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.
"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).
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.
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.