AP
NASA scientist Adam Steltzner explains the Curiosity rover's path to the
surface of Mars in Pasadena, California, on Thursday. (Right) An artist’s rendering of Curiosity on Martian surface.
When NASA’s Mars Science Laboratory (MSL) lands its
rover, Curiosity, on Mars on Monday it will be the latest in a series of
missions to the red planet that began more than three decades ago. The
MSL mission isn’t a search for life. Curiosity will sniff for chemicals
that could be relevant to life, but it won’t be looking for biological
organisms as such.
Why is this? Given the huge public
interest in life on Mars, why doesn’t NASA just go and look for it
directly? To understand, you have to go back to 1976, and NASA’s
trailblazing Viking mission. Two identical spacecraft landed on Mars to
look for microbes in the topsoil. Several experiments were performed.
One consisted of adding a nutrient broth to soil to see if anything
consumed it and gave off carbon dioxide. To scientists’ surprise,
something did — repeatedly — on both spacecraft. When the soil was
heated, the response stopped. To this day the designer of the
experiment, Gilbert Levin, insists he found life on Mars.
Few
scientists agree. The other Viking experiments did not give such
clear-cut results, and NASA’s official position is that the mission did
not detect life. So what caused the broth to emit gas? Nobody really
knows. It may indeed have been life, but it may also have been complex
soil chemistry.
Conditions on the surface of Mars
are very harsh. Radiation is intense. Water exists, but almost never in
liquid form. Reactive chemicals such as oxidants can accumulate over
immense durations without being washed away or neutralised. So it’s
perhaps no surprise that adding liquid broth made the soil fizz.
Because
of the inconclusive Viking results, the direct search for life on Mars
has effectively stalled, as it’s hard to know precisely what to look
for. What would be an unambiguous signature of life anyway? Scientists
can’t even agree on a definition of life as we know it, let alone a
possibly different form of life. And when the soil chemistry is
unfamiliar, the problems are compounded.
Clarification
could come, however, by seeking out Mars-like surfaces on Earth and
studying what, if anything, lives there. Mars is very cold and very dry
but, of the two, the dryness is the more serious obstacle: water is
crucial to known life.
The driest place on Earth is
the Atacama Desert in Chile, and for years astrobiologists have been
sifting the soil there, looking for hardy microbes able to eke out an
existence in the hyper-arid terrain. For a while it looked as if no life
could withstand the desiccating conditions of the Atacama’s core, but
then in 2006 a visiting chemist from the University of Lleida in Spain,
Jacek Wierzchos, made a discovery.
Projecting out of
the parched dusty surface of the desert are countless natural sculptures
made of common salt. Mr. Wierzchos broke one open and was puzzled to
find a distinctive dark layer inside. He dissolved the salt rock and
found the colouration was caused by several new species of microbe
living inside.
How do Mr. Wierzchos’ bizarre
microbes survive? It seems that enough light penetrates the salt to
permit photosynthesis. But what supplies the all-important water? This
was the most astonishing part. A distinctive property of salt, known as
deliquescence, is its ability to suck in moisture directly from the air.
The microbes scavenge this sparse resource, ingesting tiny quantities
of water from microscopic pores inside the crystalline matrix. Although
the fierce desert sun bakes the water out of the salt in the daytime,
there is enough humidity in the air at night to replenish it by
deliquescent absorption. So even if it never rains, life can go on.
Could
Mars harbour microbes in a similar setting? It’s not impossible. There
are salt deposits there too, and although the Martian atmosphere is much
thinner and holds less water vapour than the Atacama Desert, there may
be niche environments in which deliquescent absorption could still
operate. Cocooned in salt, protected from the oxidising soils and the
intense ultraviolet radiation, Martian microbes may be able to
photosynthesise, and support a Lilliputian ecosystem sustained by traces
of water permeating salt rocks.
Unfortunately, MSL
won’t be targeting such environments. It will, however, be looking for
organic compounds that could hint at some form of Martian biology.
Meanwhile,
the Atacama Desert hosts the closest analogue of what a real, live
Martian might be like. We can only wait expectantly for signs of
something similar on the red planet. — © Guardian Newspapers Limited, 2012
(Paul Davies is director of the Beyond Centre for Fundamental Concepts in Science at Arizona State University)
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