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Scientists have long speculated about the possibility of life on Mars, due to that planet's proximity and similarity to Earth. It remains an open question whether life exists on Mars now, or existed there in the past.

History of the debate

Of all the planets of the Solar system (other than Earth), Mars was the first one whose solid surface was observed with certainty, and its physical features determined with any accuracy.

The most obvious peculiarity of its surface -- its polar ice-caps -- were seen in the mid-17th century, but they were first proved 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 similarities to Earth. They knew that the length of a day on Mars was almost the same as a day on Earth, and 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 facts gave the impulse to the idea of Mars as a true earth on a smaller scale, which the recognition of darker albedo features as water, and brighter ones as land, further increased. It was therefore natural to suppose that it must be inhabited, and that we should some day obtain evidence of the fact.

Speculation about life on Mars exploded in the late 19th century, following telescopic observation of apparent canals — which were later found to be optical illusions. In 1854, William Whewell, a fellow of Trinity College, Cambridge University who popularized the word scientist, theorized that Mars had seas, land and possibly life forms. 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.

Better telescope imagery, and especially the photos taken by the Mariner 4 probe in 1965 showed an arid Mars without rivers, oceans or visible plants. Intense UV radiation made the planet extremely hostile to life. Although the Viking lander's tests for microbes in 1976 were inconclusive, most scientists hold that their findings can be explained on the basis of chemical reactions alone.

Modern findings

In recent years speculation has grown again, however – prodded by a study of the ALH84001 meteorite which concluded that it contained fossilized microbes. Other scientists have subsequently sought to explain these findings on the basis of chemical processes and they remain controversial within the scientific community.

Another glimmer of hope for past and present life on Mars has been revealed with the ongoing research into extremophiles on Earth which survive under the harshest conditions. Evidence for present water under the surface of Mars has been discovered in the form of flood-like gullies in June 2000. Deep subsurface water deposits near the planet's liquid core might form a present-day habitat for life. The Mars Express probe carries a subsurface radar that will test for the existence of water or ice in the upper crust of 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. 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, carbon dioxide oceans, etc. Thus, the mission of the Mars Exploration Rovers of 2004 was not to look for life (not even in the form of fossils), but for evidence of liquid water on the surface of Mars in the planet's ancient past.

In March 2004, NASA announced that its rover Opportunity had discovered evidence that Mars was, in the ancient past, a wet planet. This has raised hopes that evidence of past life might be found on the planet today. Later that same month, the orbiting ESA probe Mars Express confirmed the presence of methane in the martian atmosphere, which had earlier been suggested by observations of the UKIRT Infrared telescope on Hawaii and the Gemini South observatory in Chile in 2003. As methane cannot persist in the martian atmosphere for more than a few hundred years, this suggests that either Mars has recently been volcanically active, or that some kind of extremophile life form similar to some present on Earth is metabolising carbon dioxide and hydrogen and producing methane. A NASA scientist has also said that there are no known ways for ammonia to be present in the Martian atmosphere that do not involve life .

In January 2005, two NASA scientists reported that they had found strong evidence of present life on Mars (Berger, 2005). The two scientists, Carol Stoker and Larry Lemke of NASA’s Ames Research Center, based their claims on methane signatures found in Mars’ atmosphere that resemble the methane production of some forms of primitive life on Earth, as well as their own study of primitive life near the Rio Tinto river in Spain. NASA officials soon denied the scientists’ claims, and Stoker herself backed off from her initial assertations (www.spacetoday.net, 2005). However, only a few days after Stoker and Lemke made their claims, scientists from the European Space Agency reported that their own measurements of methane on Mars suggested an organic origin (Michelson, 2005).

Though such findings are still very much in debate, support among scientists for the belief in the existence of life on Mars seems to be growing. In an informal survey of scientists attending the conference at which the European Space Agency presented its findings, 75 percent of the scientists at the conference reported to believe that life once existed on Mars; 25 percent reported a belief that life currently exists there (Michelson, 2005).

Fringe viewpoints

Among the more extreme beliefs held in mainstream academia is that of Dr. Courtney Brown, an associate professor of political science at Emory University, who has written two books saying an ancient race of humanoid Martians are living below the surface of Mars. He believes they survived a catastrophic natural disaster on their planet eons ago, destroying their atmosphere. Dr. Brown bases his conclusions on data he supposedly gathered by remote viewing.

See also

• Face on Mars

External links

• NASA - The Mars Exploration Program
• Arthur C. Clarke sees life on Mars
• Scientists have discovered that Mars once had saltwater oceans
• BBC News: Methane on Mars could signal life
• BBC News: Ammonia on Mars could mean life
• Scientists say that life on Mars is likely today
• Ancient salty sea on Mars wins as the most important scientific achievement of 2004 - Journal Science
• Mars meteor found on Earth proves microbial life once existed on Mars

See also: the song by David Bowie: Life on Mars?

References

• Wallace, Alfred Russel. Is Mars habitable? A critical examination of Professor Percival Lowell's book "Mars and its canals," with an alternative explanation, by Alfred Russel Wallace, F.R.S., etc. London, Macmillan and co., 1907.
• Berger, Brian (2005). Exclusive: NASA Researchers Claim Evidence of Present Life on Mars. Posted Feb. 16, 2005.
• spacetoday.net (2005). NASA denies Mars life reports. Posted Feb 19, 2005.
• Michelson, Marcel (2005). European Scientists Believe in Life on Mars. Posted Feb 25, 2005.

• Astrobiology
• Mars