From November 25-27, 2009, I attended a workshop on the topic of "Methane in the Mars Atmosphere", held at the site of the European Space Research Institute ESRIN in Frascati, Italy. This is my summary of what went on there and what I learnt.
First of all, I have to tell you that I am not a scientist. I am a spacecraft engineer and a mission analyst, and I have worked on more space missions than I care to remember. A major portion of these were missions to Mars. Not all of these missions actually happened - it's normal in the space research business to study dozens of missions, out of which, if you're lucky, two may proceed any further than the initial planning stage. That's the way it goes, like it or not. I do know a fair bit about the science, though obviously not nearly as much as someone who is active in the science proper. But perhaps my knowledge makes up in breadth at least partly what it lacks in depth.
I also have a German blog, and mostly, my English articles are more or less close translations of the original German ones. Not this one, though, because I wrote the German account of the workshop as a kind of diary, during the conference breaks. The present English article however is more like a digest of the salient facts, or rather, my interpretation of them. Feel free to point out any errors to me, I will gladly correct them.
So, what's it all about then?
The title says it all. The issue is methane on Mars, or more precisely: methane in the Martian atmosphere, because it because it hasn't yet been found anywhere else than in the atmosphere. In fact, so far, no organic matter (i.e., no carbohydrates) have been found in the Mars surface or subsurface. The existence of gaseous methane in the Mars atmosphere has been proven by three different sources. It's been measured independently and repeatedly by spectroscopes attached to terrestrial telescopes and Mars-orbiting spacecraft.The average abundance in the atmosphere can be in the low tens of parts per billion (ppb).
Since 2004, several papers have been published on the topic. In 2004, this one by Formisano et al. on observations of methane by an instrument called the Planetary Fourier Spectrometer (PFS) riding on the European Space Agency (ESA) Mars orbiter Mars Express created a stir. So did the latest one by Michael Mumma et al. on a plume of methane observed with a spectroscope peering through a terrestrial telescope over specific locations for a few months before it subsided. Both Dr. Formisano and Dr. Mumma and their collaborators were present and gave presentations at the workshop.
So there's methane on Mars, and so what? There's plenty of it in the Earth atmosphere, so why should we get excited if it's found on Mars? Well, that's precisely why we should get excited. 90% of the methane present in the Earth atmosphere is there due to biotic processes, i.e., because there is life on Earth and living organisms are releasing methane. We might in fact be seeing a sign of alien life here. If so, it would be the most significant scientific finding ever. Good reason to get excited. Very excited. But precisely because of the enormous implications, it is imperative be be extra-careful and explore all possible alternative explanations.
Where does the methane come from?
Methane, once released, has a limited lifetime. Ultraviolet radiation, which on Mars is not filtered by an ozone layer, would completely destroy (photochemically dissociate, for those who like big words) it in just 300 - 600 years. So there must be a source that replenishes it. There aren't all that many processes that could plausibly explain what scientists have been observing. Comets contain methane, among lots of other things. So a large comet impact a few decades ago might be the source?
Well, no, because observations show that the abundance of methane varies considerably with time over spans of just a few months. In fact, what is observed is most accurately described as plumes: Methane appears to emanate from limited regions and then is dispersed over wide areas by winds before subsiding. The Mars year lasts 687 Earth days, not quite two Earth years, and Mars has seasons because its equator is tilted with respect to its orbit, as is Earth's. There seems to be more methane around in northern spring and summer; it then disappears in Northern autumn and winter, though then it is spring and summer on the Southern hemisphere. So we're talking about a cycle here, and one that spans a few months, not hundreds of years. The observed cycle appears to be related to seasons (and thus to correlate with sunlight or temperatures?) and one might also be tempted to conjecture that it is more likely to be released on the northern hemisphere. But comets that periodically hit in Northern spring and summer? Nah. That just doesn't sound right.
