Boulder,
CO (8/25/98)- While the basic geological conditions on Mars could provide
the minimal requirements for life to exist, new geochemical models suggest
the biological potential of the red planet may be minimal. Chances of life
on the Jovian moon Europa are even slimmer, say geobiologists.
"There has been a revolution in biology that has changed ideas about
early life on Earth. These new ideas came after recent discoveries of life
on Earth in extreme conditions where organisms use geochemistry rather
than photosynthesis for energy," Professor Bruce Jakosky of the University
of Colorado's Laboratory for Atmospheric and Space Physics
For example, recent reports show that some organisms live on chemical
energy obtained from near-boiling
water created by mid-ocean rifts and continental hot springs like Yellowstone
while other studies reveal microbial life feeding off rocks below Antarctic
ice lakes.
"We used this new knowledge as a springboard to estimate the amount
of chemical energy available on Mars," said Jakosky, a science team member
on NASA's Mars Global Surveyor spacecraft. "This is the first modern estimate
for the potential amount of life on Mars, past or present, and was calculated
by using what we know about the planet's geological history," he said.
The new research modeled geochemical reactions from rock weathering.
The researchers estimated Martian volcanic activity over time and the associated
activity of hydrothermal vents. The data suggests that only a surprisingly
small amount of life could have been produced through chemical reactions
over billions of years.
Exobiologists, those who look for life forms on other planets and who
study the origin of life on Earth, work under the assumption that life
requires water, access to elements like carbon, hydrogen, oxygen and sulfur
to build complex molecules, and a source of energy. The energy source
can be either natural chemical reactions or heat from the sun.
Earth can produce about 20 grams of organisms per square centimeter
of land every 1,000 years because of the powerful forces of photosynthesis.
Yet it would take Mars four billion years to produce that same 20 grams,
assuming the organisms were using chemical energy. This massive difference
in the possible biomass produced on Earth relative to Mars is due almost
entirely to the occurrence of photosynthesis on Earth, says Professor Everett
Shock of Washington University's Department of Earth and Planetary Sciences.
As yet, there is no evidence of life on Mars, much less photosynthesis.
Jakosky and Shock estimated the amount of geochemical energy that has
been available through time on Mars from evidence involving volcanism,
the circulation of water on Mars that once flowed through its hydrothermal
systems, and the weathering on the planet's surface and crust.
These analyses indicate that Mars has spewed several hundred times less
volcanic rock in its lifetime than has the Earth. This suggests that the
global amount of energy accessible through hydrothermal vents would likely
have been proportionately less on Mars, Jakosky said.
NASA now has plans underway to recover soil and rock samples from Mars
with remote equipment and return these samples to Earth to look for evidence
of life. Given the geology of Mars, this may prove challenging, said
Jakosky said:
"The chance of picking up rocks containing fossils or even life during
sample-return missions is small. Our best hope lies in targeting and exploring
fossil or active hydrothermal systems, aqueous systems that could be exposed
in walls of Mars' deep canyons, or active springs discharging at the surface."
The researchers also evaluated the potential biomass that could have
been created on Europa, a moon of Jupiter. Europa gained attention
in recent months when Galileo spacecraft scientists discovered what they
believe to evidence for liquid water under thick sheets of ice.
If there is even a slim chance for finding life there, it would
not be in the water but in rocks underlying the water where internal heat
sources may transfer energy for life. However, the estimated
energy available on Europa is even lower than on Mars, noted Jakosky.
The research appears is in the August 25, 1998 issue of the Journal
of Geophysical Research.
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