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.

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.

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.

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

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.

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?

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.

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.

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

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.

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 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!

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”?

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

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.

If it helps, at this time I would have to say I think
serpentinization is a more realistic explaination for
the methane. But it is hard to ignore that evidence
for life will be looked for in the Phoenix data.
At this time, there is no strong evidence for life
on Mars. We should however be able to discuss how
to rule out -- or in -- what evidence does exist as it
comes in.

And hibernating bacteria would have a rate next to zero.

On earth fixed carbon is fairly cheap, and can be excreted without much cost. It's plausible that Martian bacterial analogs have optimized their ability to sequester fixed carbon, so that the average rate of atmospheric methane production is much less than it is on earth.

I'm not sure what you mean by that last sentence but it's important to note that Phoenix is not designed as a life detection mission, unless "life" happens to walk in front of the cameras. In any event, one should bear in mind that while TEGA and RA are to be subjected to some heightened level of cleaning and dry heat treatment to reduce bioload, my understanding is that funding was never available for complete, Viking-level sterilization of the lander or its constituent parts. So any detection of "life" would face that not insubstantial hurdle.

Just basing it on this quote I already posted from the Phoenix web site:
"...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

Although Phoenix is not designed to look for life, it will be interesting to see what
the isotope ratio data tells the researchers. I'm sure it will be far from conclusive
and there will be many questions raised for future missions to look into. Phoenix
will be gathering types of data that have never before been gathered on Mars
-- very exciting.

What will the data tell to the operators of current Mars missions? Can Phoenix data supply additional instructions to the MERs or the orbiters?

Of course, the Phoenix data will be of great help in providing ground truth
for the orbiters. The predictions of the orbiter data concerning amount of ice,
and its closeness to the surface can be compared with what Phoenix finds.
This will make orbiter observations of other areas not visited by Phoenix more
meaningful.

I can't think of any way that Phoenix discoveries can affect the MER mission.
Come to think of it, is there any way that one MER has affected the mission of the other?
The data of all the missions of course compliments each other in advancing
our understanding of Mars.

If Phoenix succeedes, it will cast MUCH light on the chemical reactions that occured in the Viking Biology Experiments. There clearly is *INTERESTING* chemistry in the "soils" that is not really hinted at by bulk minerology and elemental abundance information. Few people believe Gil Levin's special-pleading for biological interpretations of the experiment's results, but while the preferred explanation is abiotic chemistry, we only have "preferred models" of what the soils did under different experimental protocals. Phoenix is going to do more wet chemistry tests in the MECA (I think) instrument that will be powerfully diagnostic of soil components. Sample return would be far far better, but is far far in the future, compared with Phoenix's "real soon now".

I'm really looking forward to a ramp-up of info about the Phoenix payload before launch ( the typical Science briefing etc etc ) as I think some people - even people here - will be suprised just how much they've got going on in there.

Doug

I wonder how much the soil chemistry of the Phoenix site will have in common with with
the Viking sites. Does the presence of much water (even as ice) and less UV radiation
result in radically different chemistry, particularly of the so called super oxides?

Thanks. I thought you were referring to Phoenix's goals in searching out paleohabitats but I wasn't sure. Of course, that's a whole different kettle of fish than looking for signs of extant life.


Although Phoenix, due to its lack of mobility, may not garner the same public attention as MER, it will indeed offer some very interesting science; in fact, the science portion of Phoenix's proposal was rated very high during the Mars Scout AO selection process. Assuming it succeeds, Phoenix will serve as a nice precursor for MSL.

Interesting comment. Reminds me of two factoids I recently encountered. Housecats have more concentrated urine than people, dogs, etc., because as desert natives, they try to conserve water. Snakes take this even further, excreting the same waste as a solid with no water loss. Life... adapts.

"I wonder how much the soil chemistry of the Phoenix site will have in common with with the Viking sites"

Probably a lot in the top centimeter. Stuff in contact with the ice.. that's another story.

I get the impression that many of the chemical characteristics of the
surface soil are a result of interactions with UV radiation. I wonder
how much less of an impact UV radiation has at the Phoenix site on
the chemistry of the surface. If not much, Phoenix may have the
best of both worlds: Studies of the surface that can be useful in further
understanding of results from other sites, like Viking, and studies of the
totally new environment of the icy subsurface.

Of course any thoughts of how life may have (theoretically) adapted
for life on today's Mars should probably be restricted to the tactics
of microbes, which in any case seem to be even more adaptable
than multicelled life forms.

Here's a recent paper from Geophysical Research Letters:

Elwood Madden, M. E.; Ulrich, S. M.; Onstott, T. C.; Phelps, T. J.
Salinity-induced hydrate dissociation: A mechanism for recent CH4 release on Mars
Geophys. Res. Lett., Vol. 34, No. 11, L11202
10.1029/2006GL029156
08 June 2007
Abstract

Excellent hypothesis! CH4 hydrates are not that stable to start with. Increasing the salinity, the polarity, alienates non-polar molecules. This hypothesis assumes that subsurface water is still in a dynamic, wicking phase and is being depleted from the Martian regoth...I assume that is a known(?)

Could there be a similar process at Titan?

There is an interesting 'antilog' on Earth: The Great Salt Lake has been collecting and pickling organics (yes, including raw sewage), for many millenian. If the spring is exceptionally wet, the influx of fresh water 'unpickles' the sewage and the bacteria have a field day - releasing nearly explosive levels of methane and noxious aromatics.

For those without access to Scientific American, Sushil Atreya has made available an excerpt (5.9 Mb PDF) from the article. Of course, the most interesting part of the article is not in the excerpt.

You know I would offer up photochemical pathways for producing methane gas in the Martian atmosphere. We had a nice thread on this a year ago in UMSF.com, and a couple of good threads on this subject in space.com forums.

Personally, I favor the photoreductive pathways, which only require metal oxide dust catalysts (TiO2) on the surface or suspended in air, CO2 and H20 vapor. Such dusts are common on the Martian surface, and dust devils continually re-suspend fresh dusts. Read the prior threads for citations.

I do want to finally note that CH4 production on Mars is highest in the surface areas with the highest insolation - a result one would expect to see if CH4 was being produced by a photochemical pathway.

Moved this post to new topic called Local Methane Plumes On Mars