This topographic map (Click to enlarge) shows regions where Mumma et al detected large methane plumes. According to Mumma et al., an increased abundance of methane correlates with a geologically ancient surface below, though not everyone agreed with this statement
Then there is a geological process called serpentinization. This refers to a series of chemical reactions, starting out with volcanic minerals, olivine or pyroxene being subjected to liquid water - lots of it - and creating another mineral, serpentine, along the way, and also methane. There we have a problem already, as liquid water cannot exist on or near the Martian surface. The atmospheric pressure is less than 1% of the Earth's. Liquid water would evaporate, ice would sublimate. Deep below the surface, water can exist in the liquid state, so if the process of serpentinization takes place, it has to be due to a significant subsurface reservoir of water. This has not been found so far, although two spacecraft are looking for it with a radar that can penetrate the ground, in one case up to a depth of several kilometers: ESA's Mars Express with its MARSIS and NASA/JPL's MRO with its SHARAD. But the fact that it hasn't been found yet doesn't mean it's not there.
A variant of the serpentinization theory is that this took place very long ago and that resulted in extended subsurface reservoirs of methane, from which some portions are periodically released. There is abundant evidence that large quantities of liquid water once existed on the Mars surface, when the planet was young, warm, volcanically active and clad in a much denser atmosphere than the tenuous wisp of today. As volcanism ceased and the planet cooled and froze, so did that water. Water ice is still plentiful, and sporadic volcanism seems to have existed until fairly recent times, which might have melted some of the ice at least for limited periods. All the ingredients for serpentinization were there, so if this is the source of atmospheric methane, we should look for the reservoirs and the vents.
The reservoirs could be in the form of clathrates: methane molecules trapped in "cages" inside ice crystals. One of the presenters addressed this issue, which by the way also is of relevance for the Earth. Some theorists have conjectured horror scenarii involving massive releases of methane from submarine clathrates. Such theories appear to ignore the fact the methane release is an endothermic and therefore self-limiting process, releasing part of the methane also lowers the ice temperature and thus inhibits the release of further gas. Apart from the implications for terrestrial doomsday scenarii, this effect seems to indicate that clathrates might not be a plausible type of reservoir that would support short-term release of a massive plume of methane.
The case for life
And then of course, there is the possibility that methane might be generated by life. If life does exist on Mars, it is unlikely that one could expect anything more than primitive microorganisms. On Earth, we know Archaea, a class of
very primitive microbes microorganisms that do not even contain cell membranes a cell nucleus. Some of these are anaerobic methanogens, meaning that they thrive on the absence of oxygen (they would even be killed by the presence of free oxygen) and their metabolism produces methane.
Conditions on Mars are not exactly benign. Ultraviolet radiation, as mentioned, likely sterilizes the upper few millimeters of ground. On top of that, as Mars is not protected by a magnetic field nor a dense atmosphere, cosmic radiation, mostly charged particles from the Sun but also much heavier and faster particles from elsewhere in the galaxy, sleets through the ground up to several meters of depth. This bombardment, which goes on continuously, day after day, aeon, after aeon, easily packs enough punch to break up complex molecules which are a prerequisite for life.
And then there are aggressive oxidants, mostly hydrogen peroxide, which is triboelectrically created through strong electrostatic charges induced by the frequent massive dust storms that engulf large portions of the planet. Oxidants could oxidize (burn) carbohydrates and thus break them up, ultimately, to carbon dioxide and water. In fact, that's why we use hydrogen peroxide as a disinfectant, because it efficiently kills germs.
