March 29, 2005

Crossing the Treeline

Planetary scientist Chris McKay describes one of the driest place on Earth and how one might test for the equivalent of a bacterial 'treeline', a region so harsh in the Andes Mountains where even microbes cannot thrive.

Astrobiology Magazine -- Chris McKay, a planetary scientist at the Ames Research Center, has long been investigating the coldest and driest places on Earth. These harsh environments - and the ability of life to adapt there - could point the way to finding life on Mars.

McKay presented this lecture, entitled "Drilling in Permafrost on Mars to Search for a Second Genesis of Life," at a NASA Astrobiology Institute Director's Seminar on November 29, 2004.

In this part of his lecture, McKay describes one of the driest place on Earth and how one tests for the equivalent of a bacterial 'treeline' where microbes cannot thrive in the Andes Mountains.


I want to move from frozen ground to dry places. On the west coast of Chile and Peru is the Atacama desert, the driest place on Earth. It has a double rain shadow. On the western side, the cold Pacific fog is blocked out, and on the eastern side, rain is blocked out by the towering Andes. There are also mountains in the middle, which block out the runoff from the Andes. This is a very dry, virtually lifeless place.

In a normal desert, which gets about 25 millimeters of rain per year, there are organics. You get bushes and even some ants.

But the Mars Viking detector of organics - the GCMS - would not be able to find life organics in the Atacama. The level of organics is so low that we have to heat the sample to find any. Viking heated samples to 500 degrees Celsius, but to find organics at Atacama, we have to heat the sample to 750 degrees C.

The concentration of organics in the really dry part of Atacama is interesting, because there seems to be a microbial "tree line." At the tree line on a mountain, the trees suddenly stop, as though the mountain had been shaved. In the driest part of the Atacama, the number of bacteria falls by four orders of magnitude.

One of the Viking experiments added nutrients to the martian soil. There were some soil reactions, and people today still argue about what happened. Viking added both right and left-handed amino acids to the soil. Since life on Earth is composed of left-handed, or L-chiral, amino acids, you can think of the nutrients as good soup for life and bad soup.

The idea was that if something biological is there, you would expect it to eat the good, "left-handed amino acid" soup. Life would show a higher uptake of good biology soup. But in the Viking experiment, the two soups were mixed and we could not tell if one was favored over the other.

We decided to duplicate this experiment on Earth, in the Atacama, adding a similar soup of amino acids. Our experiment found no clear signs of life, but had some interesting carbon reactions.

At Atacama both kinds of soup are consumed with equal gusto. This indicates a photo-chemically produced oxidant, and not a biological reaction.

Atacama is like Mars because the addition of nutrients caused oxidized organics to increase. Also, virtually no organisms are detectable, and no DNA.

At the Atacama, we don't drill, we dig. There is no detectable life on the surface, but in pits, there is a layer where the bacterial count shoots up. We don't know what is happening there. It could be a relic from the past. It could be a water layer. It could be a habitable zone.

This may be relevant to Mars. The martian surface is cooked or bleached, but relic organic material may be below the topsoil.


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Viking Biology Experiments