Two thousand cows

(This is the fifth of the posts on methane, and will probably make more sense in the context of the first such post, and maybe also the second, third and fourth)

A quick post with some news. Here, courtesy of Vladimir Krasnopolsky, is the abstract for the paper he and his colleagues have just submitted to the planetary science journal Icarus. A previous version was submitted to Science. This is the material that will form the basis for Dr Krasnopolsky's talk in Nice.

Detection of Methane in the Martian Atmosphere: Evidence for Life?

Vladimir A. Krasnopolsky
Department of Physics, Catholic University of America, Washington, DC 20064, USA

Jean Pierre Maillard
Institute d'Astrophysique de Paris, CNRS, 75014 Paris, France

Tobias C. Owen
Institute for Astronomy, University of Hawaii, Honolulu, HI 96822, USA

Submitted to Icarus on March 29, 2004

Abstract. Using the Fourier Transform Spectrometer at the Canada-France-Hawaii Telescope, we observed a spectrum of Mars at the P-branch of the strongest CH4 (methane) band at 3.3 microns with resolving power of 180,000 for the apodized spectrum. Summing up the spectral intervals at the expected positions of 15 strongest Doppler-shifted martian lines, we detected the absorption by martian methane at a 3.7 sigma level. This absorption is exactly centered at its expected position in the sum spectrum. The observed CH4 mixing ratio is 10 ± 3 ppb. Total photochemical loss of CH4 in the martian atmosphere is equal to 2.2 x 10^5 cm-2 s-1, and the CH4 lifetime is 340 years. Heterogeneous loss of atmospheric methane is probably negligible, while the sink of CH4 during its diffusion through the regolith may be significant. There are no processes of CH4 formation in the atmosphere, so the photochemical loss must therefore be balanced by abiogenic and biogenic sources. The abiogenic outgassing of CH4 from hydrothermal systems is 4000 cm-2 s-1 on the Earth and may be smaller by an order of magnitude on Mars. Volcanic eruptions are at least 10 Myr old on Mars, and the erupted methane disappeared since that time. The calculated production of CH4 by cometary impacts is 2% of the methane loss. There are no effective ways in the martian atmosphere for conversion of the organic matter delivered by meteorites to methane. Methane cannot originate from an extinct biosphere, as in the case of "natural gas" on Earth, given the exceedingly low limits on organic matter set by the Viking landers and the dry recent history which has been extremely hostile to the macroscopic life needed to generate the gas. Therefore, methanogenesis by living subterranean organisms is a plausible explanation for this discovery. Our estimates of the biomass and its production using the measured CH4 abundance show that the martian biota may be extremely scarce and Mars may be generally sterile except for some oases.

(I imagine either the authors or Icarus hold the copyright on this abstract; it's probably best to assume it cannot be used without permission.)

I really can't speak to the spectrometer details, other than to note that 3.7 sigma is definitely statistically significant, though not earth shatteringly so. But here are a few other quick reactions.

If I understand the numbers right, and if methane is being produced at the same rate as they calculate that it's destroyed in the atmosphere, the figures seem to me to correspond to a rate of methane production of about 230 tonnes a year.

I'd be interested to know where the figure for abiogenic methane from hydrothermal systems comes from, and whether there's a consensus among geochemists that it's really a good indicator of the total abiogenic methane production on the earth. What’s more, I'd imagine that the claim there had been no volcanic activity on Mars for ten million years is one that might be open to question. I assume it refers to the fact that the youngest lava fields on Mars, according to the people who calculate ages on the basis of the number of craters, are about ten million years old. (There's some disagreement about just how young some of the young lava is, though, as discussed in this pdf abstract by Alfred McEwen and a colleague about the Cerberus plains.) Anyway, I don't see why volcanic activity that injected molten rock into the subsurface, rather than erupting out on to the surface proper, might not have taken place more recently. I think it's going to take some quite strong arguments to make the believers in an abiogenic source give up.

Lastly, if the rate is indeed 240 tonnes a year, then it does, as the authors conclude, sound like a pretty sparse biosphere. According to the figures here and some more of my (probably ropey) calculations, it's the equivalent of the annual methane production of about 2,000 cows, or rather of the methane-producing bacteria inside two thousand cows. 2,000 cows makes a moderately impressive stampede, I'd imagine, but a pretty tiny planetary biosphere.

However, there may be some caveats here. One is that the production of methane would be a measure of the biosystem's activity, not of its absolute size. If the methanogens involved had very low metabolic rates, then the actual biomass of the system might be surprisingly large. A second is that if the methane seeps up from below, as it surely must, I'd have thought there would be a chance that some might be oxidised on the way up, making the rate at which it gets into the atmosphere less than the rate its produced in the subsurface. After all, it's widely believed that the martian regolith has a lot of oxidising power (see the discussion of the Viking lander experiments in the previous post), at least near the surface. For this to work over the long term you'd have to have some sort of process in play that renewed the oxidising power or the regolith -- maybe something driven by sunlight? A third possibility is that the methane producers are not the only life forms in the subsurface; there may be methane consumers, too. In that case the methane being seen might represent the small fraction of total production that escapes from the subsurface biosphere; the total production might be a good bit higher. None of this need be true, and even if some of it is, the biosphere would probably still be a pretty paltry one, but the possibility of something a bit bigger than it might seem at first may be worth bearing in mind.

A final point made by Dr Krasnopolsky separately is that he’s dubious as to whether localisation of the source or sources will be possible, given that the time it takes for the gas to mix itself thoroughly throughout the atmosphere is just a month. We shall see.

Comments

[Joe Knapp]

Since the Martian atmospheric mass is 2.5e16 kg, and methane is 10 ppb, wouldn't the methane mass there be 2.5e8 kg, and a lifetime of 340 years imply a sink rate of 2.5e8/340 or 740 metric tons per year (about triple the rate quoted above)?

If so, I raise you 4,000 cows.

[Oliver Morton]

Joe, you may be right, but the sink rate I give in this post was derived from the figures in their abstract on the rate of destruction of methane molecules, rather than being calculated from the starting point of the total atmospheric mass. That said, I was pleased it came to about the same as the estimate in the previous post, which was made in much the way you outline. The discrepancy may be because parts per billion is, I think, a volume measurement, and the methane will be lighter than the bulk of the atmosphere (carbon dioxide) by a ratio of 16 to 44. Do you think that would get our figures straight?

[Rick L. Sterling]

Is Vladimir Krasnopolsky's Mars Methane research funded by NASA?

[Joe Knapp]

I'm unsure of whether "ppb" is weight or molar percent. If the latter, then a mole of CH4 weighs only 16 grams compared to a mole of CO2, which is 44 grams, so that would explain the discrepancy neatly.

BTW, if you prefer, one source notes "recent research by Zimmerman et al. (1982) found average CH4 production rates of 0.425 ug CH4/termite/day for the lower termite species.." [url]http://www.iitap.iastate.edu/gcp/studentpapers/1996/atmoschem/brockberg.html[/url]

So 240 metric tons CH4 per year (2,000 cows worth) would be produced by 2.4e8/(0.425e-6*365) or 1.5e12 termites (1.5 teratermite).

The surface area of Mars is 1.44e14 m^2, so that's about one termite per hundred square meters. Not exactly a case for the exterminators!

[Oliver Morton]

Joe -- LOL. But bear in mind those are lower termites. Lord knows what the higher ones can do.
Rick -- I don't know. None of the observers is NASA staff, and the instrument isn't a NASA facility. But some of the work may be supported by NASA.