Not a situation that sounds conducive to life. On the other hand, lately more and more extremophile biots have been found on Earth - life forms that live and thrive near submarine volcanic vents, deep down in rocks, in enclosed Antarctic lakes, in crude oil reservoirs, even inside the Chernobyl reactor, thriving on radiation. A geologist at the workshop presented findings from the Rio Tinto region in Spain, where methanogens were found to exist in very salty, acid water. Is it unthinkable that life originated on Mars, when it was still a more benign place, and now continues to exist somewhere below the surface, safe from the most harmful radiation and oxidants? It wouldn't even need a continuous supply of liquid water. A sporadic one might do. Terrestrial biots have shown their ability to survive amazing stretches of time, especially if they can form spores. But even higher life forms, even actual animals such as tardigrades have proven their skills as survival artists.
It is in fact possible to discern between a geological and biological origin of methane by studying the isotope ratios (deuterium vs. hydrogen, 13C vs. 12C, 18O vs. 16O). But this imposes added requirements on the measurement accuracy and probably will only be feasible with solar occultation measurements (more on those later) or with dedicated lab equipment on landed craft.
Where does the methane go?
As stated, the observed plumes disperse and the methane abundance decreases much faster than would be explained by photochemical dissociation. There must be another process, one that removes methane - tens of thousands of tons - in months, rather than hundreds of years. You ma
zy have heard of the Saturn moon Titan, which was shown to have an actual methane cycle similar to the water cycle on Earth. But on Titan, surface temperatures are around 90 K (-180 C), the pressure is 1.5 bar, close to the methane triple point. On Mars, conditions are nowhere close to that. Methane condensation is not an option. Water, carbon dioxide, those, yes, but methane, no. Likewise, chemical decomposition in the regolith or re-introduction of the methane to underground reservoirs were regarded, but experimental evidence suggests that this is not a plausible explanation either.
So what else? There appears to be some correlation between the abundances of water vapour and methane, though increased amounts of water vapour do not necessarily coincide with increased amounts of methane. So water may have to do with the release or generation of methane. But it could also have to do with the methane depletion, perhaps via intermediate products such as hydrogen peroxide and hydroxyl. Hydrogen peroxide alone could do it, as one presenter pointed out, the available quantities should be largely sufficient. But then, there should be methane decomposition products such as formaldehyde (which was claimed to have been observed in 2004, but that claim was never substantiated) and carbon monoxide, which is present, but in much lesser quantities than that methane decomposition process would suggest.
So we still have a problem here and much research is still needed. There clearly is hydrogen peroxide in the atmosphere, and the fact that the methane tends to disappear in southern spring and summer, which is the global dust storm season on Mars, promoting triboelectric hydrogen peroxide generation, also seems to be more than just a coincidence. There may be other agents involved, in particular, hydroxyl, which is known to be highly reactive and could accelerate the chemical decomposition to the observed levels.
Just how do they measure a few ppb, anyway?
That is a good question and it also explains why methane was not discovered earlier. On distant Titan, the dense atmosphere contains several percents of methane. That's a lot of methane, and that's fairly easy to measure. The much lower abundances on Earth are a different matter. In fact, several recent new technologies made these measurements possible.
In general, the chemical composition of a planetary atmosphere, down to the trace gas level, is measured via spectroscopy. This comes in different flavours. One is absorption spectroscopy using terrestrial telescopes. There, you observe the sunlight that shines on the Mars atmosphere and is reflected to your detector. If you analyze the spectrum of the received electromagnetic radiation (visible, infrared or ultraviolet light), some discrete frequencies will show decreased intensity. The frequencies are typical for given chemical elements and compounds. However, you have to watch out because certain frequencies might already not have been present in the spectrum of the light that shone on the planet, and other frequencies may be absorbed in the Earth atmosphere.
You have to take into account that the observed absorption bands will appear at frequencies that are different from their actual values because of Doppler shift: Earth and Mars are moving on their different orbits, and the Earth is rotating, and all of that adds up to significant relative velocities. That is in fact a good thing, because the same compounds you are trying to detect in the Mars atmosphere also exist on Earth, and the signal from the Earth atmosphere would completely blot out the minute radiation you receive from Mars. But thanks to the Doppler shift, the "signature" of the Mars atmosphere moves to different apparent frequencies, where there is no interference with terrestrial gases.
