Scientists have long speculated about the possibility of life on Mars owing to the planet's proximity and similarity to Earth. It remains an open question whether life currently exists on Mars, or has existed there in the past. The range of opinions spans a broad spectrum from those who believe that the conditions on Mars make life impossible, to those who accept life on Mars as a demonstrated fact.
Mars' polar ice caps were observed as early as the mid-17th century, and they were first proven to grow and shrink alternately, in the summer and winter of each hemisphere, by William Herschel in the latter part of the 18th century. 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 the increase in speculation that the darker albedo features were water, and brighter ones were land. It was therefore natural to suppose that Mars may be inhabited by some form of life.
In 1854, William Whewell, a fellow of Trinity College, Cambridge, who popularized the word scientist, 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 however soon 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. By 1909 better telescopes and the best perihelic opposition of Mars since 1877 conclusively put an end to the canal theory.
The primary mission of the Viking probes of the mid-1970s was to carry out experiments designed to detect microorganisms in Martian soil. The tests were formulated to look for life similar to that found on Earth. Of the four experiments, only the Labeled Release experiment returned a positive result, showing increased 14CO2 production on first exposure of soil to water and nutrients. All scientists agree on two points from the Viking missions: that radiolabeled 14CO2 was evolved in the labeled release experiment, and that the GC-MS detected no organic molecules. However, there are vastly different interpretations of what those results imply.
One of the designers of the LR experiment, Gilbert Levin, believes his results are a definitive diagnostic for life on Mars. However, this result is disputed by many scientists, who argue that superoxidant chemicals in the soil could have produced this effect without life being present. An almost general consensus discarded the Labeled Release data as evidence of life, because the gas chromatograph & mass spectrometer, designed to identify natural organic matter, did not detect organic molecules. The results of the Viking mission concerning life, are considered by the general expert community, at best, as inconclusive.
Since Mars lost most of its magnetic field about 4 billion years ago, the Martian ionosphere is unable to stop the solar wind or radiation, and it interacts directly with exposed soil, making life, as we know it, impossible to exist. Also, liquid water, necessary for life and for metabolism, cannot exist on the surface of Mars with its present low atmospheric pressure and temperature, except at the lowest shaded elevations for short periods and liquid water never appears at the surface itself.
On 2007, during a Seminar of the Geophysical Laboratory of the Carnegie Institution (Washington, USA), Gilbert Levin's investigation was assessed once more. Levin maintains that his original data were correct, as the positive and negative control experiments were in order.
Ronald Paepe, an edaphologist (soil scientist), communicated to the European Geosciences Union Congress that the discovery of the recent detection of phyllosilicate clays on Mars may indicate pedogenesis, or soil development processes, extended over the entire surface of Mars. Paepe´s interpretation views most of Mars surface as active soil, colored red by eons of widespread wearing by water, vegetation and microbial activity.
A research team from the Salk Institute for Biological Studies headed by Rafael Navarro-González, concluded that the equipment used (TV-GC-MS) by the Viking program to search for organic molecules, may not be sensitive enough to detect low levels of organics. Because of the simplicity of sample handling, TV–GC–MS is still considered the standard method for organic detection on future Mars missions, Navarro-González suggests that the design of future organic instruments for Mars should include other methods of detection.
The claim for life on Mars, in the form of Gillevinia straata, is grounded on old data reinterpreted as sufficient evidence of life, mainly by professors Gilbert Levin, Rafael Navarro-González and Ronalds Paepe. The evidence supporting the existence of Gillevinia straata microorganisms relies on the data collected by the two Mars Viking landers that searched for biosignatures of life, but the analytical results were, officially, inconclusive.
On 2006, Mario Crocco, a neurobiologist at the Hospital Municipal José Tiburcio Borda in Buenos Aires, Argentina with expertise in the state of the water adsorbed and flowing within very slender biophysical interstices (such as between brain cells) and a relative of Gaetano Crocco who in 1956 proposed trajectories for advantageous Mars and Earth-Mars-Venus-Earth flyby missions, proposed the creation of a new nomenclatural rank. It classified these responses as 'metabolic' and, therefore, belonging to a form of life. Crocco proposed to create new biological ranking categories (taxa), in the new kingdom system of life, in order to be able to accommodate the genus of Martian microorganisms. Crocco proposed the following taxonomical entry:
As a result, the Gillevinia straata would not be a bacterium (which rather is a terrestrial taxon) but a member of the kingdom 'Jakobia' in the biosphere 'Marciana' of the 'Solaria' system.
