MainlyMartian

Sedna update

In a nice interview with Newsweek International's Fred Guterl, Mike Brown touches on Sedna's planetary status and talks a little about its surprising bright redness. Oddly he doesn't mention the most obvious explanation for a lump of ice being bright red: alien art. (Iceberg via Electrolite and YAWL, whose webmistress clearly remembers her John Varley stories...)

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.

Meteoritic methane

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

Here's a little more on the chances that the methane is produced by the breakdown of organics that come to the surface of Mars from space. On the back of my envelope, 10 parts per billion in the martian atmosphere looks like about 90,000 tonnes of methane. If methane lasts for three centuries in the martian atmosphere, then keeping 90,000 tonnes in the atmosphere means adding about 300 tonnes a year. That's a very rough estimate, but probably in the right sort of ballpark, to an order of magnitude. More than 30 tonnes, less than 3,000.

The best estimate I've been able to find of the rate at which organic material (meaning carbon bearing molecules) arrives at the surface of Mars is in a 1996 paper by George Flynn of SUNY-Plattsburgh. ("The delivery of organic matter from asteroids and comets to the early surface of Mars", Earth, Moon and Planets 72 469-474 -- not as far as I can see available online.) Most of the material that falls to Mars from space is in the form of little dust particles; based on the flux of these particles measured on earth, Flynn estimates that today the average amount of matter delivered this way is 8,600 tonnes a year. Some of that material will be heated up so much as it comes through the atmosphere that any organic molecules in it will be destroyed, but at least 2,400 tonnes a year should make it to the surface without having its organic matter blow-torched away. The amount of this dust made up of organic molecules is a subject of some debate, as I understand it; Flynn goes with 10%, which is apparently fairly mainstream. Thus he suggests that the amount of unaltered organic matter arriving on the Martian surface each year is about 240 tonnes.

(A neat aside from the Flynn paper: over the course of Mars's history 240 tonnes a year would add up to a trillion tonnes of carbon, which is roughly as much as is found in the earth's entire biosphere. This doesn't mean 240 tonnes a year is a lot -- it means that the planet is really incredibly old.)

If my calculations above are correct, a dustfall of 240 tonnes a year means that that organic matter would have to be converted into methane really very efficiently -- yields of between 10% and 100% -- if that dust were the source. That sort of yield sounds unlikely. But it seems undeniable that something happens to those organic molecules -- because when the Viking landers looked for organics in the martian soil that dust lands on, they found nothing. That's one of the reasons why, when one of the life detection experiments showed a life-like chemical response from the soil, it was discounted -- life in soil that has no measurable level of organic compounds in it seems highly unlikely. For an account of all this, see the section on Viking in Michael Caplinger's article here; for Gil Levin's minority view that life was detected, have a look here. (Technically, by the way, the stuff on the surface of Mars is regolith, not soil, since "soil" carries connotations of the possibility of growth, the presence of nutrients, etc. This is a technicality I am currently ignoring.)

The mainstream answer to what happens to the organic material is that it gets oxidised in some way by chemicals in the soil. (This is taken to explain the "life-like" response of the soil in that Viking experiment; fed with organic molecules the soil produced carbon dioxide, just as a lifeform would.) Oxidation is not something that produces methane, though I suppose it's conceivable that secondary reactions might produce a little. Another possibility is that the organics are smashed apart by ultraviolet light, with no need for weird soil chemistry. Experiments by Carol Stoker at NASA Ames seem to show that the harsh ultraviolet at the martian surface would be able to destroy organics at a rate that vastly exceeds that at which they could fall from the sky (abstract here). I don't know much about photochemistry, but it seems highly unlikely that this process would have methane as a primary product. Where would the necessary hydrogen come from? (See update below)

Another possibility is that the organics are not destroyed, but that instead they are converted chemically into a form sufficiently inert that it could not be detected by the Vikings' Gas Chromatograph Mass Spectrometer instruments (a spare part for which, incidentally, I was once shown by Jim Lovelock in his workshop -- it was one of the experiments he worked on when consulting with NASA). This is an intriguing idea, expounded by the wonderfully imaginative biochemist Steven Benner in an article in the proceedings of the National Academy of Sciences a couple of years ago (abstract here with link to full text). This possibility is undoubtedly interesting -- but Benner says it wouldn't give off methane.

So at the moment, it seems to me, the chances of the methane being made from stuff that comes to Mars from elsewhere looks pretty slim.

Presswatch: This story has now (Monday evening) been dealt with by much of the mainstream media in the UK, including the BBC. The PA wire service has a story that's been picked up by a number of papers; it includes the opinion of Colin Pillinger, who "said the discovery could be evidence of 'past life or even current life'. He said he believed the gas was unlikely to be the production of volcanic activity alone." There's a nice account at New Scientist that covers all the bases. In all honesty, though, there's not much out there that's not also here (though I'll admit other people are being more concise...) As far as I can see, the American press has yet to chime in, though Slashdot is on the case.

UPDATE: I've now managed to get a copy of the paper on Carol Stoker's experiments ("Organic degradation under simulated martian conditions", Carol R. Stoker and Mark A. Bullock, JGR 102 E5, pages 10,881 to 10,888 -- thanks Kara!) and it shows that, as I said, I don't know much about photochemistry. In fact there is methane given off as a primary product. This doesn't necessarily mean that's actually happening on Mars, though, certainly not at the rate necessary to explain the methane measurements. For one thing some or all of the breaking down of organics may be done by soil chemicals, not ultraviolet light. For another the organics in meteoritic dust are more graphite-like than the glycine used in the experimental set up, which might make a difference. As Carol and Mark Bullock point out, some of the organics might be tough enough neither to be knocked to pieces by ultraviolet light nor to be picked up by the Viking GCMS. Still, it's important to note that this is an abiogenic mechanism that might be producing some methane out of some of the stuff that falls from the sky.

