With the detection a few years ago
of methane in the atmosphere of Mars, astrobiologists are keen to discover if
this gas is derived from living things or not. Researchers are developing a new
detector that can trace the origin of martian methane from its weight.
Methane was the first organic
compound to be discovered on Mars, and the implication of this
could be huge.
[Today, scientists announced a fresh analysis of the data, again speculating that the methane could indicate biological activity on Mars.]
"On Earth most of the methane
is made biogenically," said Tullis Onstott of Princeton University.
Microbes called methanogens
produce this greenhouse gas as part of their metabolism.
Although it is possible that similar
organisms live in Martian soil, Martian methane could be produced
geochemically, without the need for life.
Onstott and his colleagues are
building an optical device for a future rover mission that could solve the
Martian methane mystery. The project is part of the Astrobiology Science and
Technology Instrument Development and Mission Concept Studies (ASTID).
Out-sourcing methane
Methane was discovered on Mars in
2003 using powerful
telescopes on Earth, and its presence was later confirmed with the ESA spacecraft
Mars Express.
Although the amount of Martian
methane is small (10 parts per billion compared to 1,800 parts per billion on
Earth), it appears to be concentrated in regions around the equator. Because
these methane "clouds" only last a year before dispersing, the
methane sources must be fairly localized and constant.
"In order for methane to be
present on Mars, it needs to be repetitively generated," Onstott said.
Onstott estimates that this
localized generation is comparable to that of Earth's Arctic permafrost, which
is one of our planet's main sources of this greenhouse gas.
What could be generating methane at
these high rates on Mars? A variety of possibilities exist, but ultimately the
choice comes down to a biogenic or abiogenic source.
Abiogenic methane is made under the
high temperatures and pressures found deep below the surface. Although a
variety of geochemical pathways exist, the basic reaction combines hydrogen gas
and some carbon-carrying molecule like CO2 to form methane (CH4).
The methane may come out directly
through volcanoes or fault lines. Or it may get temporarily trapped in ice-like
deposits before escaping during a warm period.
The other main alternative is that
Martian methanogens are creating methane from the same molecular ingredients on
Mars (i.e. hydrogen and carbon dioxide) but with the help of enzymes.
"Enzymes are capable of
performing the same fundamental chemistry at much colder temperatures and at
higher rates," Onstott said.
Biogenic methane-production could be
going on now. Or it could have ended long ago, and we are simply seeing the
release from stored methane reservoirs.
Light eaters
There is a way to determine the
origin of Mars' methane without sifting through the soil for signs of life. It
involves using the fact that not all methane is made the same.
The building blocks of methane
(carbon and hydrogen) exist in different forms, called isotopes, that differ in
mass. Geochemistry isn't picky and will use whatever isotopes it finds to make
methane. Life, however, prefers to consume lighter
isotopes.
"Enzymatic processes work
faster on compounds of lighter weight," Onstott said.
In the case of methanogens, they
will select molecules with hydrogen (rather than its heavier isotope deuterium)
and carbon-12 (rather than the heavier carbon-13).
The result is that biogenic methane
should be lighter than abiogenic methane.
One confounding factor is that
organisms that eat methane may also inhabit Mars. These so-called methanotrophs
have a preference for light-weight methane, thereby removing the evidence of
methanogen activity.
However, Onstott thinks that
variations in the methane isotopic abundances could signal the presence of a
biological methane cycle.
Give it a ring
There are in general two ways to
detect isotopic abundances. The first involves a mass spectrometer, which
separates the different isotopes using electric and magnetic fields. Although
great for a lab, a mass spectrometer sensitive enough to detect a biological
signal in Martian methane would be too large for a rover, Onstott said.
The alternative is to use an optical
spectrometer, which measures the frequencies at which a gas absorbs light.
These so-called resonant frequencies depend on which isotopes make up the
molecules in the gas.
The Mars Science
Laboratory (MSL) - now scheduled to launch in 2011 - will carry such an
optical spectrometer (the Tunable Laser Spectrometer, or TLS). This device may
be able to measure the carbon isotope ratio in Martian methane, but Onstott
does not think it will be able to say unequivocally whether life or geology is
the source.
For this reason, he and his
colleagues are designing a special kind of optical spectrometer, called a
cavity ring-down spectrometer (CRDS), that will be 1,000 times more sensitive
than TLS. The CRDS works by illuminating an atmospheric sample with a laser
whose frequency can be tuned to resonate with methane molecules of a particular
isotopic configuration.
The cavity's walls are partially
mirrored, so light cannot easily escape. Once the laser is turned off, the
light keeps bouncing back and forth for several microseconds before it finally
peters out — or "rings down."
The time it takes for ring down is a
measure of the amount of the target molecule inside the cavity. In this way,
the CRDS can determine the ratios of the different isotopic abundances in
Martian methane. Because the light passes through the gas thousands of times
before escaping, the CRDS is much better at measuring low concentrations than
normal "one-pass" optical spectrometers, Onstott said.
Although the CRDS is a mature technology,
Onstott and his group need to develop a portable device that can reach a high
sensitivity. They have already built a test version that weighs 70 pounds,
about a fifth of what a typical mass spectrometer weighs.
The goal now is to make the instrument
smaller and more compatible for space missions — such as the next rover mission
after MSL.
"We plan to make modifications
that will ensure it functions on Mars, where there's lower pressure and lots of
dust," Onstott said.
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