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 canals — which were however soon found to be optical illusions. Despite this, in 1895, Australian 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.
 Mariner 4
The photographs taken by the Mariner 4 probe in 1965, showed an arid Mars without rivers, oceans or any signs of life. Further it revealed that the surface (at least the parts that it photographed) was covered in craters, indicating a lack of plate tectonics and weathering of any kind for the last 4 billion years. The probe also found that Mars had no protective magnetic field, meaning that intense UV radiation makes the planet extremely hostile to life as we know it. The probe was also able to calculate the atmospheric pressure on the planet to be between 4 and 7 millibars, meaning that liquid water could not exist on the planet's surface. After Mariner 4, the search for life on Mars changed to a search for bacteria-like living organisms rather than for multicellular organisms, as the environment was clearly too harsh for these.
 Viking experiments
Main article: Viking biological experiments
The primary mission of the Viking probes of the mid-1970s was to carry out experiments designed to detect microorganisms in Martian soil. The big difficulty of this mission was that NASA's knowledge about conditions on Mars' surface was limited to the data returned by Mariner 4, and so the tests were formulated to look for life similar to the life found on Earth. Nevertheless, of the four experiments carried out, the labeled release experiment returned a frightful result showing increased CO2 production on exposure to water and nutrients. However this sign of life was disputed by many scientists, who argued that superoxidant chemicals in the soil could have produced this effect without life being present. To counter this it has been argued that the labeled release experiment detected that there were so few metabolising organisms in the martian soil that it would have been impossible for the gas chromatograph to detect them. This view is put forward by one of the designers of the LR experiment, Gilbert Levin, who believes the results of the Viking landers are diagnostic for life on Mars.
A re-analysis of the now 30 year old Viking data in the light of modern knowledge of extremophile forms of life has suggested that the Viking tests were not sophisticated enough to detect these forms of life, and may even have killed it in the testing procedure. The central idea here is that instead of being destroyed by Mars' high levels of hydrogen peroxide and other oxidants, life on Mars may use these chemicals to help them survive. For example hydrogen peroxide would stop water in a cell from freezing down to −50 °C and is hygroscopic, a useful trait on such a dry planet. The researchers cite Acetobacter peroxidans as a known example of a microbe that uses hydrogen peroxide in its metabolism.
There are some that now argue that, if there was life at the Viking lander sites, it may have been killed by the exhaust from the landing rockets.
 Modern findings
A series of artist's conceptions of hypothetical past water coverage on Mars.
A series of artist's conceptions of hypothetical past water coverage on Mars.
Observations made in the late 1990s by the Mars Global Surveyor confirmed the suspicion that Mars, unlike Earth, no longer possessed a substantial global magnetic field, thus allowing potentially life-threatening cosmic radiation to reach the planet's surface. Scientists also speculate that the lack of shielding due to Mars's diminished global magnetic field helped the solar wind blow away much of Mars's atmosphere over the course of several billion years.
In recent years speculation has grown as a result of studying the ALH84001 Mars meteorite which concluded that it contained fossilized microbes. Other scientists have subsequently sought to explain these findings on the basis of chemical processes. Both remain highly controversial within the scientific community. Other Mars meteorites such as the Nakhla meteorite were suggested to have evidence of life also but were later shown to contain no suggestion of evidence.
Another suggestion of evidence for extremophiles on Mars comes from analysis of satellite images. It has been proposed that there is a biological origin for the annual appearance and disappearance of dark dune spots near the polar regions of Mars.
Haloarchaea have been proposed as a kind of life that could live on Mars; because the Martian atmosphere has a pressure below the triple point of water, freshwater species would have no habitat..
 Liquid water on Mars
No Mars probe since Viking has tested the Martian soil 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 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.
Possible evidence of water flowing on Mars
Possible evidence of water flowing on Mars
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. The findings were published in the December 8, 2006 issue of the journal Science.
 Methane on Mars
As methane cannot persist in the Martian atmosphere for more than a few hundred years, its presence suggests either that it is being replenished by some unidentified volcanic or geologic process, or that some kind of extremophile life form similar to some existing on Earth is metabolising carbon dioxide and hydrogen and producing methane.
In March 2004, the orbiting ESA probe Mars Express reported detecting methane in the Martian atmosphere, which had earlier been suggested by observations of the United Kingdom Infrared Telescope on Hawaii and the Gemini South observatory in Chile in 2003.
Others have proposed that a process called serpentinization, wherein the mineral olivine is converted into serpentine in the presence of liquid water, may be occurring somewhere in the subsurface of Mars and releasing enough methane to explain the observations.
 Formaldehyde on Mars
In February 2005, it was announced that the Planetary Fourier Spectrometer (PFS) on the European Space Agency's Mars Express Orbiter detected substantially more formaldehyde than anyone had reasonably expected, strongly pointing to other explanations such as microbial life. This claim continues to be widely debated in the scientific community. Scientists skeptical to the measurements say that the data from the PFS has been misinterpreted.
 Ammonia on Mars
In the Martian atmosphere ammonia would be unstable and only last for a few hours. In fact a NASA scientist has said "There are no known ways for ammonia to be present in the Martian atmosphere that do not involve life". For this reason, the detection of ammonia would be extremely important for the debate of whether there is life on Mars.
In July 2004 rumors began to circulate that Vittorio Formisano, the scientist in charge of the Planetary Fourier Spectrometer (PFS), would announce their discovery of ammonia at an upcoming conference. It later came to light that none had been found; in fact some noted that the PFS was not precise enough to distinguish ammonia from carbon dioxide anyway.
 Phoenix lander, 2008
The Phoenix mission will land a telerobot in the polar region of Mars in May 2008. One of the mission's two primary objectives is to look for a 'habitable zone' in the martian soil where microbial life could exist, the other mission being to study the geological history of water on Mars. The lander will have a 2.5 meter robotic arm that is capable of digging a 0.5 meter trench in the soil. The arm is fitted with an arm camera that will be able to verify that there is soil in the scoop when returning soil 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 organic volatiles up to 10ppb, an optical microscope and an atomic force microscope. There is an electrochemistry experiment which will tell scientists about ions in the soil and show the amount and type of antioxidants on Mars, if the device works. NASA scientist Carol Stoker reports that oxidants on Mars vary with latitude, noting that Viking 2 saw less oxidants than Viking 1 because of its more northerly position. Phoenix will land further north still . Rates of sedimentation at the Phoenix landing site are hoped to allow the probe to 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 will land the same way that the Viking landers did, despite the claims that rocket exhaust may have contaminated the Viking landing sites enjoy liveleak 3v1ld34d
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