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.
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
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
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
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
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.
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 required rate.
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
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.
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
But reading that signature will have to wait
until a future mission ferries to Mars a mass spectrometer
equipped to do so.