Biosignature

[1][2][3] Measurable attributes of life include its physical or chemical structures, its use of free energy, and the production of biomass and wastes.

One of the theories on the origin of life involves molecules developing the ability to catalyse geochemical reactions to exploit the energy being released by them.

Oxygen is also readily detectable in spectra, with multiple bands across a relatively wide wavelength range, therefore, it makes a very good biosignature.

One possibility is that oxygen can build up abiotically via photolysis if there is a low inventory of non-condensable gasses or if the planet loses a lot of water.

[15][16][17] Finding and distinguishing a biosignature from its potential false-positive mechanisms is one of the most complicated parts of testing for viability because it relies on human ingenuity to break an abiotic-biological degeneracy, if nature allows.

Some related disciplines such as geochemistry, geobiology, and geomicrobiology often use biosignatures to determine if living organisms are or were present in a sample.

Another example is the presence of straight-chain lipids in the form of alkanes, alcohols, and fatty acids with 20–36 carbon atoms in soils or sediments.

Another possible biosignature might be morphology since the shape and size of certain objects may potentially indicate the presence of past or present life.

[29] The possible biomineral studied in the Martian ALH84001 meteorite includes putative microbial fossils, tiny rock-like structures whose shape was a potential biosignature because it resembled known bacteria.

[30] A consensus that has emerged from these discussions, and is now seen as a critical requirement, is the demand for further lines of evidence in addition to any morphological data that supports such extraordinary claims.

[34] For example, membrane lipids left behind by degraded cells will be concentrated, have a limited size range, and comprise an even number of carbons.

[39] The abundance of petroleum biomarkers in small amounts in its reservoir or source rock make it necessary to use sensitive and differential approaches to analyze the presence of those compounds.

[42] The ratio of pristane to phytane (pr:ph) is the geochemical factor that allows petroleum biomarkers to be successful indicators of their depositional environments.

[43] Geologists and geochemists use biomarker traces found in crude oils and their related source rock to unravel the stratigraphic origin and migration patterns of presently existing petroleum deposits.

[51][52][53][54][55] Scientists use the Earth as an example of this when looked at from far away (see Pale Blue Dot) as a comparison to worlds observed outside of our solar system.

[56] Ultraviolet radiation on life forms could also induce biofluorescence in visible wavelengths that may be detected by the new generation of space observatories under development.

In 2002, Steven A. Benner and Daniel Hutter proposed that for a linear genetic biopolymer dissolved in water, such as DNA, to undergo Darwinian evolution anywhere in the universe, it must be a polyelectrolyte, a polymer containing repeating ionic charges.

[65][66] Since 2004, there have been several detections 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.

[80] However, recent measurements using the ACS and NOMAD instruments on board the ESA-Roscosmos ExoMars Trace Gas Orbiter have failed to detect any methane over a range of latitudes and longitudes on both Martian hemispheres.

[85] This nondetection is a major contradiction to what was previously observed with less sensitive instruments and will remain a strong argument in the ongoing debate over the presence of methane in the Martian atmosphere.

Furthermore, current photochemical models cannot explain the presence of methane in the atmosphere of Mars and its reported rapid variations in space and time.

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.

[3] 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.

It delivered the Schiaparelli EDM lander and then began to settle into its science orbit to map the sources of methane on Mars and other gases, and in doing so, will help select the landing site for the Rosalind Franklin rover to be launched in 2022.

NASA's Dragonfly[100] lander/aircraft concept is proposed to launch in 2025 and would seek evidence of biosignatures on the organic-rich surface and atmosphere of Titan, as well as study its possible prebiotic primordial soup.

[103][104] In addition, scientists believe that Titan may have the conditions necessary to promote prebiotic chemistry, making it a prime candidate for biosignature discovery.

Europa is one of the best candidates for biosignature discovery in the Solar System because of the scientific consensus that it retains a subsurface ocean, with two to three times the volume of water on Earth.

In addition, the instruments will be used to map and study surface features that may indicate tectonic activity due to a subsurface ocean.

[116][117] In 2014, more evidence was presented using gravimetric measurements on Enceladus to conclude that there is in fact a large reservoir of water underneath an icy surface.

In addition, their large collecting area will enable high angular resolution, making direct imaging studies more feasible.

False positive mechanisms for oxygen on a variety of planet scenarios. The molecules in each large rectangle represent the main contributors to a spectrum of the planet's atmosphere. The molecules circled in yellow represent the molecules that would help confirm a false positive biosignature if they were detected. Furthermore, the molecules crossed out in red would help confirm a false positive biosignature if they were not detected. Cartoon adapted from Victoria Meadows ' 2018 oxygen as a biosignature study. [ 10 ]
Electron micrograph of microfossils from a sediment core obtained by the Deep Sea Drilling Program
Some researchers suggested that these microscopic structures on the Martian ALH84001 meteorite could be fossilized bacteria. [ 25 ] [ 26 ]
Structures of prime examples of biomarkers (petroleum), from top to bottom: Pristane, Triterpane, Sterane, Phytane and Porphyrin
Biogenic methane production is the main contributor to the methane flux coming from the surface of Earth. Methane has a photochemical sink in the atmosphere but will build up if the flux is high enough. If there is detectable methane in the atmosphere of another planet, especially with a host star of G or K type, this may be interpreted as a viable biosignature. [ 64 ]
Methane (CH 4 ) on Mars - potential sources and sinks.
Europa Clipper
An image of the plumes of water and ice coming from the surface of Enceladus. Future missions will investigate these geysers to determine the composition and look for signs of life.