Talking about terraforming

The first mainstream media reference to the methane story, as mentioned in the comments to the previous post, was Steve Connor's in The Independent on Saturday. One interesting detail is that he says the longitude band in which Michael Mumma thinks it's possible the methane is enriched runs over Meridiani. Meridiani is on the edge of one of the equatorial hydrogen-enriched area in the Mars Odyssey GRS data, which I think Mumma mentioned in his original poster at last year's DPS meeting. It will be interesting to hear more about this.

The second reference I've come across is in a half-page story in today's Observer by Robin McKie. (Note for non Brits puzzled by why this link leads to the Guardian's website; the Observer is a Sunday paper owned by the same trust as the better known daily paper, the Guardian; though the editor of the Guardian is also the executive editor of the Observer, in print the two papers have distinct if compatible identities. Their online presence, though, is all under the Guardian brand.)

Robin's story mentions the methane in passing (strangely, he says it was "announced yesterday", by which I assume he means that Steve ran a story on it on Saturday), but for the most part it's a curtain raiser for Tuesday's terraforming debate at NASA Ames, which will be part of the Third Astrobiology Science Conference that starts today. The debate features Chris McKay of Ames, Mars Underground veteran and lead author of the terraforming paper that made it to the cover of Nature a decade or so ago, John Rummel, NASA's head of planetary protection (meaning he worries about bacteria on spacecraft, both outward bound and incoming), Jim Kasting, an expert on planetary atmospheres and their greenhousing potential, and Lisa Pratt, an expert on, among other things, underground microbes. (It will be obvious to anyone who's read the last few posts that putting these people on a stage and not asking them to talk about methane would be a crime; Jim Kasting has speculated about biogenic methane as the missing ingredient for a strong greenhouse effect in the planet's past; Lisa Pratt really knows about the kind of bugs that might do this stuff; John Rummel is alive to the way that planetary protection issues might change if you really believed their were microbes on the other planet; and Chris was one of the authors of the first paper to discuss a deep biosphere persisting on Mars today.)

They'll be joined by a trio of science fiction luminaries: Kim Stanley Robinson, Greg Bear and Sir Arthur C. Clarke. This reflects the fact that the event's being cosponsored by the new Experience Science Fiction museum and hall of fame being set up in Seattle by Paul Allen. Greg Bear's the chairman of Experience, Stan and Sir Arthur are on its board of advisors and the wonderful Donna Shirley, who's going to chair the terraforming debate, is its director. (Donna ran the Sojourner rover project and was then head of the JPL Mars Exploration Program from 1994 to 1998).

Given this, it's a bit of a pity that Robin's article takes a classically dismissive attitude to science fiction: "The idea -- known as terraforming -- sounds like science fiction. But turning Mars into an Earthly paradise is being taken seriously by increasing numbers of researchers." The implication is that the fact that terraforming is being taken seriously by increasing numbers of researchers (something I'm not sure I believe) means that though it may "sound like" science fiction it must be something else, since science fiction is by definition not serious. So let's be straight about this: terraforming doesn't just sound like science fiction. It is science fiction. It's a term invented by the science fiction writer Jack Williamson in the course of writing science fiction about themes that only science fiction was then concerned with. It has become a significant trope in the genre, and for fifty years or so it has dominated the fiction devoted to Mars. This doesn't preclude its consideration by serious scientists (or, more accurately, by practitioners of serious science); it was the pre-requisite for that consideration.

The problem is that saying all this goes against the fairly constrained forms in which it is considered permissible to talk about science fiction in a newspaper or magazine science story. When you invoke science fiction, you have to make an explicit opposition between it and science proper, thereby keeping it in its place. You can say "It sounds like silly old SF, but it's not", and you can say "science stranger than any science fiction" (a claim which normally just shows the writer hasn't read much SF, and certainly not much of the stranger variety). The more complex reality, in which science fiction and science are continuous as well as opposed, partaking in strange and multilayered conversations about what the world is, might be, should be, shouldn't be and isn't, is just not something it's felt that the reader wants to face. And that's a pity.

Robin goes on to quote from Paul Murdin, a very distinguished astronomer and space science administrator whom I like a lot. Paul's take on terraforming also depends on the "great divide": "If it was just a silly science-fiction notion, you could laugh it off. But the idea is terribly real. That is why it is dreadful. We are mucking up this world at an incredible pace at the same time that we are talking about screwing up another planet." Leaving aside the dissing of my beloved SF, though, this simply seems both silly and self-contradictory. It's silly because of the many things that terraforming may be, "terribly real" just doesn't seem like one of them. Vast sulphur hexafluoride plants on Mars (and the strip mines to feed them) are not something a civilization that strains its capabilities just to send golf buggies there is going to be trying anytime soon. It's self-contradictory because if you do think that the threat to Mars is terribly real, isn't that exactly why we should be talking about it? Isn't discussion our friend here? Isn't the gist of a discussion on Paul's terms highly likely to be "Should we screw up another planet?" "On balance, no"? Aren't the worst things we tend to do, including the things that are mucking up this planet, the ones we don't discuss before we get started?

