not only uninhabited by intelligent beings such as Mr. Lowell postulates, but is absolutely uninhabitable." Historian Charles H. Smith refers to Wallace's book as one of the first works in the field of astrobiology.

Spectroscopic analysis of Mars's atmosphere began in earnest in 1894, when U.S. astronomer William Wallace Campbell showed that neither water nor oxygen were present in the Martian atmosphere. The influential observer Eugène Antoniadi used the 83-cm (32.6 inch) aperture telescope at Meudon Observatory at the 1909 opposition of Mars and saw no canals; the outstanding photos of Mars taken at the new Baillaud dome at the Pic du Midi observatory also brought formal discredit to the Martian canals theory in 1909, and the notion of canals began to fall out of favor. The two current ecological approaches for predicting the potential habitability of the Martian surface use 19 or 20 environmental factors, with an emphasis on water availability, temperature, the presence of nutrients, an energy source, and protection from solar ultraviolet and galactic cosmic radiation.

Scientists do not know the minimum number of parameters for determination of habitability potential, but they are certain it is greater than one or two of the factors in the table below. There are no full-Mars simulations published yet that include all of the biocidal factors combined.

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! style="align: center; background: lavender;" colspan="2" | Habitability factors Transiently warm conditions related to impacts or volcanism could have produced conditions favoring the formation of the late Noachian valley networks, even though the mid-late Noachian global conditions were probably icy. Local warming of the environment by volcanism and impacts would have been sporadic, but there should have been many events of water flowing at the surface of Mars.

thumb|left|[[List of craters on Mars: A-G|Alga crater is thought to have deposits of impact glass that may have preserved ancient biosignatures, if present during the impact.]]

The loss of the Martian magnetic field strongly affected surface environments through atmospheric loss and increased radiation; this change significantly degraded surface habitability. When there was a magnetic field, the atmosphere would have been protected from erosion by the solar wind, which would ensure the maintenance of a dense atmosphere, necessary for liquid water to exist on the surface of Mars. The loss of the atmosphere was accompanied by decreasing temperatures. Part of the liquid water inventory sublimed and was transported to the poles, while the rest became

trapped in permafrost, a subsurface ice layer.

Soil and rock samples studied in 2013 by NASA's Curiosity rover's onboard instruments brought about additional information on several habitability factors. The rover team identified some of the key chemical ingredients for life in this soil, including sulfur, nitrogen, hydrogen, oxygen, phosphorus and possibly carbon, as well as clay minerals, suggesting a long-ago aqueous environment—perhaps a lake or an ancient streambed—that had neutral acidity and low salinity. The confirmation that liquid water once flowed on Mars, the existence of nutrients, and the previous discovery of a past magnetic field that protected the planet from cosmic and solar radiation, together strongly suggest that Mars could have had the environmental factors to support life. The assessment of past habitability is not in itself evidence that Martian life has ever actually existed. If it did, it was probably microbial, existing communally in fluids or on sediments, either free-living or as biofilms, respectively.

Impactite, shown to preserve signs of life on Earth, was discovered on Mars and could contain signs of ancient life, if life ever existed on the planet.

On June 7, 2018, NASA announced that the Curiosity rover had discovered organic molecules in sedimentary rocks dating to three billion years old. The detection of organic molecules in rocks indicate that some of the building blocks for life were present.

Research into how the conditions for habitability ended is ongoing. On October 7, 2024, NASA announced that the results of the previous three years of sampling onboard Curiosity suggested that based on high carbon-13 and oxygen-18 levels in the regolith, the early Martian atmosphere was less likely than previously thought, to be stable enough to support surface water hospitable to life, with rapid wetting-drying cycles and very high-salinity cryogenic brines providing potential explanations.