The intended effect of the new nomenclature was to reverse the burden of proof concerning the life issue, but the controversy remains. While the taxonomy proposed by Crocco has not been widely accepted by the scientific community and some even considered it a nomen nudum, Crocco contended that exobiology ought to live with this epistemological condition. Namely, that as biosecurity constraints turn extremely hazardous bringing to our planet any extraterrestrial holotype, i.e. instances to be displayed in a museum, exobiology should follow the path of particle physics (which not either can directly detect its objects) and establish as biological holotypes the data radioed by remote equipment from outside the Earth. Further, in July 2008 the Phoenix mission has corroborated the existence of dirty water ice on Mars' surface, entailing the microscopic source of liquid water capable to act as solvent of Jakobia's metabolism. Phoenix also corroborated the inexistence in the soil of superoxidant chemicals presumably lethal to life.
The NASA maintains a catalog of at least 57 Mars meteorites, which are extremely valuable since these are the only physical samples available of Mars. Speculation has grown as a result that studies show that at least three of them may have evidence of possible past life on Mars. Although the scientific evidence collected is reliable, its interpretation varies. To date, no fatal strikes have been made to any of the original lines of scientific evidence despite several misconstrued press releases.
Over the past few decades, eight criteria have been established for the recognition of past life within terrestrial geologic samples. Those criteria are:
For general acceptance of past life in a geologic sample, essentially most or all of these criteria must be met.ALH84001 meteorite
The ALH84001 meteorite was found on December 1984 on Antarctica by members of the ANSMET project; the meteorite weighs 1.93 kg. The sample was ejected from Mars about 17 million years ago and spent 11,000 years in or on the Antarctic ice sheets. Composition analysis by NASA revealed a kind of magnetite that on Earth, is only found in association with certain microorganisms; Then, in August 2002, another NASA team led by Thomas-Keptra published a study indicating that 25% of the magnetite in ALH 84001 occurs as small, uniform-sized crystals that, on Earth, is associated only with biologic activity, and that the remainder of the material appears to be normal inorganic magnetite. The extraction technique did not permit determination as to whether the possibly biologic magnetite was organized into chains as would be expected. The meteorite displays indication of relatively low temperature secondary mineralization by water and show evidence of preterrestrial aqueous alteration. Evidence of polycyclic aromatic hydrocarbons (PAHs) have been identified with the levels increasing away from the surface.
Some structures resembling the mineralized casts of terrestrial bacteria and their appendages (fibrils) or by-products (extracellular polymeric substances) occur in the rims of carbonate globules and preterrestrial aqueous alteration regions. The size and shape of the objects is consistent with Earthly fossilized nanobacteria, but the existence of nanobacteria itself is controversial.Nakhla Meteorite
The Nakhla meteorite fell on Earth on June 28, 1911 on the locality of Nakhla, Alexandria, Egypt.
In 1998, a team from NASA's Johnson Space Center obtained a small sample for analysis. Researchers found preterrestrial aqueous alteration phases and objects of the size and shape consistent with Earthly fossilized nanobacteria, but the existence of nanobacteria itself is controversial. Analysis with gas chromatography and mass spectrometry (GC-MS) studied its high molecular weight polycyclic aromatic hydrocarbons in 2000, and NASA scientists concluded that as much as 75% of the organic matter in Nakhla "may not be recent terrestrial contamination".
This caused additional interest in this meteorite, so on 2006, NASA managed to obtain an additional and larger sample from the London Natural History Museum. On this second sample, a large dendritic carbon content was observed. When the results and evidence were published on 2006, some independent researchers claimed that the carbon deposits are of biologic origin. However, it was remarked that since carbon is the fourth most aboundant element in the Universe, finding it in curious patterns is not indicative or suggestive of biological origin.Shergotty meteorite The Shergotty meteorite, a 4 kg martian meteorite, fell on Earth on Shergotty, India on August 25, 1865 and was retrieved by witnesses almost immediately. This meteorite is relatively young, calculated to have been formed in Mars only 165 million years ago from volcanic origin. It is composed mostly of pyroxene and thought to have undergone preterrestrial aqueous alteration for several centuries. Certain features in its interior suggest to be remanents of biofilm and their associated microbial communities. Work is in progress on searching for magnetites within alteration phases.
No Mars probe since Viking has tested the Martian regolith directly for signs of life. NASA's recent missions have focused on another question: whether Mars held lakes or oceans of liquid water on its surface in the ancient past. Scientists have found hematite, a mineral that forms in the presence of macroscopic amounts of liquid water. Many scientists have long held this to be almost self-evident based on various geological landforms on the planet, but others have proposed different explanations—wind erosion, oxygen oceans, etc. Thus, the mission of the Mars Exploration Rovers of 2004 was not to look for present or past life, but for evidence of liquid water on the surface of Mars in the planet's ancient past.
