Miércoles 14 de Diciembre de 2005, Ip nº 139

Extreme bugs back idea of life on Mars
Por Kelly Young and David L Chandler

Methane-producing microbes have been discovered in two extreme environments on Earth - buried under kilometres of ice in Greenland and living in hot, dry desert soil. The findings lend weight to the idea that similar organisms may have lived on Mars.

Live microbes making methane were found in a glacial ice core sample retrieved from three kilometres under Greenland by researchers from the University of California, Berkeley, US. It is the first time such archaea have been found at that depth, says Buford Price, one of the research team, which published its results in the Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.0507601102).

Scientists had already noticed that the concentrations of methane in the lowest 90 metres of the ice core was 10 times as high as that at other depths. Now the Berkeley scientists have found the likely cause - correspondingly higher levels of microbes that produce methane, known as methanogens.

Areas of high methane concentration in the Martian atmosphere have been spotted by Europe’s Mars Express spacecraft, but its origin is uncertain. A renewable source of methane would be needed, as otherwise ultraviolet light from the Sun would have destroyed it within 340 years. The methane could come from a geological source, such as unseen volcanic activity, or biological sources such as methanogens.

Poor-man’s borehole
Price’s group used the data from Greenland to devise a scenario on Mars, in order to guide future missions. The methanogens in the ice cores existed at -10°C, but could produce more methane in warmer conditions. In order to account for the methane seen on Mars, they calculated the bugs would have to live at 0°C or above.

This temperature is likely to occur between 150 metres and 8 kilometres beneath the Martian surface, depending on the rock type. It would then take between 15 years and 30,000 years for that methane to percolate up to the surface.

“In my opinion, there’s no way in my lifetime that NASA will find a way of drilling a borehole 150 metres deep [on Mars],” Price told New Scientist. “But the poor-man’s borehole is just looking at what’s been thrown out of a crater.”

So a spacecraft could be sent to a large crater where higher levels of atmospheric methane have been detected. The lander could then drill a shorter distance down into the crater to try to find evidence of life.

Desert dwellers
Another new study, reported in the journal Icarus (vol 178, p 277), has also discovered methanogens in a harsh environment on Earth. Researchers studied dozens of soil and vapour samples from five different desert environments in Utah, Idaho and California in the US, and in Canada and Chile.

Of these, five soil samples and three vapour samples from the vicinity of the Mars Desert Research Station in Utah were found to have signs of viable methanogens. The methane in the vapour samples was 300 times higher than background levels.

One of the team, Timothy Kral of the University of Arkansas, US, told New Scientist that dry conditions usually kill this type of microbe: "So finding them in a dry place is not what everyone would have expected."

Methanogens require anaerobic (oxygen-free) environments to survive, and most combine carbon dioxide and hydrogen to generate energy. Kral explains that their samples were collected from possible anaerobic settings such as dry-channel deposits 70 centimetres underground.

The surface of Mars is also very dry, so the finding helps support the idea that the methane detected there could be an indicator of current microbial activity.

The lesson for Mars, says Andrew Knoll, a biogeochemist at Harvard University, US, and a member of the Mars rover science team, is that methanogenesis may be possible there, but only when the right conditions of a limited-oxygen environment and availability of nutrients occur.

  07/12/2005. New Scientist Magazine.