Present

Conceivably, if life exists (or existed) on Mars, evidence of life could be found, or is best preserved, in the subsurface, away from present-day harsh surface conditions. Present-day life on Mars, or its biosignatures, could occur kilometers below the surface, or in subsurface geothermal hot spots, or it could occur a few meters below the surface. The permafrost layer on Mars is only a couple of centimeters below the surface, and salty brines can be liquid a few centimeters below that but not far down. Water is close to its boiling point even at the deepest points in the Hellas basin, and so cannot remain liquid for long on the surface of Mars in its present state, except after a sudden release of underground water.

So far, NASA has pursued a "follow the water" strategy on Mars and has not searched for biosignatures for life there directly since the Viking missions. The consensus by astrobiologists is that it may be necessary to access the Martian subsurface to find currently habitable environments.

Cosmic radiation

In 1965, the Mariner 4 probe discovered that Mars had no global magnetic field that would protect the planet from potentially life-threatening cosmic radiation and solar radiation; observations made in the late 1990s by the Mars Global Surveyor confirmed this discovery. Scientists speculate that the lack of magnetic shielding helped the solar wind blow away much of Mars's atmosphere over the course of several billion years. As a result, the planet has been vulnerable to radiation from space for about 4 billion years.

Recent in-situ data from Curiosity rover indicates that ionizing radiation from galactic cosmic rays (GCR) and solar particle events (SPE) may not be a limiting factor in habitability assessments for present-day surface life on Mars. The level of 76 mGy per year measured by Curiosity is similar to levels inside the ISS. <!-- In the 2014 Findings of the Second MEPAG Special Regions Science Analysis Group, their conclusion was: -->

Cumulative effects

Curiosity rover measured ionizing radiation levels of 76 mGy per year. This level of ionizing radiation is sterilizing for dormant life on the surface of Mars. It varies considerably in habitability depending on its orbital eccentricity and the tilt of its axis. If the surface life has been reanimated as recently as 450,000 years ago, then rovers on Mars could find dormant but still viable life at a depth of one meter below the surface, according to an estimate. Even the hardiest cells known could not possibly survive the cosmic radiation near the surface of Mars since Mars lost its protective magnetosphere and atmosphere. After mapping cosmic radiation levels at various depths on Mars, researchers have concluded that over time, any life within the first several meters of the planet's surface would be killed by lethal doses of cosmic radiation. The team calculated that the cumulative damage to DNA and RNA by cosmic radiation would limit retrieving viable dormant cells on Mars to depths greater than 7.5 meters below the planet's surface.

Data collected by the Radiation assessment detector (RAD) instrument on board the Curiosity rover revealed that the absorbed dose measured is 76 mGy/year at the surface, Regardless of the source of Martian organic compounds (meteoric, geological, or biological), its carbon bonds are susceptible to breaking and reconfiguring with surrounding elements by ionizing charged particle radiation.

UV radiation

On UV radiation, a 2014 report concludes that "[T]he Martian UV radiation environment is rapidly lethal to unshielded microbes but can be attenuated by global dust storms and shielded completely by < 1 mm of regolith or by other organisms." In addition, laboratory research published in July 2017 demonstrated that UV irradiated perchlorates cause a 10.8-fold increase in cell death when compared to cells exposed to UV radiation after 60 seconds of exposure. A recent study found that photosynthesis could occur within dusty ice exposed in the Martian mid-latitudes because the overlying dusty ice blocks the harmful ultraviolet radiation at Mars's surface.

Perchlorates

The Martian regolith is known to contain a maximum of 0.5% (w/v) perchlorate (ClO₄⁻) that is toxic for most living organisms, but since they drastically lower the freezing point of water and a few extremophiles can use it as an energy source (see Perchlorates - Biology) and grow at concentrations of up to 30% (w/v) sodium perchlorate by physiologically adapting to increasing perchlorate concentrations, it has prompted speculation of what their influence would be on habitability.