Lisa Pratt is then quoted as saying "It is very depressing. Before we have even discovered if there is life on Mars -- which I am increasingly confident we will find -- we are talking about undertaking massive projects that would wipe out all these indigenous lifeforms, all the strange microbes that we hope to find buried in the Martian soil. It is simply ethically wrong." Again, this seems to me to confuse the talking with the doing. Wiping out some Martian bacteria, it seems to me, is probably ethically wrong and certainly scientifically disastrous; talking about modifying the Martian environment is not. As Chris McKay will undoubtedly point out at the debate, the Martian bacteria, if they are there, are not doing terribly well, and a bit more warmth and running water would probably be welcome. Large amounts of oxygen might not be such a good idea, but a) not all people entertaining the idea of changing the martian environment want to build up oxygen there and b) getting serious oxygen into the system is a multi-millennium idea.

For a more temperate run through some of the ethical issues, this article by John Carter McKnight is a good place to start. It also points out that though the "terraform at all costs, martian life has no worth" position feared by Paul Murdin and Lisa Pratt is a pretty marginal one, some form of terraforming has support in some forums. That's why debates are a good idea. That said, one of the topics such debates do need to discuss is why science fiction has not presented us with any serious terraforming-free narrative of the Martian future. (A partial exception is Aldiss and Penrose's "White Mars"; but its opposition to terraforming shapes the book so much that it's more a negative of a terraforming novel than an independent alternative.) It's probably true that it would be easier to have a really good debate if the imaginative space provided by the "unacknowledged legislators" of science fiction were a bit broader, and it would nice to have some thought on why it isn't.

The thing that's interesting me most about terraforming at the moment, though, is not the ethical dimension (undoubtedly important, but neither fresh, at least to someone who's tracked these things, nor pressing) but the practical dimension. With ESA showing that there's relatively little frozen carbon dioxide in the southern polar region, and the Mars Odyssey GRS showing that the pore spaces in the high latitude regolith are full of ice and thus can't contain a huge amount of carbon dioxide, quick early warming looks much harder for terraforming to achieve. The idea -- one which dates back to Carl Sagan in the 1970s -- that there is an easily triggered positive feedback in which a little warming releases more carbon dioxide which in turn leads to more warming which in turn leads to more carbon dioxide depends on there being a fair amount of accessible carbon dioxide to flush out into the atmosphere. It may be that there just isn't. (Martin Fogg has been arguing this way for some time, as this paper on his excellent terraforming pages shows.) The idea that Mars cycles through climatically distinct "ice ages" also suggests that its current climate is not, as some terraformers wanted to believe, poised in a metastable state where just a little push would set vast changes in motion. The modelling result (discussed in this post) that a thicker atmosphere leads to a lower possibility of liquid water at the surface also seems a bit of a blow to terraformers, though since liquid water might have formed a sink for the liberated carbon dioxide, it may be that the less water there is around to begin with the better.

The broader point, though -- and my apologies for reaching it so far down the post -- is that this all seems decreasingly relevant. To some extent, I think, the terraforming debate was a creature of the eighties and nineties. This is why I tend to doubt the idea that "increasing numbers of researchers" are taking the time to think seriously about terraforming. They're too busy with Mars as it is to work hard on ideas about how to engineer its future. As far as I know, no-one has looked at the sulphur-and-halogen rich rocks of Meridiani and cried out with glee "wow - raw materials for super-greenhouse gases".

Ideas about terraforming flourished at a time when there seemed a fairly solid model of Mars based on the Viking data, and people interested in Mars had the time to engage in thrilling thought experiments on the basis of that model, with its carbon dioxide southern cap, and buried aquifers, and relatively simple warm-and-wet past. Now there is a torrent of new data and no time for thought experiments. The new Mars isn't semi-static, as the Viking Mars was for a decade or so. Our understanding of it is in constant flux. And so, we are beginning to see, is the planet itself. You don't have to imagine climate change -- we are seeing its effects right now. You want a changing planet? We have one beneath our eyes

(Musical note: while I've been writing this, iTunes has randomly thrown up both Joe Jackson's "Sunday Papers" and the Divine Comedy's cover of Bowie's "Life on Mars". What a fine thing coincidence is.)

Updates: In the original post I got Donna Shirley's career history wrong. It's now been put right. The Pathfinder mission was managed by Tony Spear. And Greg Bear points out that in "Moving Mars" no-one terraforms the planet in any normal sense. But given the title it's not much of a spoiler to point out that they do move it, which Greg admits might seem a little intrusive...

Yet more on methane

This morning I spoke to Vittorio Formisano, the principal investigator on the Mars Express PFS, about the methane results. (If you're just coming to this topic, best take a look at my first post about Dick Kerr's subscription-required online report for Science, available in a slightly fuller form here now the magazine has been published).

Of the follow-up strategies, the one Formisano says he is pursuing most keenly (given the constraints of time and the fact that there is a lot of other work for PFS to do) is localising the source. PFS averages out the atmosphere, taking data pretty much continuously when the satellite is near Mars (Mars Express has a quite eccentric orbit, swinging from 11,500km away to just 260km away and back on each orbit). So, because the orbit is south-to-north, at every close approach PFS sees a swathe of the planet’s daytime side along a given line of longitude. Formisano is currently looking to see if some longitude bands provide a stronger methane signature than others, as a first step towards seeing where the methane is coming from. He gave the impression that it is possible there might be some preliminary findings on this to report by the time he gives his invited talk at the European Geophysical Union's Nice meeting at the end of April. He also said that he understood ESA would be putting out a press release around then.

