Richard Quinn of Ames Research Center delivered one talk summarizing a study which his team has since reported in the Nov. 7 "Science", regarding the number of microbes in the driest, most lifeless core of northern Chile's Atacama desert -- the oldest and driest desert on Earth, sealed off from rainfall on all sides by mountain ranges which limit the rainfall in its Yungay region to only about 0.6 mm per year.

All the Atacama desert is very low on water and thus on soil microbes; but the team discovered that soil from the upper 10 cm of the Yungay region was virtually devoid of any culturable germs at all -- unlike soil from anywhere else on Earth, including the rest of the Atacama or Antarctica's "dry valleys".

The team also discovered that, had the Viking landers touched down there, their organic molecule detection experiment would have come up as empty for organic compounds as it did in Mars' upper soil. Faint traces of organics were detected only when the Atamaca soil was roasted at considerably above the 500 deg C. top limit of the Viking experiment's extraction ovens, indicating that any traces of organics in both places must be in forms very resistant to breakdown by heat.

And a duplicate of the Viking landers' "labeled release" experiment, to culture Mars soil in a nutrient broth and detect any carbon dioxide produced by microbes in it, produced the same results here as on Mars -- a sudden burst of CO2 apparently produced by a reaction with some powerful oxidant chemical in the soil that broke down the broth's organics, and which must also exterminate any local soil microbes. Various tests confirmed that this reaction in the Yungay soil must indeed be purely chemical, due in no way to the existence of microbes -- and there is nothing like it in any soil anywhere else on Earth.

But the team's tests also opened up yet another intriguing mystery with relevance to Mars: just what is Yungay's oxidant? Tests of the Yungay soil failed to detect hydrogen peroxide, metal superoxides or peroxynitrite -- the obvious suspects long suspected as the soil oxidants on Mars.

And all the theories of the origin of Martian soil oxidants have involved them being produced by the solar UV light hitting Mars, thousands of times stronger than that which punches through Earth's ozone layer -- yet the Yungay desert is only one km above sea level and does not have any higher UV level than most Earth locations.

So -- even without Mars' very high levels of solar UV light -- some as-yet unknown chemical is being produced in the Yungay soil by ordinary Earth-level sunlight, and has accumulated to high levels there over millions of years in which it hasn't been consumed by contact with liquid water or organic compounds, so that it serves as an antiseptic which further retards the survival of any germs that DO happen to drift into the region. This same as-yet unknown reaction obviously makes it even more plausible that a similar germ-destroying chemical has been produced in the upper fraction of a meter of Mars' soils and mixed deeper into the surface soil by its winds.

The question remains whether the occasional brief exposures of Mars' soils to small traces of liquid water every few tens of thousands of years thanks to the obliquity cycle may have broken down this chemical enough to allow microbes to exist as spores in the near-surface soil and then come back to life and reproduce during those brief periods when the soil is warmed enough to produce those traces of liquid water -- but the Yungay finding is not at all good news for Mars life fans.

Click for Part Seven

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