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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Biosignature | 4/6 | https://en.wikipedia.org/wiki/Biosignature | reference | science, encyclopedia | 2026-05-05T13:15:42.723741+00:00 | kb-cron |
===== Ozone in the Martian Atmosphere ===== Mars has traces of ozone (including a seasonal ozone layer over the south pole in winter) and oxygen in its atmosphere both byproducts of life on Earth but explained by photochemistry on Mars. Mariner 7 detected ozone in 1971 and in 1976 by the Viking biological experiments. Significant levels of oxygen were detected in Gale Crater by Curiosity Rover in 2019 with seasonal variability that has not fully been explained. Studies indicate that the Martian atmosphere was once oxygen-rich. Today they are no longer considered valid biosignatures and are proposed to be the result of photodissociation of carbon dioxide.
===== Methane in the Martian Atmosphere ===== Martian methane is an area of ongoing research. With life being the strongest source of methane on Earth, continued observation of such a disequilibrium could be a viable biosignature. Current photochemical models cannot explain the reported rapid variations in space and time. Neither its fast appearance nor disappearance have been explained. Because of its tendency to be destroyed in the atmosphere by photochemistry, excess methane could indicate that there must be an active source. Since 2004 there have been several detection claims of methane in the Mars atmosphere by a variety of instruments onboard orbiters and ground-based landers on the Martian surface as well as Earth-based telescopes. However 2019 measurements put an upper bound on the overall methane abundance at 0.05 p.p.b.v contradicting previous observations. The Curiosity rover's Tunable Laser Spectrometer (TLS) has detected methane on the Martian surface. However, this data is inconclusive due to methane leaks in the TLS that have most likely contaminated the methane readings from the surface of Mars. To rule out a biogenic origin for the methane, a future probe or lander hosting a mass spectrometer will be needed first to prove its presence, and second, to use the isotopic proportions of carbon-12 to carbon-14 in methane to distinguish between a biogenic and non-biogenic origin, similarly to the use of the δ13C standard for recognizing biogenic methane on Earth.
===== CO and H2 in Martian atmosphere ===== The Martian atmosphere contains high abundances of photochemically produced CO and H2, which are reducing molecules. Mars' atmosphere is otherwise mostly oxidizing, leading to a source of untapped energy that life could exploit if it used by a metabolism compatible with one or both of these reducing molecules. Because these molecules can be observed, scientists use this as evidence for an antibiosignature. Scientists have used this concept as an argument against life on Mars.
==== Possible Bioorganic Chemistry on Mars ====
Organic chemistry has been discovered on Mars, some of which can be explained by geochemical processes. Chlorobenzene (C6H5Cl), for example, was detected as early as the Viking lander biological experiments and later in sedimentary rocks by Curiosity likely from perchlorate reactions with organic matter. Some abiotic source, such as a Fischer–Tropsch process, could also have produced alkanes. Some discoveries have been found in areas confirmed previously be wet, adding weight to their significance. In 2018 at Gale Crater, Curiosity discovered Thiophene (C4H4S) and polymers (Polythiophene). Natural sulfur reduction has been proposed as a possible abiotic source. Dimethyl sulfide (CH2S) was also detected. In Cheyava Falls discovered by Perseverance in July 2024, organic matter was detected. Also found were millimeter-sized splotches resembling "leopard spots" containing iron and phosphate, elements often associated with microbial life. In 2025, analysis of rocks from Gale Crater by SAM found decane (C10H22), dodecane (C12H26) and undecane (CH3(CH2)9CH3), collectively known as fatty acids, which terrestrial cell membranes are made of. However these have formed on meteorites, which may have delivered them to Mars. An analysis of 2020 data from Mary Anning3 (MA3) from SAM TMAH found evidence for more than 20 complex molecules including Nitrogen Heterocyclic compounds considered precursors to RNA and DNA along with many other volatile compounds. Molecules detected included Trimethylbenzene, Tetramethylbenzene, Methyl benzoate, Dihydronaphthalene, Naphthalene, Benzothiophene and Methylnaphthalene. While there is currently no abiotic explanation for the complex organics found, exogenous sources must first be ruled out.
==== Mars Missions ====
===== The Viking missions to Mars =====
The Viking Landers (1976) performed the first in situ biological experiments on Mars, testing for metabolic activity using gas-exchange and labeled-release assays. Although some experiments produced responses initially interpreted as possible metabolic signatures, the absence of organic molecules in Viking’s gas chromatograph–mass spectrometer led to widespread disagreement over the biological interpretation. The results remain inconclusive, and the experiments are often cited as an example of the need for multiple, independent lines of evidence when evaluating biosignatures. The Viking findings also highlighted the importance of characterizing the inorganic chemistry of the environment, as biosignatures cannot be reliably interpreted without understanding the abiotic context in which they may occur.
===== Mars Science Laboratory =====
The Curiosity rover from the Mars Science Laboratory mission, with its Curiosity rover is currently assessing the potential past and present habitability of the Martian environment and is attempting to detect biosignatures on the surface of Mars. Considering the MSL instrument payload package, the following classes of biosignatures are within the MSL detection window: organism morphologies (cells, body fossils, casts), biofabrics (including microbial mats), diagnostic organic molecules, isotopic signatures, evidence of biomineralization and bioalteration, spatial patterns in chemistry, and biogenic gases. The Curiosity rover targets outcrops to maximize the probability of detecting 'fossilized' organic matter preserved in sedimentary deposits.