My guess would be that a source localised in one (or more) bands of longitude would favour methane made by life or something like volcanism over methane made from meteoritic debris, which you'd expect to find scattered evenly all over the surface (see previous post). Meteoritic organics might behave differently at different latitudes -- different temperatures and sun intensity could effect their stability, I suppose -- but I can't see why they'd care about longitude.

Another possible follow-up strategy we talked about is looking for other volcanic gases that you'd expect to see if the methane was coming from a geothermal/volcanic source. Formisano thinks this may well be a good avenue to pursue, but hasn't had the time to start work on it as yet.

He says he is pretty much agnostic as to the source, but seems to have a certain sympathy for volcanism. As he points out, Mars has a long, proud history of volcanism. It would be a bit strange to think that this history, having endured for four and a half billion years since the planet's creation, would have come to a complete stop just before we arrived and started taking measurements. A very small amount of volcanism might be an a priori expectation for Mars, and might explain what's being seen. It would also (this is me, not Formisano, though I've no reason to think he wouldn't agree) argue for pockets of warmth and wetness in the crust.

As it happens, Formisano went on to point out, one of PFS's original objectives is to look for hotspots on Mars that might be associated with geothermal activity, so it can tackle both ends of the question. (Yesterday I asked Phil Christensen of Arizona State University, who's the principal investigator on NASA's two orbiting infrared instruments, TES and Themis, whether his data was able to put any limits on current hotspots. He said they hadn't seen anything clearly geothermal as yet, but that there's a lot of variability in the night-time Themis data that could either be internal heat or something explicable through surface properties -- different materials having different cooling rates, and so on. Phil says that looking for geothermal heat with Themis is a high priority for him -- but with two orbiting instruments and the mini-TES instruments on Spirit and Opportunity to deal with, being a high priority doesn't necessarily get something done right away...)

Looking for hotspots is something PFS will be able to do particularly well after mid-May, when Mars Express's orbit will be shifted so that it is closest to the planet in the nighttime, rather than the daytime. That shift is also good for the radar, Marsis (which is why it's being done), and I imagine that Marsis might pick up any subsurface water associated with a hotspot. The orbital shift means there will be no close-up observations with the HRSC camera for a while, though, and it means that PFS will stop doing methane measurements for a while.

On another track, I think it's worth pointing out (and I should have done so before) that none of the work on detecting methane I've been talking about here has yet been published in a peer-reviewed journal. The PFS results came out via a press conference, and have not yet been submitted for publication. Michael Mumma and his colleagues presented a poster at a conference last September and have since been working on a piece for peer-reviewed publication that will include some new observations; I don't know if it's been submitted yet. Vladimir Krasnopolsky and his colleagues have provided an abstract of their work in advance of presentation at the EGU meeting in Nice, and I would guess they've submitted it somewhere but I don't know for sure. Making this clear isn't a way of saying the data's no good. People produce abstracts and speak at conferences and sometimes give press conferences before peer review all the time. It’s the way things are done. But as I said, I think it’s worth noting simply because in general I think science writing should take note of such things.

More on methane

(This probably won’t make too much sense if you read it before the original post, "Methane (and thus life?) on Mars")

First, some more details on non-biological sources.

I asked for a little more info on methane coming out of Martian volcanoes from John Eiler, a geochemist at Caltech who’s thought a lot about these things, and he generously sent a full and very interesting reply. He says his best bet for the amount of methane associated with martian volcanism -- based on how oxidized martian rocks seem to be, and on the elemental ratios in the martian meteorites -- would be that lava at the surface, or magma injected into sub-surface intrusions, is probably significantly richer in methane on Mars than it is on earth: between 0.1% and 1% methane by volume, “with a less plausible upper bound of up to 10%”. This may mean that a fairly moderate amount of lava or magma giving up the gas dissolved in it could pump out the observed low level of methane. In that case, the methane would not mean life, but would mean active geothermal systems with lava/magma.

As I understand him, Eiler notes that water vapour, carbon dioxide, carbon monoxide and hydrogen would all be more abundant in the lava than methane, which means that if the methane comes from volcanism, there should be other shortlived gases present in the artmosphere as well, such as carbonyl sulfide, carbon monoxide and “various other sulfur gases and hydrocarbons”. Obviously these would provide new targets for spectrometers to seek out.

He also points out that there were other ways to get methane out of rock, without the benefit of bacteria. Water rich in carbonic acid (H2CO3 -- what you get when you dissolve carbon dioxide in water) running over or through iron-bearing minerals in which the iron isn’t fully oxidized may oxidize those minerals, giving off methane (and hydrogen) in the process. “These reactions,” Eiler says, “are believed to be a significant source of methane in gas seeps from the Canadian shield, parts of Indonesia, and seafloor hydrothermal vents, and it is quite plausible that they generate large amounts of methane in the martian crust.”

If this is actually going on without any bacteria to help it, and constitutes a source large enough to account for the methane that has been observed, then the methane would not be evidence of life. But it would be evidence for quite copious amounts of water in the martian crust today; that’s a pretty good consolation prize, it seems to me. (It would also show that what are called redox reactions are going on in the crust, and those are the sorts of reactions that life uses to get its energy)

It would, I imagine, be possible to tell the hypotheses apart if you were on Mars with a good mass spectrometer. If martian bacteria were anything like earthly bacteria, they would be choosy about carbon isotopes, using the lighter form of carbon rather than the heavier form. Reactions between acidic water and iron-bearing rocks, though, will not care about the weight of the carbon involved. So biogenic methane (methane made by living things) can be expected to weigh less than methane made by any other process. Beagle 2’s Gas Analysis Package would, I think, have been able to tell light methane from heavy methane. I don't know whether any instrument currently slated for Mars landing can do this with methane present only at about ten parts per billion. Maybe the TEGA gas analysis instrument on the 2007 Phoenix lander.