In June 2000, evidence for water currently under the surface of Mars was discovered in the form of flood-like gullies. Deep subsurface water deposits near the planet's liquid core might form a present-day habitat for life. However, in March 2006, astronomers announced the discovery of similar gullies on the Moon, which is believed never to have had liquid water on its surface. The astronomers suggest that the gullies could be the result of micrometeorite impacts.
In March 2004, NASA announced that its rover Opportunity had discovered evidence that Mars was, in the ancient past, a wet planet. This had raised hopes that evidence of past life might be found on the planet today.
In December 2006, NASA showed images taken by the Mars Global Surveyor that suggested that water occasionally flows on the surface of Mars. The images did not actually show flowing water. Rather, they showed changes in craters and sediment deposits, providing the strongest evidence yet that water coursed through them as recently as several years ago, and is perhaps doing so even now. Some researchers were skeptical that liquid water was responsible for the surface feature changes seen by the spacecraft. They said other materials such as sand or dust can flow like a liquid and produce similar results.
Recent 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 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.
See also: Methane in the Atmosphere of Mars
Trace amounts of methane in the atmosphere of Mars was discovered. The presence of methane on Mars is very intriguing, since as an unstable gas, it indicates that it must have a source on the planet in order to keep such levels in the atmosphere. It is estimated that Mars must produce 150 ton/year of methane, but asteroid impacts account for only 0.8% of the total methane production. Although geologic sources of methane are possible, the lack of current volcanism, hydrothermal activity or hotspots are not favorable for geologic methane. The existence of life in the form of microorganisms such as methanogens are among possible, but as yet unproven sources.
In February 2005, it was announced that the Planetary Fourier Spectrometer (PFS) on the European Space Agency's Mars Express Orbiter, detected traces of formaldehyde in the atmosphere of Mars. Vittorio Formisano, the director of the PFS, has speculated that the formaldehyde could be the byproduct of the oxidation of methane, and according to him, would provide evidence that Mars is either extremely geologically active, or harbouring colonies of microbial life. NASA scientists consider the preliminary findings are well worth a follow-up, but have also rejected the claims of life.
NASA's current theory is that the patches are composed of either basaltic ash fragments or aggregates of a minor, dark dust component of the layered deposits that forms as a sublimation residue.
On 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.
The Phoenix had great potential for solving this age-old mystery of life on Mars. While it did not find organic matter in the top soil of Mars, it observed dirty water ice on the surface. By entailing the microscopic source of liquid water pointed by Crocco as capable to sustain metabolism, this would remove the major impediment to acceptance of the Viking LR results as evidence for living organisms, and would likely swing the consensus to that conclusion. However, epistemological reasons prevent any finding of organic material to be identified with a finding of life. The Phoenix mission landed a telerobot in the polar region of Mars on May 25, 2008. One of the mission's two primary objectives is to search for a 'habitable zone' in the Martian regolith where microbial life could exist, the other goal being to study the geological history of macroscopic water masses on Mars. The lander has a 2.5 meter robotic arm that is capable of digging a 0.5 meter trench in the regolith. The arm is fitted with an arm camera able to verify that there is material in the scoop when returning samples to the lander for analysis – this overcomes an important design flaw in the Viking landers The craft has a mass spectrometer capable of detecting volatile organic compounds up to 10ppb, an optical microscope and an atomic force microscope. There is an electrochemistry experiment which tells scientists about ions in the regolith and show the amount and type of antioxidants on Mars, if the device works. Phoenix has hitherto found not any strong oxidant, less even one letha to life. Yet NASA scientist Carol Stoker reports that oxidants on Mars vary with latitude, noting that Viking 2 saw fewer oxidants than Viking 1 because of its more northerly position. Phoenix has landed further north still. Based on rates of sedimentation at the Phoenix landing site, it is hoped that the probe may sample layers that date back at least 50,000 years, and maybe up to a million years. This is important because the climate of Mars has been much warmer in the past and any life could have been more active and widespread, says Stoker. Unlike the Mars Pathfinder Sojourner rover and the Mars Exploration Rovers, which used airbag-cushioned capsules to land on Mars, the Phoenix lander landed using retro-rockets, as the Viking landers did, despite claims that rocket exhaust may have contaminated the Viking landing sites.