The
possibility of life on Mars has been debated almost since the invention of the
telescope.
Annual
growth and shrinkage of the martian ice caps and seasonal changes in color were
observed by astronomers such as Herschel and Whewell in the 18th and
19th centuries. Late in the 19th century, Schiaparelli
reported the existence of linear
features he called canali, and this led to speculation by Lowell
(among others) that the Red Planet might host a civilization
more advanced than our own.
The arrival
of instrumented spacecraft, beginning with Mariner IV in 1964, showed a
cratered, dry, cold planet that looked almost as inhospitable as our Moon; but
later missions provided evidence
of liquid water on Mars in the recent past. This has led to speculation
about whether simple forms of life, like bacteria, might exist on Mars. The
thin atmosphere of Mars does little to block out damaging radiation from the
sun, and the surface of Mars seems to be sterilized by caustic chemicals like hydrogen
peroxide, but scientists still hold out hope that life
on Mars could survive protected below the surface.
Now,
building on a tradition of ground-based simulation that extends back to 1958, a
new series of experiments, conducted by an interdisciplinary research team from
the Faculty of Natural Sciences of the University of Aarhus, Denmark, suggests
that indeed bacteria could survive beneath the martian soil.
The team
constructed a Mars Environmental Simulation Chamber (MESCH), from which air is
removed with a vacuum pump, and replaced with a thin mixture of gases
equivalent to those in the martian atmosphere. The chamber has a double wall
cooled with liquid nitrogen to simulate the cold temperatures experienced in
the martian night.
Samples of
soil in glass cylinders are placed in the chamber's steel sample tubes through
a small air-lock, then rotated on a carousel; exposing one sample at a time to
ultraviolet-rich light from a xenon-mercury arc lamp. Cycling samples through
the light beam produces a temperature rise and fall that mimics the diurnal
temperature variation on Mars. The atmosphere in the chamber is sampled and
subjected to molecular gas analysis to determine how the soil (and any bacteria
it may contain) interacts with the atmosphere.
Shining
a light on Mars
Clifford A.
Cerbus is a research physicist with the University of Dayton Research
Institute, and has worked for 20 years on a space environment simulation system
at the U.S. Air Force Research Laboratory Materials and Manufacturing
Directorate in Dayton, Ohio. He expressed concern about the high intensity
of the ultraviolet light generated by the xenon-mercury lamp used in MESCH,
which is approximately 35 times as intense as the sun at Mars' surface. While
this enables accelerated testing, in which a few hours in the simulator
simulates several days on Mars, Cerbus wonders if the results would be
representative of what actually takes place on the martian surface.
"Yes,
that is a problem," concedes Professor Kai Finster, a member of the MESCH
team. "We have plans to reduce the intensity by filters to levels more
representative of those on Mars. We are also thinking of other light sources
that represent a different spectrum and/or intensity, such as a solar simulator
or other types of UV lamps. However, in the first round of experiments we were
interested in seeing whether we could see any effect at all."
So far, the
University of Aarhus team has only published results with samples of sand,
which demonstrated that the system produces temperature variations comparable
to those on the martian surface. However, Finster says that additional tests
have since been conducted on samples containing freeze-dried bacteria from
Arctic permafrost.
While
exposure to 80 days in the simulated martian environment essentially sterilized
the topmost two centimeters of the simulated sample core, bacteria were
"relatively unaffected" in the rest of the 30 centimeter sample
tubes. This result, presented in a paper which is due to appear soon in the
journal Astrobiology, suggests that some form of life could exist below
the martian surface.
The Faculty
of Natural Sciences supported the construction of the MESCH instrument, and the
experiments are supported by the Danish Natural Science Research Council.
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