(It is obviously not very likely that, if the biological methane production process on Mars was due to some form of life completely unlike earthly methanogens, they would change the carbon isotope signature in the same way. But I think it’s a fair assumption that any living process would fractionate the methane to some extent, one way or another, and so produce methane with a different isotopic balance to that seen in the planet’s carbon dioxide reservoirs.)

A possible non-biological source suggested by another expert who's thought a lot about detecting life on Mars is that the methane might be due to sunlight breaking down carbon-bearing molecules that have been carried to the martian surface by meteoritic dust. The back of a friendly meteorite expert’s envelope suggests 3,000 tonnes of organics might be delivered to Mars every year in meteoritic dust, and the back of my (considerably less reliable, and likely to be wrong) envelope suggests that only 30 tonnes would have to be turned into methane every year to maintain the 10 parts per billion that seem to have been detected in the atmosphere. I have no idea whether that yield is plausible or not, or what other chemicals might be given off at the same time that we might have a chance of detecting. So far, this explanation seems the least exciting, since it keeps the whole process on or above the surface, and thus has no implications for heat or water below, let alone life. That clearly doesn't mean it's wrong.

Finally, here’s some information on earthly methanogens from phylogenomics wizard Jonathan Eisen of the Institute for Genomic Research.

All known methanogens so far are members of the archaea – the kingdom of single-celled microbes that look very much like the “prokaryotic” bacteria we’re all more familiar with, but which are genomically quite distinct. That said, Eisen warns that properties previously considered unique to archaea have now been found in prokaryotes, too, and says he thinks it “reasonably likely” that methanogenesis will eventually be found in bacteria. As I understand it, though, the presence of archaean traits such as the halorhodopsin genes in bacteria is normally taken as evidence of horizontal gene transfer between species (or in this case kingdoms), and most people (maybe not Jonathan) probably do think that methanogenesis arose originally among the archaea.

When people make family trees for the archaea, methanogens tend to turn up on a number of different branches, suggesting that the ability to make a living by making methane evolved on a number of separate occasions. However, according to Eisen, in some of the most recent trees they appear to be collecting together into one subgroup, which might mean it was something archaea were able to do from quite early on.

As to whether archaea on earth might find Mars at all hospitable, Eisen says that, as far as he knows, no one has yet identified methanogens adapted to very cold conditions or to high salt concentrations, though he wouldn’t be surprised to find some cold-adapted methanogens. Some acid tolerant methanogens have recently been described, and acid tolerance might well be handy on Mars.

That’s about it for the methanogens, as of now.

I hope to have more on various aspects of this tomorrow.

Methane (and thus life?) on Mars

Update: this was my first post about methane: for a fuller version of the story you might want to try this post written a week later

Dick Kerr of Science magazine, who's been writing planetary science a good bit longer than most of us in this game, has a remarkable story up on the Science Now site -- something potentially far more striking than the crossbedding announcement. The team on the Mars Express Planetary Fourier Spectrometer (PFS) has announced the discovery of what look like methane absorption lines in the Martian atmosphere at 3.3 microns. Kerr quotes the PFS principal investigator, Vittorio Formisano saying it's "A very little amount," -- 10.5 parts per billion -- "but the result is clear." If this is indeed methane, then it's evidence that something is going on: either volcanic activity or life.

Methane is not a stable molecule in the Martian atmosphere. Left in the sun it will fairly quickly react with hydroxyl ions in the atmosphere; estimates suggest that it has an atmospheric lifetime of a few hundred years. And while some chemicals can be made in one part of the atmosphere and used up in other parts, methane isn't one of them, at least not to any appreciable degree. So if there is an even moderately substantial level of methane in the Martian atmosphere, then something on or below the surface is putting it there. Kerr mentions two possibilities; volcanic activity or bacteria. Methane in the atmosphere suggests that one or other of these processes is going on now. (A third possibility comes from above, not below: the recent impact of a comet, since comets contain methane. But there doesn't seem to be any sign of the very fesh looking crater that would be associated with such an event in any of the datasets, and it seems unlikely that a comet small enough to be destroyed by Mars's thin atmosphere, and thus not leave a crater, would be big enough to leave a global methane signature for any length of time.)

On the earth, methane from rocks is for the most part methane that derives ultimately from life -- natural gas, and the like, formed from the bacterial decay of organic matter. There is apparently also, though, a very low level of methane from non-biological stuff, which I believe is thought to be methane left over from the planet's formation. (Some people think there are vast amounts of this primoridal methane, and there would be on Mars, too. But most don't, and I'm not going to get into that here.) The average outgassing flow of methane from the martian depths expected by most people would be a lot less than the flow from the deep earth, and it would not be able to put substantial amounts of methane into the atmosphere. But during active volcanism, or other geothermal activity, the flow would be higher. So that's a possible source.

If it's volcanoes or similar sources, though, we have to wonder where they actually are. Nothing that looks like an active volcano has been seen. And even if the activity were more subtle and less splashy -- the injection of lava below the surface somewhere, say -- it's hard to see how it would avoid giving off heat that TES on Mars Global Surveyor or Themis on Mars Odyssey would have picked up. A small and not very warm spot of geothermal energy might escape TES, which divides the surface up into fairly big parcels. But with a resolution of 100 metres in the infrared Themis should be able to pick up such things, and so far it hasn't, even though it's taken infrared data on large parts of the planet.