A way to avoid these difficulties is to get closer to the red planet. Then you can apply extinction spectroscopy. To do that, you have to observe the light from a known object, such as a star as it shines through the planetary atmosphere. Ideally, you observe sunlight as it traverse the Mars atmosphere, which is obviously not possible from the Earth, as Mars is farther from the Sun than we are. Ideally, the spectroscope is on a spacecraft orbiting Mars. In particular if sunlight is used, this form of spectroscopy will allow the precise measurement of even minute quantities of trace gases. If in orbit around Mars, you can also observe the thermal emission of the atmospheric gases and analyze this spectrum, if the appropriate hardware is installed on your spacecraft. This is referred to thermal emission spectroscopy.So, what will come next?
The workshop also gave an overview on Mars missions looking for methane in the near future.
NASA'S MSL will be launched in the 2011 launch window. The landing site of this 800 kg monster still is to be decided; there are five possible options.
The 2013, NASA's aeronomy mission "Maven" will be launched. This is an orbiter that will be placed in an orbit with a pericentre altitude of only 150 km, so it will
grave graze the upper atmosphere during each orbital revolution. The primary mission aim is to study the loss of atmospheric matter through interaction with the interplanetary medium.
The launch windows in 2016, 2018 and 2020 will see launches of joint ESA-NASA missions. In 2016, an ESA-led orbiter mission will be launched. This shall specifically study trace gas abundances via extinction measurements during occultation events, when the spacecraft enters or exits the planet's eclipse cone and therefore "sees" sunlight filtered by the atmosphere.
As stated, this type of measurement allows accurate detection of even the rarest trace gases, so will will finally find out about methane's decomposition products such as formaldehyde or methanol.
In 2018, a NASA-led mission will send two rovers to Mars. One will be provided by ESA, the other by NASA. The ESA rover's task will be to search for traces of extant or previous life. Both rovers will land at the same location and then pursue their separate mission goals. The fact that for the first time two rovers will be active that the same location is expected to
load lead to considerable synergies. The orbiter that will have been launched in 2016 will serve as data relay.
Finally, in 2020, a network of landers will be placed on the Martian surface. The exact scientific aims yet remain to be defined, but they are likely to cover a wide range, including meteorology, soil chemistry and atmospheric research.
The low down
The most important thing I learnt during this conference was that, though it indubitably is interesting and important to find out what process releases methane into the Martian atmosphere with a local and seasonal variability, in the methane concentration on Mars, the really big question is how the methane is so rapidly removed from the atmosphere. There clearly is a very efficient process at work here, but science currently has no idea what that may be.
I think it is likely that we will find that both methane source and sink are not single, but rather combinations of different processes, as is the case on the Earth. It will be very interesting to find out how the competing processes interact, perhaps synergetically. And of course, there remains the biggest question of all: whether a biological process is involved.
Workshop web site on the ESA web pages
Article on the workshop on the ESA Science web pages. The presentations held at the workshop are available through this page via hyperlinks in PDF format.
Web site of NASA/JPL's MSL rover mission
Michael J. Mumma, Geronimo L. Villanueva, Robert E. Novak, Tilak Hewagama, Boncho P. Bonev, Michael A. DiSanti, Avi M. Mandell, Michael D. Smith: Strong Release of Methane on Mars in Northern Summer 2003, Science 20 February 2009: Vol. 323. no. 5917, pp. 1041 - 1045
V. Formisano, S. Atreya, T. Encrenaz, N. Ignatiev, M. Giuranna: Detection of Methane in the Atmosphere of Mars, Science, 3 December 2004: Vol. 306. no. 5702, pp. 1758 - 1761
E. Chassefiere: Metastable Methane Clathrate Particles as a Source of Methane to the Martian Atmosphere, Icarus Volume 204, Issue 1, November 2009, Pages 137-144