Life on Mars?
Reports of methane on Mars first seeped out in 2004. Three separate
groups had detected traces of the greenhouse gas in the red planet's
atmosphere. One group relied on spectrometry readings from the
European Space Agency's orbiting Mars Express. Two others pulled
their data from powerful telescopes on Earth.
When the news reached Mukul Sharma, the Dartmouth geochemist
immediately thought through all of the obvious possible sources.
There were comet or meteor impacts to be considered, as well as
magmatic activity. And then of course there were the most
tantalizing explanations of all, capable of changing the way we view
the universe and our place in it—those that signaled past or
even present life. Bacteria produce most of the methane found on
Earth and thus could be a subtle marker for life on Mars.
Sharma was familiar with the slate of suspects, having taught for
several years a class called "Life on Mars?" But he
believed the simplest of all these potential explanations to be an
inorganic chemical reaction known as serpentinization.
Here on Earth, "there are several places on the continents and
in the ocean basins where abiotic methane is being produced by
serpentinization reactions," Sharma says. The process requires
the mineral olivine, water, carbon dioxide and some
catalysts—all well documented to be present on Mars.
Serpentinization had already entered the flurry of possibilities
that scientists put forward, but no one had worked out how the
reaction could produce the levels of methane that had been observed
in the Martian atmosphere.
So, Sharma and a colleague, Dartmouth postdoctoral fellow Chris Oze,
set out to calculate just how easy it would be for serpentinization
to produce the gas.
Olivine hides out in what geochemists call ultramafic
rock—rock high in magnesium- and iron-containing olivine and
pyroxenes, which are silicate minerals. During serpentinization,
Sharma explains, water attacks olivine and alters it to another
mineral, called serpentine. At the same time, the hydrogen molecules
are cleaved from the water. In the presence of certain catalysts,
those hydrogen molecules combine with the carbon from carbon dioxide
to form methane (CH4).
For the reaction to occur, the water must not be frozen, so
serpentinization could not take place on the surface of Mars today.
But Sharma said subsurface hydrothermal activity is a possibility.
"Chris and I reasoned that the reactions could occur below the
surface, such as close to the bottom of Helas basin, where the
normal thermal gradient of the planet would predict the temperatures
to be high enough for the water to flow."
Temperatures there could not heat the water to the mark, roughly 300
degrees Celsius, at which serpentinization is most efficient. But
Sharma says that should not matter—the reaction can take place
at room temperature and would still spit out enough methane to
sustain the levels that had been detected.
A key consideration is that methane on Mars must be replenished by a
current or recent source because it's an unstable gas broken down by
ultraviolet radiation. On Mars, methane molecules typically survive
about 340 years. Achieving a balance between the rate of
methanogenesis and the rate that methane breaks down would be the
crux of any calculation.
Writing in Geophysical Research Letters in May, Sharma and
Oze determined that it would take just 80,000 tons of olivine each
year to sustain the amount of methane observed in the Martian
atmosphere. Orbiter studies suggest that there are huge amounts of
olivine on Mars, more than enough to replenish the gas at the
Although Sharma and Oze's work shows that the mere presence of
methane is not enough to justify claims of life on Mars—some
of them shouted prematurely from media accounts that amplified the
initial methane detections—neither does the adequacy of
serpentinization rule out biogenic sources. After all, the
methanogens that generate the stuff are found in virtually every
place on Earth where oxygen is not—from the intestines of cows
to the hot springs of Yellowstone to the farthest depths of glacial ice.
It was this last location where Berkeley physicist Buford Price
joined the search for the source of methane on Mars.
Years ago, Price said, "I became fascinated by the question,
‘How can microorganisms live for hundreds of thousands of
years while frozen in deep ice?" To answer that question, he
and colleagues determined the metabolic rate for microbes trapped in
a 3,053-meter-deep ice core pulled from the Greenland Ice Shelf. The
core revealed at its greatest depths striking variations in
concentrations of methane. Clusters of the ancient organisms called
archaea were found "at exactly the depths where there was
excess methane," Price explained.
As soon as Price read that methane had been detected on Mars, he
"got very excited" and immediately began calculating
whether the metabolic rate he had established for ice-locked
methanogens on Earth might apply to the conditions as scientists
understand them to be on Mars, thereby contributing to the necessary
atmospheric balance of methane. In a paper that appeared in
Proceedings of the National Academy of Sciences in
December 2005, Price posited that it could.
He observed that the metabolic rate on Earth rises with temperature,
which increases with depth, both on Earth and on Mars. The
concentration of microbes and the thickness of ice would also vary
the rate of methane production.
All of that means that there are different plausible biogenic
scenarios in play, but Price picked a favorite: "In my opinion,
if the methane is biogenic, the methanogens are likely to be at a
depth of hundreds or even a thousand meters, where they have access
to ice that is warm enough to contain aqueous veins," Price
said. The temperature would be somewhere between -10 and -40 degrees.
Sharma and Price both point out that the best way to determine
whether the methane on Mars is biogenic or abiogenic would be to
measure the ratio of carbon-12 to carbon-13 found in methane.
Methanogens produce a gas much higher in carbon-12 than that
produced by serpentinization, and this distinctive isotopic
composition persists throughout the life of individual methane molecules.
But reading that signature will have to wait until a future mission
ferries to Mars a mass spectrometer equipped to do so.
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