The other obvious possibility is life. On earth, almost all the methane in the atmosphere is produced by bacteria, specifically methanogenic archae. These are anaerobic organisms, which would suit them to Mars. Their metabolisms depend on making methane from carbon dioxide and hydrogen (the hydrogen itself sometimes a product of other bacterial life). These have long been seen as the most likely forms of life to be found on Mars.

It would be a jump to go from a single announcement of a single piece of evidence from a single instrument to assuming life on Mars. But there is not just one piece of evidence. As Bruce Moomaw pointed out in the comments to this post (which made some remarks about the role of methane and life early on in martian history), Michael Mumma of NASA's Goddard Space Flight Center presented a poster on methane on Mars at the American Astronomical Society's Division of Planetary Sciences (DPS) meeting last fall. The poster (abstract here) presented results from observations of the Martian atmosphere using two different telescopes, NASA's Infrared Telescope Facility in Hawaii and the international Gemini South observatory in Chile. These showed some evidence for methane absorbing infrared light in the 3.3 micron wavelength, which is what the PFS observations show, too. Mumma and his colleagues have since refined their analysis of the data from Gemini, and taken more data from IRTF. They have also taken data using the NIRSPEC spectrometer on on of the vast Keck telescopes, but not yet reported results from those observations, as far as I know.

In the DPS poster, Mumma and his colleagues note that in one set of observations the strongest methane signal seems to come from the equatorial regions of Mars where the GRS has shown surprising amounts of hydrogen, and thus possibly water, in the surface. That fits the possibility of life quite nicely.

In his piece on Science Now, Dick Kerr quotes Mumma as saying that the PFS result "bears additional confirmation" -- which I assume the work he currently has in hand might provide. (That's if the PFS data don't come to be seen as confirming the initial observations he and his colleagues have already reported). In an email to me earlier this month, Mumma pointed out some of the difficulties of observing at these wavelengths when, as is the case for PFS, you have a relatively poor ability to make fine distinctions between wavelengths; ozone, water vapour and carbon dioxide can all complicate your results. The big spectrometers on earthly telescopes make such fine distinctions rather better. But he also said the PFS team was expert in dealing with these issues, and that he was "guardedly optimistic" they'd be able to measure methane were it there.

As Dick also mentions, there is another set of observations. Vladimir Krasnopolsky and his colleagues have been using the Fourier Transform Spectrometer at the Canada-France-Hawaii Telescope, and they too have detected evidence of methane on Mars. They'll be presenting it at the European Geophysical Union's meeting in Nice next month. Their results are right in line with the PFS measurement -- 11 parts per billion. In their abstract (pdf here) they argue that the best explanation for this is very sparse bacterial life on, or rather in, Mars. With three independent sets of observations, and with no evidence of current geothermal activity, that looks like the best explanation to me as well.

Nor need the life necessarily be quite that sparse. While some bacteria produce methane, others eat it up. If I were a martian ecosystem living in the shallow or deep subsurface, I would want to lose as little methane as I could, and so I would have layers of methane eaters above the methane producers, keeping the net flow to the atmosphere as low as possible. (Yes, that's an absurdly anthropomorphic way of putting it. So sue.) Earthly bacteria are very good at organising themselves this way.

Another possibility is that the methane output might be sporadic; the amount given off might change, for example, with the cycle of the Martian ice ages, or some other ongoing climate change. We might be seeing fallow-period methane levels. It's also possible that we are seeing methane that has been in storage for some time. This could conceivably be the methane from the martian equivalent of natural gas, laid down as biomass and stored in a deep geological formation for hundreds of millions of years ago. That seems unlikely, though -- you need a really productive biosystem to lay down vast fossil fuel reserves (probably one that photosythesises, too) and there's no evidence that anything like that could have been going on on Mars in the recent past. It would basically mean that there would have to have been an earthlike biosphere on Mars in the past 500 million years, and that just didn't happen.

A more plausible reservoir that might have stored up methane in the past would be the ice. Methane can be stored in water ice in the form of something called a clathrate. This is what happens to quite a lot of the methane produced by earthly bacteria -- there are methane-storing clathrates scattered all over the sea floors of the earth. These clathrates may be a potential source of trouble in a greenhouse world; they may also be a potential energy source. Martian ice might be a methane reservoir, storing it up under some circumstances, releasing it under others. So if methane production on Mars was spasmodic, methane stored in clathrates at the time when it's being made might be released from them more slowly later on.

That's probably enough speculaton for this first post. Except to note that even if the methane is coming from a gethermal source and not from bacteria, that itself would be big news. And it would have implications, too. If there's geothermal energy being pumped into a Martian crust which we now know to contain a lot of water ice, then that means there is almost certainly some liquid water down below. So even if the methane is not evidence of bacteria, it sort of has to be evidence of a wet and therefore habitable niche on Mars right now.

On the beach

So today's announcement at NASA HQ was pretty much what I (and a lot of other people) expected, given the specialities of the people on the panel doing the announcing. There's evidence of cross bedding in the thinly layered sediments that Opportunity has been studying, and the size and shape of the cross bedding makes it look pretty conclusive that the sediments were laid down in water. They're seeing ripples from the floor of some sort of shallow pond or lake or sea. How shallow is not clear: "up to your neck or up to your ankles? I don't think we have a good answer to that right now." But it was shallowish. How large an area the water covered is not clear, either, but if all the haematite seen from orbit is in the form of "blueberry" beads that have been eroded out of this sort of rock then there was once a great deal of it. (That said, it's not a done deal that all the haematite comes from rocks like these.)

