The possibility of life on Mars is a subject of interest in astrobiology due to its proximity and similarities to Earth. To date, no proof 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, the existence of habitable conditions does not necessarily indicate the presence of life.
Scientific searches for evidence of life
began in the 19th century and continue today via telescopic investigations and
deployed probes. While early work focused on phenomenology and bordered on
fantasy, the modern scientific inquiry has emphasized the search 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 so since Mars has a cold climate and lacks plate tectonics or continental
drift, so has remained almost unchanged since the end of the Hesperian period.
At least two thirds of Mars' surface is more than 3.5 billion years old,
and Mars may thus hold the best record of the prebiotic conditions leading to
life, even if life does not or has never existed there, which might have
started developing as early as 4.48 billion years ago.
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, taphonomy
(related to fossils), and organic compounds on Mars is now a primary NASA and
ESA objective.
The findings 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 will drill and analyze subsurface samples, 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 February 8, 2021, an updated status of studies
considering the possible detection of lifeforms on Venus (via of phosphine) and
Mars (via methane) was reported.
Early Speculation
Mars' polar ice caps were discovered in
the mid-17th century. In the late 18th
century, William Herschel proved they grow and shrink alternately, in the
summer and winter of each hemisphere. By the mid-19th century, astronomers knew
that Mars had certain other similarities to Earth, for example that the length
of a day on Mars was almost the same as a day on Earth. They also knew that its
axial tilt was similar to Earth's, which meant it experienced seasons just as
Earth does—but of nearly double the length owing to its much longer year. These observations led to increase in
speculation that the darker albedo features were water and the brighter ones
were land, whence followed speculation on whether Mars may be inhabited by some
form of life.
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.
Spectroscopic analysis of Mars'
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.
Potential for Habitability
Chemical, physical, geological, and
geographic attributes shape the environments on Mars. Isolated measurements of
these factors may be insufficient to deem an environment habitable, but the sum
of measurements can help predict locations with greater or lesser habitability
potential. 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. Similarly, for each group of parameters, the
habitability threshold for each is to be determined. Laboratory simulations show that whenever
multiple lethal factors are combined, the survival rates plummet quickly. There are no full-Mars simulations published
yet that include all of the biocidal factors combined. Furthermore, the possibility of Martian life
having a far different biochemistry and habitability requirements than the terrestrial
biosphere is an open question.
Liquid Water on Mars
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.
Water on Mars exists almost exclusively
as water ice, located in the Martian polar ice caps and under the shallow
Martian surface even at more temperate latitudes. A small amount of water vapor is present in
the atmosphere. There are no bodies of
liquid water on the Martian surface because its atmospheric pressure at the
surface averages 600 pascals (0.087 psi)—about 0.6% of Earth's mean sea
level pressure—and because the temperature is far too low, (210 K
(−63 °C)) leading to immediate freezing. Despite this, about
3.8 billion years ago, there was a denser atmosphere, higher temperature,
and vast amounts of liquid water flowed on the surface, including large oceans.
It has been estimated that the
primordial oceans on Mars would have covered between 36% and 75% of the planet. On November 22, 2016, NASA reported finding a
large amount of underground ice in the Utopia Planitia region of Mars. The
volume of water detected has been estimated to be equivalent to the volume of
water in Lake Superior. Analysis of
Martian sandstones, using data obtained from orbital spectrometry, suggests
that the waters that previously existed on the surface of Mars would have had
too high a salinity to support most Earth-like life. Tosca et al. found
that the Martian water in the locations they studied all had water activity, aw
≤ 0.78 to 0.86—a level fatal to most Terrestrial life. Haloarchaea, however, are able to live in hypersaline
solutions, up to the saturation point.
In June 2000, possible evidence for
current liquid water flowing at the surface of Mars was discovered in the form
of flood-like gullies. Additional
similar images were published in 2006, taken by the Mars Global Surveyor, that
suggested that water occasionally flows on the surface of Mars. The images
showed changes in steep crater walls and sediment deposits, providing the
strongest evidence yet that water coursed through them as recently as several
years ago.
There is disagreement in the scientific
community as to whether or not the recent gully streaks were formed by liquid
water. Some suggest the flows were merely dry sand flows. Others suggest it may be liquid brine near
the surface, but the exact source of the water and the mechanism behind its
motion are not understood.
In July 2018, scientists reported the
discovery of a subglacial lake on Mars, 1.5 km (0.93 mi) below the southern
polar ice cap, and extending sideways about 20 km (12 mi), the first
known stable body of water on the planet.
The lake was discovered using the MARSIS radar on board the Mars
Express orbiter, and the profiles were collected between May 2012 and
December 2015. The lake is centered at
193°E, 81°S, a flat area that does not exhibit any peculiar topographic
characteristics but is surrounded by higher ground, except on its eastern side,
where there is a depression.
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