[X] My Assistant

[Guest_AlexBlackwell_*]

The May 2007 issue of Scientific American has a fairly informative article ("The Mystery of Methane on Mars and Titan") by Sushil Atreya, although at the moment it's behind a pay-per-view wall for non-subscribers. If you cannot obtain the magazine, check out Professor Atreya's website, especially the publications section, which contains links to several of his papers, talks, etc.

[Gray]

I finally got around to reading the article. I thought I'd note a few points here. Forgive me if I'm stating what everybody already knows.

Using the Mars Express Orbiter they measured the concentration of methane in the Martian atmosphere as 0-35 ppbv(v?). The concentration varied with location.

They estimated the residence time of methane in the Maratian atmosphere as about 600 years (I presume Earth years).

Using a conservative concentration estimate of 10 ppbv of methane in the martian atmosphere, they calculated that an annual production rate of 125 tons of methane was needed to sustain that concentration.

They suggested that the two most likely sources of the methane were biologic production or an inorganic process called serpentinization, which involves reactions between water and ultramafic rocks. At the present time there is not enough data to determine which is the most likely source.

[Cugel]

Using a conservative concentration estimate of 10 ppbv of methane in the martian atmosphere, they calculated that an annual production rate of 125 tons of methane was needed to sustain that concentration.

They suggested that the two most likely sources of the methane were biologic production or an inorganic process called serpentinization, which involves reactions between water and ultramafic rocks. At the present time there is not enough data to determine which is the most likely source.

On planet Earth it is estimated that biological sources have a production rate of 1.000.000.000 ton CH4 per year
(10×10^14 g CH4/year) source: http://adsabs.harvard.edu/abs/1982JGR....87.1305S

Suggesting a biological source on Mars for 250 tons per year seems pretty absurd to me.

[centsworth_II]

Suggesting a biological source on Mars for 250 tons per year seems pretty absurd to me.

If Mars had a biomass, I do not think it is absurd that it might be 1/4,000,000th that of Earth's.

[Gray]

One more bit of information from the article I did not include is that they estimated the residence time of methane in the terrestrial atmosphere as 10 years, vs 600 years for Mars.

Perhaps their key point is that since there are chemical reactions in the Martian atmosphere that destroy methane, the methane that is detected cannot be primordial methane, but methane that is continually produced by some ongoing process occurring in the planet. And the fact that it has a patchy distribution suggests that there are localized centers of production.

[marsbug]

Serpentinization can involve water and heat, which spectulative martians would need, just to confuse things a bit more. Is there an applicable method of distinguishing between biologically produced methane and methane produced by serpentinization?

[Gray]

Well, on Earth, biologic processes tend to favor carbon 12 over carbon 13. So biogenic compounds tend to be reletively enriched in C-12. But all of the measurements used in terrestrial studies are, of course, based on a terrestrial standard which is of known biogenic origin.

Even if there were a way of measuring C-13/C-12 ratios in martian hydorcarbons, I'm not sure what would be used as an objective standard for assessing them.

[rogelio]

Cugel wrote:
“On planet Earth it is estimated that biological sources have a production rate of 1.000.000.000 ton CH4 per year (10×10^14 g CH4/year) source: http://adsabs.harvard.edu/abs/1982JGR....87.1305S
Suggesting a biological source on Mars for 250 tons per year seems pretty absurd to me.”


How much of that annual gigaton of Terran methane is produced deep underground from a crustal biosphere? I don’t know offhand, but if it were, say, even 1%, then we might expect an analogous submartian biosphere to produce quite a lot, too, buffered as it is from killing radiation... So I agree that if there WERE deep Martian bugs, they’d produce a lot more than 250 tons of methane per annum.

[centsworth_II]

Even if there were a way of measuring C-13/C-12 ratios in martian hydorcarbons,
I'm not sure what would be used as an objective standard for assessing them.

Someone better get started on this because they'll be needing that
information about a year from now.

"...scientists will be able to determine ratios of various isotopes of hydrogen,
oxygen, carbon, and nitrogen, providing clues to origin of the volatile
molecules, and possibly, biological processes that occurred in the past."
http://phoenix.lpl.arizona.edu/science_tega.php

[Gray]

On Earth, serpentinization occurs on the sea floor in association with hydrothermal activity at the oceanic ridges. None of these are known to exist on Mars.

Perhaps the Martian methane is generated by a uniquely Martian process.

[centsworth_II]

Cugel wrote:
...if there WERE deep Martian bugs, they’d produce a lot more than 250 tons of methane per annum.

Why would you expect a martian biosphere to be of a scale similar
to Earth's? Why would not a biosphere one billionth the size of Earth's,
commensurate with available resources, be possible?

[marsbug]

I seem to remember something in the rules about discussions about astrobiology and, although it's hardly possible to avoid the topic on a thread like this, I worry we might be pushing at the boundaries. I only mention this because I rate the detection of martian methane as one of this decades great unsolved mysteries and would like this discussion to continue! If its not appropriate I hope doug will let us know, or move the thread somewhere more appropriate.

Is there anything about the serpentinization process that is readily identifiable? If that hypothesis could be confirmed or eliminated it would be a big hint, although not conclusive. Also; as I understand it organisms only produce methane when active (although I could be wrong). The martian mantle is largely solid, so theres no heat supply to keep subsurface microbes active, and no new materials being brought from the surface. Given the scarcity of liquid water, radiation , low temperatures and pressures at the surface I would expect any martian biomass to be in hibernation across the planet with activity only in the rare locations and times where conditions are more clement. If that is true the 250 tons per year figure only indicates the total active biomass, and sites of activity might not be fixed adding further difficulty to detection. The more we learn about this puzzle the more I think it will take a dedicated mission to resolve this!

[rogelio]

Centsworth II wrote:

“Why would you expect a martian biosphere to be of a scale similar
to Earth's? Why would not a biosphere one billionth the size of Earth's,
commensurate with available resources, be possible?”

I guess I’m assuming that any extant underground Martians are the remnants/descendents of an era (or series of eras) when conditions for life were much more favorable (more liquid at or near the surface, for example. Therefore, given time and the current abundant subsurface water, why shouldn’t the extant biota be widespread, fairly common, and within a few orders of magnitude of the biomass of the current Terran subsurface “fauna”?

[centsworth_II]

...given time and the current abundant subsurface water, why
shouldn’t the extant biota be widespread, fairly common...

I don't think water would be the limiting factor. Lack of nitogen
may be the most severe obstacle to overcome.

[helvick]

I don't think that we can infer much about the actual size of a theoretical Martian methanogenic biosphere just from measurments that say there is some cycle producing ~250ton/annum. After all, the range of metabolic rates in earth based animal life forms covers at least 7 orders of magnitude (from hibernating brine shrimp to hovering hummingbirds) so unless you make dangerous assumptions and restrictions about these theoretical martian life forms you cannot really make any sensible estimate of how "big" such a biosphere would need to be to produce 250 tons.

Personally as much as I'd love to see some real evidence of martian life the way I see it the current balance of probabilities seems to point to serpentinization as much more likely.