Again, the take-home is that that there was a habitable environment on Mars a few billion years ago. In theory, there could be microfossils -- fossilised bacteria -- in the very rocks that Opportunity is focusing on, evidence that the environment was not just habitable, but inhabited. Unfortunately, as Steve Squyres explained, the MER instruments are not sharp eyed enough to discern such microfossils. So though Ed Weiler, NASA's science chief, enthused that "If you have an interest in searching for fossils on Mars this is where you want to go", you can't do that hunting with this rover. You should be able to do it with the next generation rover that is slated to be launched in 2009, though, and as Weiler went on to say, Meridiani now looks like the prime landing site for that mission. (How the science community will feel about such a strong steer from the boss on what could be a hardfought decision has yet to be seen.)

The question of whether the water was actually a blue-water sea is still open. It could have been more marshy or pondy, at least some of the time (the earth analogue showed by Dave Rubin of the USGS, which I think was from the Trimed region of Tibet, was dried out ripples covered by a salt crust a very long way from any sea). And it could have been frozen. Steve Squyres outlined the intriguing possibility that ice, rather than evaporation, might account for the high salinity. Imagine an ice covered lake getting colder and colder. As the ice cover gets thicker, the residual water at the bottom gets more and more briney. So ice could make the rocks salty - they don't have to have sat around in the sun having their water evaporated out. The fact that Steve mentioned this makes me think he believes there could have been enough water movement under the ice to account for the rippling seen in the cross beds.

The next step is to take Opportunity to the lip of nearby Endurance crater, where it should be able to see outcrops of the same rock ten or a hundred times thicker, thus putting together a better picture of what was going on and how it changed over time. (According to some estimates, based on looking at outcrops all over Meridiani, the layer of yellowish rock Opportunity has been looking at is in fact about 800 metres deep; the stuff looked at so far is near the top of this column.) Various people on the panel also mentioned that this sort of sulphurous sedimentary rock would be easily spotted from orbit by the CRISM spectrometer on NASA's 2005 Mars Reconnaissance Orbiter. They didn't mention that much the same science, at a slightly less sophisticated level of resolution, can be done by the OMEGA instrument on ESA's Mars Express -- indeed the OMEGA team is probably doing it right now.

No time for more right now -- I have to go and cook. But there is one last thought. Steve Squyres was adamant that the rovers couldn't see microfossils. But those aren't the only fossils that microscopic organisms leave. In the shallow water of Shark Bay, in Australia, there are decidedly macroscopic bacterial colonies called stromatolites, fossils of which are found through much of the earth's early history. On the bottoms of permanently ice-covered antarctic lakes -- lakes I'm pretty sure Steve has swum in -- there are extensive mats built by microbes. Fossils of things like that, I'd have thought, the MER cameras would be able to see. Whether they'd be able to recognise them, of course, is a different matter. It might be a hard call, one you wouldn't want to even raise the possibility of until you had to.
NB, haematite is also known as hematite

Arizona in the sky (with water)

So we're waiting for the latest news from Opportunity at the big press conference later today. My guess is that it's about the setting for the water and that they'll argue that it was a big old lake or sea on the basis of the sedimentology, a guess based mainly on the presence of sedimentologists John Grotzinger and David Rubin on the panel; Rubin is an expert on the cross bedding structures in sedimentary rocks that reveal currents, wave action and the like. (If it was about life they'd have Andrew Knoll on the platform). Since people have been kind of assuming that the rock was laid down in water, rather than altered hydrothermally, for a while, the announcement may not come as a great shock. As I suggested before, it would be nice to know whether the water was ice-covered or not, and evidence of wave action might settle that. But even there the chemical similarities to evaporite sequences Ben Clark talked about at the last big announcement seem already to have edged us towards the open water idea. (I say this because it's not obvious to me how an evaporite could be formed under ice; but then, on earth, evaporites are diagnostic of high temperatures anyway, so it wouldn't occur. Mars may have tricks that make it possible. And if it *was* obvious to me how these things worked, then it would all be much much less fun.)

Meanwhile Spirit is taking a poke at a more interesting looking rock than any it's seen so far, inasmuch as I can tell. It's quite big, perched on a crater rim and looks to have an unusual texture, which all sounds good.

This new rock has been named Mazatzal, after the mountains in Arizona (which, it must be admitted, are prettier). People are always seeing Arizona on Mars, and vice versa, adding romance, misconception and relevance to our view of the planet in various proportions over the years. This predilection that was the subject of the aptly named John Carter McKnight's thoughtful talk entitled "Barsoom's legacy: Mars Through the Lens of the American Southwest", given at the Contact conference held at NASA Ames ten days ago. Although I was only able to be there for one day, I thought it was a terrific meeting, with many good talks and some great visual presentations. It was particularly nice to see Kees Veenenbos (check out his lovely topographically correct renderings and animations), who provided some gorgeously frosty art for my National Geographic piece about ice ages, as well as the cover. If today's announcement does talk about open water in Meridiani, here's Kees's view of it.

And as a Mac user, I can't say for sure how good the Mars globe software you can download from Geofusion is, but when Chuck Stein demo'd the full-up version it was stunning (this too has a National Geographic link -- it was developed for a display in the society's Hall of Explorers).

Update: Rupert Goodwins, who knows about such things, says in the comments that the Geofusion Mars download is very good indeed, if you have the right graphiocs cards and a lot of memory

Is Pluto a planet?

