Feb. 19, 2007 — Hunting for traces of life
on Mars calls for two radically different strategies, says Arizona
State University professor Jack Farmer. Of the two, he says, with
today’s exploration technology we can most easily look for evidence for
past life, preserved as fossil "biosignatures" in old rocks.
Farmer is a professor of geological sciences in ASU’s School of
Earth and Space Exploration, where he heads the astrobiology program.
He is reporting on his work at the annual meeting of the American
Association for the Advancement of Science in San Francisco.
"Searching for extraterrestrial life must follow two alternative
pathways, each requiring a different approach and tools," Farmer says.
"If we’re looking for living organisms, we are doing exobiology. But if
we are seeking traces — biosignatures — of ancient life, it’s better
to call it exopaleontology."
Unfortunately, he notes, "for the next 10 or 15 years, technology
limitations will force us down the exopaleontology path." The core
issue is accessibility. "To find living organisms on Mars," says
Farmer, "you need to find liquid water. Because liquid water is
unstable on the Martian surface today, that means going deep into the
Water saturates the ground in high latitudes north and south, and
around both poles, only a few inches below the surface, Farmer
explains. But this water remains frozen year round. "Environments with
liquid water will likely lie far deeper, perhaps miles below the
Organisms have been found living in fractured rock, thousands of
feet underground on Earth, Farmer notes. "But with current robotic
technology, we simply can’t drill that deep on Mars."
Terrestrial deep drilling requires complex, heavy equipment, plus
constant supervision and troubleshooting by human crews. Says Farmer,
"We’ll be lucky if, in the next decade or so, robotic drilling on Mars
reaches a depth of a couple yards."
So where does that leave us in the search for life on Mars? Farmer says our best choice is to pursue the exopaleontology path.
"Finding the signatures of an ancient Martian biosphere means
exploring old rocks that might preserve traces of life for millions or
billions of years," Farmer notes. Among the best places to look on
Mars, he says, are deposits left by springs and former lakes in the
heavily cratered highlands. "The rocks there date from a period in
Martian history when liquid water was common at the surface." In fact,
says Farmer, conditions on Mars then were likely similar to those on
the early Earth at the time when life began.
"Besides water, life also requires energy sources and organic
chemical building blocks," Farmer explains. "The Mars Exploration Rover
Opportunity found ample evidence for water in ancient rocks at
Meridiani Planum, but the rovers’ instruments can’t detect organic
materials." However, NASA’s next rover, the Mars Science Laboratory,
will carry instruments to analyze traces of organic substances. It is
due for launch in 2009.
Recognizing a Martian fossil may be difficult. "We’re not talking about stumbling over dinosaur bones," Farmer says.
Instead, the discovery may involve finding biologically formed
structures in old sedimentary deposits, perhaps like stromatolites
found here on Earth. Stromatolites are distinctive structures that form
in shallow oceans, lakes, or streams where microbial colonies trap
sediments to form thin repeating layers.
Stromatolites also contain microscopic cellular remains and chemical
traces left by the microbes that formed them. Taken together, such
structures comprise the primary record of life in ancient rocks on
For hunting Martian fossils, says Farmer, we will need robotic
microscopic imagers capable of viewing rocks in many wavelengths as
well as seeing details as small as a hundredth of a millimeter across.
Also needed are organic chemistry laboratories to analyze promising
rocks. "That will help us avoid mistaking non-biological features for
biological ones," he says.
Farmer’s fieldwork has taken him to extreme microbial habitats in
Iceland, New Zealand, Yellowstone National Park, and Mono Lake, Calif.
He has sought to understand how modern microbial communities become
preserved as fossils. Their environments, he notes, span physical and
chemical conditions believed to be representative of early Mars.
"Studying how microbes become fossils is a key step in developing an
effective strategy for exopaleontology," Farmer says. "It will help us
find the best places to explore on Mars and how to look."