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.

The photographs taken by the Mariner 4 probe fly-by 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 has no global magnetic field that would protect the planet from potentially life-threatening cosmic rays. The probe was also able to calculate the atmospheric pressure on the planet to be about 0.6 kPa (compared to Earth's 101.3 kPa), 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.

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.

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.

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.

In May 2007, the Spirit rover disturbed a patch of ground with its inoperative wheel, uncovering an area extremely rich in silica (90%). The feature is reminiscent of the effect of hot spring water or steam coming into contact with volcanic rocks. Scientists consider this as evidence of a past environment that may have been favorable for microbial life, and theorize that one possible origin for the silica must have been produced by the interaction of soil with acid vapors produced by volcanic activity in the presence of water. Another could have been from water in a hot spring environment.

See also: Terraforming

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