Since the question is raised again by the discovery of Sedna, and since the topic here is only MainlyMartian, I thought I'd post an article I did for the International edition of Newsweek a few years ago, with some very minor polishing. (I'd link to the published version, but it doesn't appear accessible). Nothing much seems to have changed -- nor will it, I expect, until a TNO larger than Pluto turns up.

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"The Little Planet that Couldn't"
Newsweek International, February 26th 2001

We live on one, we've been observing a handful of others since antiquity, we've sent space probes to almost as many as can be reached and we're currently discovering new examples around distant stars at a staggering rate. So it might come as something of a surprise that we don't really seem to know what a planet is. But we don't. The recent decision by the American Museum of Natural History to question Pluto's right to be called a planet has reopened a fascinating -- and instructive -- can of worms.

When Pluto was discovered by Clyde Tombaugh in 1930, there wasn't much doubt about it being a planet. Percival Lowell, the man who had built the observatory where Tombaugh worked, had predicted that there should be a ninth planet beyond the orbit of Neptune. Tombaugh had found something out there. What could it be other than the long sought planet? After all, the only objects in the solar system were planets (big things that orbit the sun), moons (things which orbit planets), asteroids (little stony things, mostly to be found between the orbits of Mars and Jupiter) and comets (little icy things that spend most of their time further away and which, unlike asteroids, develop a gaseous "coma" and tail when they pass close to the sun). With that menu to choose from, Pluto could only be a planet.

Today things look less clear. Since 1992, astronomers have been finding big lumps of ice in orbits very similar to Pluto's: almost 400 have been discovered to date, some as large as the largest asteroid (though in composition they're much closer to comets), and larger ones almost certainly remain to be discovered. Some say that Pluto should properly be classed as the largest of these "Trans-Neptunian Objects", or TNOs, rather than as a planet proper. There is precedent for such a move. When Giuseppe Piazzi discovered something orbiting between Mars and Jupiter 200 years ago, it was proclaimed a planet; as it became clear that there were a lot more of these objects in similar orbits, though, Piazzi's discovery was downgraded. Ceres is now seen as the largest of the asteroids and nothing more. Asteroids and comets are referred to as minor planets in some circumstances: but that doesn't mean anyone thinks of them as planets strictu sensu.

To many astronomers this is heresy. Pluto was one of the planets they learned as schoolchildren. It's undeniably larger than any other TNO so far discovered. It has great scientific interest -- and it is the only planet to have been discovered by a great American, to boot. (Pluto's demoters believe this fact is a strong, if unvoiced, factor in the Pluto-is-a-planet arguments championed by, among others, the American Astronomical Society). Changing things would be needlessly confusing to the public -- and possibly to Congress. Planetary scientists have long wanted NASA to send a mission to Pluto, but the plans have been repeatedly deferred and cancelled. A current last ditch effort at a shoestring mission is underway, and it needs all the PR it can get. Having its target demoted from its planetary status might not be helpful.

Though heartfelt, these arguments are not particularly strong. Since the Voyager missions revealed the extraordinary faces of the major moons of Jupiter, Saturn and Neptune (all six of them larger than Pluto, as is the earth's moon) no-one has been able to pretend that planets are uniquely attractive heavenly bodies; Pluto doesn't have to be a planet to be interesting. And the size question cuts both ways. Yes, Pluto may be larger than any other known TNO, but it is very much smaller than Mercury, the smallest of the eight echt planets.

As a matter of practical politics it might indeed be better to wait five years or so. By then, either a mission to Pluto will already be underway, or the once-in-many-lifetimes opportunity to mount one as good as is currently possible will be gone. For most of Pluto's two and a half century long year, its thin atmosphere, having frozen, lies on its surface. Only during its brief few decades of summer is it really worth visiting. If you think a thin envelope of gas that only appears during Pluto's closest approach to the sun makes it sound like a comet, I think you have a point. If NASA's mission to Pluto gets cancelled, Pluto will look more cometary still -- NASA has a long history of cancelling comet missions.

There is a more principled argument for allowing Pluto to stay a planet --though it is unpalatable to many astronomers. Pluto is big enough, which means it has enough gravity, to have pulled itself into a spherical shape. This differentiates it from most comets and asteroids, which are oddly shaped, and implies that it has a complex, layered internal structure. A definition of planets as gravitationally collapsed, chemically differentiated non-luminous structures would let Pluto in. But it would also let in many moons, some of the largest asteroids and quite a few of the TNOs. On this definition, the total number of planets in the solar system would rise to 30 or more: an exact tally would involve close-up looks at various asteroids and TNOs.

This is the sort of possibility that seems to worry some astronomers. They think that people are comfortable with nine planets, and don't want any more -- and that the public will think less of them if they start promoting and demoting bits of the solar system. But this is wrong. The members of the public most interested in such matters -- schoolchildren -- will be quite happy with more planets. They can keep a couple of hundred pokemons straight in their heads: a couple of dozen planets shouldn't present a problem. More important, if the denizens of the solar system are more rigorously categorized according to their natures, rather than their sizes and positions, we'll be in a better position to talk about the denizens of other planetary systems as they are discovered. But most important of all, seeing scientists change their minds is an important lesson in and of itself. It reinforces that scientific ideas are often provisional and that new data changes the way things are viewed. It shows that science is about self-criticism and growth, about being open to the new ideas a serious study of the universe will always bring with it. And that's a lesson worth learning at any age.

Update: Alan Stern provides a more detailed account of the "if it's collapsed into a sphere it's a planet" argument on Spacedaily (though his daughter seems to gainsay my point about schoolchildren). And this column from Ray Arvidson's hometown paper is cute.