Research published in July 2017 shows that when irradiated with a simulated Martian UV flux, perchlorates become even more lethal to bacteria (bactericide). Even dormant spores lost viability within minutes. The researchers concluded that "the surface of Mars is lethal to vegetative cells and renders much of the surface and near-surface regions uninhabitable." This research demonstrates that the present-day surface is more uninhabitable than previously thought, and reinforces the notion to inspect at least a few meters into the ground to ensure the levels of radiation would be relatively low.

However, researcher Kennda Lynch discovered the first-known instance of a habitat containing perchlorates and perchlorates-reducing bacteria in an analog environment: a paleolake in Pilot Valley, Great Salt Lake Desert, Utah, United States. She has been studying the biosignatures of these microbes, and is hoping that the Mars Perseverance rover will find matching biosignatures at its Jezero Crater site.

Recurrent slope lineae

Recurrent slope lineae (RSL) features form on Sun-facing slopes at times of the year when the local temperatures reach above the melting point for ice. The streaks grow in spring, widen in late summer and then fade away in autumn. This is hard to model in any other way except as involving liquid water in some form, though the streaks themselves are thought to be a secondary effect and not a direct indication of the dampness of the regolith. Although these features are now confirmed to involve liquid water in some form, the water could be either too cold or too salty for life. At present they are treated as potentially habitable, as "Uncertain Regions, to be treated as Special Regions".). They were suspected as involving flowing brines back then.

The thermodynamic availability of water (water activity) strictly limits microbial propagation on Earth, particularly in hypersaline environments, and there are indications that the brine ionic strength is a barrier to the habitability of Mars. Experiments show that high ionic strength, driven to extremes on Mars by the ubiquitous occurrence of divalent ions, "renders these environments uninhabitable despite the presence of biologically available water."

Nitrogen fixation

After carbon, nitrogen is arguably the most important element needed for life. Thus, measurements of nitrate over the range of 0.1% to 5% are required to address the question of its occurrence and distribution. There is nitrogen (as N₂) in the atmosphere at low levels, but this is not adequate to support nitrogen fixation for biological incorporation. Nitrogen in the form of nitrate could be a resource for human exploration both as a nutrient for plant growth and for use in chemical processes. On Earth, nitrates correlate with perchlorates in desert environments, and this may also be true on Mars. Nitrate is expected to be stable on Mars and to have formed by thermal shock from impact or volcanic plume lightning on ancient Mars.

On March 24, 2015, NASA reported that the SAM instrument on the Curiosity rover detected nitrates by heating surface sediments. The nitrogen in nitrate is in a "fixed" state, meaning that it is in an oxidized form that can be used by living organisms. The discovery supports the notion that ancient Mars may have been hospitable for life. It is suspected that all nitrate on Mars is a relic, with no modern contribution. Nitrate abundance ranges from non-detection to 681 ± 304&nbsp;mg/kg in the samples examined until late 2017.

In contrast, phosphate, one of the chemical nutrients thought to be essential for life, is readily available on Mars.

Low pressure

Further complicating estimates of the habitability of the Martian surface is the fact that very little is known about the growth of microorganisms at pressures close to those on the surface of Mars. Some teams determined that some bacteria may be capable of cellular replication down to 25 mbar, but that is still above the atmospheric pressures found on Mars (range 1–14 mbar). In another study, twenty-six strains of bacteria were chosen based on their recovery from spacecraft assembly facilities, and only Serratia liquefaciens strain ATCC 27592 exhibited growth at 7 mbar, 0&nbsp;°C, and CO₂-enriched anoxic atmospheres.

Liquid water

Liquid water is a necessary but not sufficient condition for life as humans know it, as habitability is a function of a multitude of environmental parameters. Liquid water cannot exist on the surface of Mars except at the lowest elevations for minutes or hours. Liquid water does not appear at the surface itself, but it could form in minuscule amounts around dust particles in snow heated by the Sun. Also, the ancient equatorial ice sheets beneath the ground may slowly sublimate or melt, accessible from the surface via caves.