By Roger Highfield, Science Editor
Scientists now believe that it is likely that life once thrived on Mars and may still exist under its surface, a seminar was told [on Jan 29].
Earlier this month, a meeting of the American Astronomical Society stirred debate about possible extraterrestrial life with the discovery of new planets outside the Solar system. Now, however, it seems increasingly likely that alien life might be on our doorstep.
"Most of us here believe that there is a significant probability that there was life on Mars," Prof. Malcolm Walter of Macquarie University, Australia, told an international meeting organised by the charitable Ciba Foundation in London. "If we didn't believe it, we would not be doing what we are doing."
This year sees the 20th anniversary of the Viking mission to Mars. It failed to find evidence of life and subsequent meetings of "exobiologists" came to the disappointing conclusion that conditions on the surface of the Red Planet are not conducive to life -- certainly not life as we know it.
Now, however, NASA is planning Mars missions that will hunt for fossil evidence. One issue already under discussion is the risk of contaminating Earth if one of these missions returned with samples containing spores or microbes.
The belief that life might exist on Mars has been supported by studies of microbes thriving in extreme conditions on Earth. Last October, the Pacific Northwest Laboratory in Richland found primitive bacteria tough enough to survive on the Red Planet.
Called hyperthermophiles, these organisms can live without oxygen or light in temperatures between 85 deg C and 113 deg C, said Professor Karl Stetter of the University of Regensburg.
The microbes derive their nutrition from water and rock.
Mars-like conditions have been found deep under north Alaska, said Professor Stetter. Although the soil is frozen to a depth of 400 metres, it is sufficiently warm at even greater depths to support huge communities of hyperthermophiles.
Scientists believe that conditions for life could have emerged on Mars between three billion and four billion years ago, when the planet was warmer and wetter.
When the atmosphere of the planet was lost, more than two billion years ago, Mars cooled and its surface dried to leave valleys, channels and polar ice caps.
"The idea is that life would have taken a dive under the surface, tracking the habitable zone of liquid water," said Dr. Jack Farmer, of NASA's Ames Research Centre.
To test this theory, in December the first in a series of missions will be launched to study a site where rapid mineral precipitation could have entombed Martian organisms.
In July 1997 a probe will enter the thin Martian atmosphere and land on the planet's cold surface, which is below freezing point. The lander will release a tethered robot which will roam around and analyse the composition of the terrain. However, conclusive evidence of traces of life might have to wait until 2005, when a mission will attempt to return samples to Earth.
Professor Paul Davies, of the University of Adelaide, proposes a further theory. He says the impact of comets could have splashed dust and rock into space, transferring life forms from Earth to Mars -- or vice versa. "We estimate that 500 tons a year of Martian material lands on Earth... this obviously complicates the issue of life on Mars," he said. In 1911, for example, an Egyptian dog was struck and killed by "a chunk of Mars."
"The discovery of life beyond Earth would transform not only our science but also religions and our entire world view," Davies said.
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