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It
was a sensation when scientists discovered methane in Mars’ atmosphere
nine years ago. Many saw the presence of the gas as a clear indication
of life on the inhospitable planet, as on Earth methane is produced
predominantly by biological processes. Others assumed geological
processes, such as volcanoes, to be the cause. What has been missing
until now is proof of where the methane actually comes from, however.
Researchers at the Max Planck Institute for Chemistry in Mainz and the
universities in Utrecht and Edinburgh have now been able to show that
methane escapes from a meteorite if it is irradiated with ultraviolet
light under Martian conditions. Since meteorites and interplanetary dust
from space, which carry along carbonaceous compounds, continuously
impact on the Martian surface, the researchers conclude that high-energy
UV radiation triggers the release of methane from the meteorites.
Since
scientists identified larger quantities of methane in the Martian
atmosphere in 2003, there has been much speculation about its source.
The best-known hypothesis states that microorganisms produce the
methane, and is thus an indication of life on the red planet. Another
hypothesis assumes the source to be geological methane sources in Mars’
interior. To date, none of the theories has been able to conclusively
explain the large quantity of 200 to 300 tonnes of methane annually
which are produced on Mars, according to projections.
Without
an expedition to Mars and with nothing more than a meteorite to help
them, researchers at the Max Planck Institute for Chemistry in Mainz and
the universities in Utrecht and Edinburgh have now found a major
source.
“Methane
is produced from innumerable, small micro-meteorites and interplanetary
dust particles that land on the Martian surface from space,” explains
Frank Keppler, lead author of the study now published in the research
journal Nature. “The energy is provided by the extremely intense ultraviolet radiation.”
UV light decomposes carbon compounds in meteoritic matter
Unlike
Earth, Mars has no protective ozone layer which could absorb most of
the UV radiation from space. Moreover, the Martian atmosphere is very
thin, so that a significantly smaller portion of the meteoritic material
burns up in the atmosphere compared to Earth.
Together
with colleagues from Great Britain and the Netherlands, the researchers
from Mainz irradiated samples of the Murchison meteorite with
ultraviolet light.
“The
meteorite contains several percent carbon and has a similar chemical
composition to most of the meteoritic matter that lands on Mars,” says
the cosmochemist Ulrich Ott.
The
4.6 billion-year-old meteorite fell to Earth in 1969 in the Australian
town of Murchison. The researchers selected conditions identical to
those on Mars for the UV irradiation, which caused considerable
quantities of methane to escape from the meteorite almost immediately.
Their conclusion: carbonaceous compounds in the meteoritic matter are
decomposed by the high-energy UV radiation, and methane molecules are
formed in the process.
The methane production from meteorites depends on temperature
Since
the temperature on the red planet varies from minus 143 C at the poles
to plus 17 C at Mars’ equator, the scientists also investigated the
meteoritic samples at appropriate temperatures. The warmer it became,
the more methane was released by the meteoritic fragments. This
temperature dependence also agrees with the different methane
concentrations at different locations in the Martian atmosphere. In
infrared spectra, the largest concentration of methane was found in the
equatorial region, the warmest place on Mars, relatively speaking.
The
results obtained by Frank Keppler’s team should bring “down to earth”
all those who firmly believe in the biological origin of the methane.
The researchers cannot fully exclude the hypothesis of Martian microbes,
however, because, although the process found here is inevitable, it is
quite possible that further processes contribute to methane production.
The researchers hope that Curiosity, the Mars Rover that NASA expects
to land on our neighbouring planet at the beginning of August, will
provide more details on the formation of methane, and maybe even final
clarification as to whether there is life on Mars.
