A common complaint of Mars scientists is that some other Mars scientist, facilitated by a major governmental space agency has declared, "the discovery of water on Mars." At last count, water has been discovered on Mars some fifty times. However, the counting is usually done under the influence of a few beers, so the number of water discoveries could be at least a hundred.
The discovery of water on Mars that would put the kibosh on the natural envy of scientists for whomever is being surrounded by friendly men and women with green press badges would involve: (1) liquid water in sufficient quantities to fill a mid-sized aquarium; (2) in sufficient purity not to inhibit the biological functioning of what you'd normally put in a mid-sized aquarium; and (3) right now, not billions of years ago. Until then, the whining about the latest "discovery" of water on Mars will continue.
For the last decade or so, the other press-winding discovery on modern Mars has been of the gas methane. This colorless, odorless gas is commonly discussed on commercial breaks between Sunday morning political shows and may be powering the computer you may be using right now. For, you see, methane is natural gas and natural gas is methane. It may or may not be America's energy future, but its presence on Mars would be of significant import for Mars: past and present. (Yes, methane is odorless. A gas called mercaptan is added to commercial natural gas, so that our noses can identify gas leaks. Flatulence contains methane and odoriferous contaminants.)
What follows is a guide to the history of methane detection on Mars, a discussion of its scientific significance, and a few things to consider when hearing about and asking about the TLS detection.
In March of 2004, ESA reported that the Planetary Fourier Spectrometer (PFS) on Mars Express had detected methane at a concentration of 10 parts per billion by volume (ppbv) in the atmosphere of Mars. For comparison, Earth's atmosphere contains roughly 1800 ppbv of methane. And, more relevantly, Mars's atmosphere is more than fifty times less massive by unit area. In other words, Mars had some methane, but not very much of it.
Skepticism of the PFS detection has been high. The biggest issue is that the PFS data products are only available to a typical investigator not associated with the PFS team in a very raw form (interferograms, I believe). Therefore, it has been difficult for anyone outside the team to check their work. In addition, the detection involved averaging large numbers of individual spectra. PFS is a fairly noisy spectrometer, so averaging multiple spectra is necessary but problematic if any instrument errors are systematic.
PFS also observed significant spatial and temporal variability in methane up to tens of ppbv. This variability was unexpected. Methane in Mars's atmosphere should be very slowly converted to CO2 by processes related to the interaction of methane and other species with sunlight, such that a high proportion of methane should become CO2 in roughly 300 years. The atmosphere of Mars mixes much faster than 300 years, so methane should be evenly distributed in Mars's atmosphere. That the methane was not evenly distributed and was temporally variable made the PFS detection seem even more spurious.
When PFS reported their methane detection, Michael Mumma of NASA Goddard probably was feeling conflicted. On one hand, he had already made ground-based measurements of Mars that suggested that methane indeed was variable on Mars. On the other hand, he wanted to make sure that this extraordinary claim would be accepted more widely in the community than the PFS detection. Except for a few coy conference presentations, he waited until 2009.
The results were extraordinary. Mumma could take individual spectra with reasonably low noise on the Martian disk. He therefore could map the spatial distribution of methane as well as its concentration. The measurement was tricky, though. Mumma needed to distinguish Martian methane lines from equivalent lines in the Earth's atmosphere. To some extent, this can be done by using the Doppler shift. Just as an ambulance siren's pitch changes as it moves toward you or away from you, the apparent frequency of light changes when the light originates from an object moving toward you or away from you. The methane spectral lines of Mars are therefore slightly shifted from those of Earth when they are observed from Earth. This shift, however, put some methane lines in the same place as spectral lines associated with water in the Earth's atmosphere. Much of Mumma's hesitancy came from wanting to ensure that those water lines could be carefully understood and modeled.
Using the Doppler shift, however, became the Achilles Heel of the method. Mumma detected methane with different spectral lines depending on whether Mars was moving toward Earth or away from Earth. He had no other choice, but the concern was that the gas detected was not strong enough a spectral match to be methane.
Time-varying methane also presented challenges to current understanding of Martian atmospheric chemistry and climate. On the chemistry side, it proved very difficult to understand how methane could be destroyed over a course of a few months rather than hundreds of years. One hypothesis involved the production of hydrogen peroxide in large quantities, but this mechanism would have been inconsistent with the measured abundances of carbon monoxide in the atmosphere. On the climate side, the rapid conversion of methane to CO2 would have resulted in a major addition of atmospheric CO2 to Mars on geological timescales.
Occasionally, when faced with discoveries as revolutionary as methane on Mars, scientists sincerely hope that some new measurements will appear, either to let them off the hook or stick them on it so firmly that they will be able to convince funding agencies to support the time they will need to complete the necessary revolution with theory and modeling. Fortunately, by 2009, it was clear that these measurements would appear when Curiosity and its TLS reached Mars.
The basic principle of the TLS is that it sucks in Mars air into a chamber, bounces a laser pulse back and forth between the chamber walls, and measures the transmission of the laser light through the gas. The laser's wavelength can be subtly altered to scan along particular spectral lines such as those for methane and varieties of methane with heavier isotopes of carbon or hydrogen. Under ideal circumstances (ask the TLS team about methane enrichment!), this method can measure methane to a few hundred pptv.
Initially, TLS measured methane concentrations of effectively zero ppbv within the uncertainties of around a ppbv. Later measurements seem to have revised this estimate. I certainly look forward to the number as well as the time series of measurements made.
So what does methane mean? Why should we care?
Methane has three principal sources: (1) geological activity, such as due to the alteration of basalt by hot fluids; (2) biological activity such as anaerobic respiration of CO2 by methanogens; and (3) condensation from the solar nebula. On Mars, the third source is either not present or would suggest something is coming out of the interior of the planet, implicating geological processes. Therefore, time-varying methane suggests life and/or active volcanism.
I should point out that methane may not mean active volcanism and/or biology today. Methane might be liberated from melting methane clathrates: a type of ice which stores gas molecules within its pores at the molecular level. In other words, methane created earlier in Mars's history might have been incorporated into clathrates as Mars's climate cooled.
The time variability also suggests a relatively large leak from the methane source as well as a very efficient destruction process. The most interesting destruction processes discussed for methane on Mars involve electrical processes in the atmosphere. When the wind lifts sand particles on Mars, they generally bounce along the surface striking dust particles so that they themselves are lifted. These collisions also generate net positive charge on the sand and net negative charge on the dust. This sets up a potential gradient. In Mars's atmosphere, CO2 is at roughly optimal pressure to be ionized by electrical fields, so Mars may experience a very large number of tiny lightning strikes during dust lifting events. Could these lightning strikes destroy methane?
The last question I'd ask about the TLS measurements is about the isotopic composition of methane. Measurements of methane on Earth suggest that methane originating from geological processes as opposed to biological processes have a distinctive signature in hydrogen and oxygen isotopes (see example). I've always wondered if we would be able to interpret what this would mean on Mars (for one thing, Mars is greatly depleted in light hydrogen versus deuterium). If TLS has measured the isotopic composition of methane, we may now finally have a data point about methane origin worth debating.