Life on Mars

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Scientists, philosophers and theologians have long speculated about the possibility of life on Mars, due to that planet's proximity and similarity to Earth.

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. They could observe ice caps at the north and south poles of Mars, which grew and shrank with the seasons.

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, theorizes that Mars has 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 are 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 are fleeing that planetÃÂs desiccation.

Better telescope imagery, and especially the photos taken by the Mariner 4 probe in 1965 showed a dry 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.

In recent years speculation has grown again, however ÃÂ 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. [1] 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 Infrared telescope on Hawaii and the Gemini South observatory in Chile in 2003. As methane could not 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 this methane.

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NASA look at knowledge about Mars and its influence on science and culture

Life on Mars? is also the title of the fourth track of David Bowie's album release, Hunky Dory, albeit the song is about a girl visiting a cinema after an argument with her parents; the final sentence "is there life on Mars?" alludes to an otherwise unmentioned film which presumably poses the same question.