CH4 + H2O + Mars = Life?
Ted S. Frost
Astronomers and planetary scientists are awaiting conclusive evidence of microbial life—past or present— on Mars. Confi rmation that life exists on the Red Planet could be the most signifi cant discovery ever made by man, philosophically and scientifi cally. Many scientific papers have come forth the past year announcing new discoveries involving life on the Red Planet. Academic seminars such as the March 14th Lunar and Planetary Science Conference have been dominated by presentations considering Martian life. Life-on-Mars articles recently appeared in The New York Times, The China Daily Press, The Economist, and The San Francisco Chronicle. Two discoveries in the past year have generated excitement. Observations and data produced by the European Space Agency’s Mars Express Orbiter, NASA’s rovers Spirit and Opportunity, and infrared spectrometer studies at observatories in Chile and Hawaii make it clear that Mars was wet and warm some 3.7 billion years ago (3.7 Ga)1, and that something is currently putting methane gas (CH4) into Mars’ atmosphere2.
Mars Express Orbiter above Mars (Image: ESA)
Evidence of methane in Mars’ atmosphere really gets scientists charged up, because methane in Earth’s atmosphere is nearly all derived from one-celled organisms—a class of Archaea called methanogens. By way of an involved process, methanogens convert H2 and CO2 into CH4 (methane) and H2O. They are extremophiles that live in harsh anoxic environments such as swamps, rice paddies, and the stomachs of cows. They are also found in termite guts, undersea hydrothermal vents, hot springs, and sewage sludge. And here’s the kicker: atmospheric methane has to be continually replenished because it is destroyed by UV radiation, which causes it to react with hydroxyl radicals. On Earth a molecule of atmospheric CH4 lasts, on average, 10 years. In the atmosphere of Mars, its life is estimated to be 400 years—still fl eeting on a geological time scale. This means that the methane showing up in Mars’ atmosphere cannot be a holdover from planetary formation. Clearly, it is being produced. Methanogens have been on Earth a long time. Ribosomal 16S RNA sequencing puts methanogens far down on the tree of life, meaning (from a time-line standpoint, anyway) they are a "primitive" life form. Because methanogens are thought of as "simple" and "primitive" extremophiles, astronomers and planetary scientists make them likely candidates for methane production on Mars3. Microbial life on Mars? Not on the surface, of course. The surface of Mars is too cold (-55o C), too dry and too subject to UV radiation to support life as we know it.
The hypothesis is that methanogenic microbes evolved in the more hospitable wet-warm period of Mars and survive today in isolated subterranean oases, possibly pockets of subterranean water liquefi ed by residual planetary internal heat, similar to continental basaltic terrains on Earth where subsurface methanogens obtain energy from H2 and CO2 generated by hydrothermal activity4. On Mars, the methane so produced would, so the story goes, migrate to the surface into the atmosphere through cracks and fi ssures associated with crater formation from bolide impacts.
Recently, NASA had to rein in two of its scientists, in response to reports that the scientists were planning to issue a paper claiming there is strong evidence for current life on Mars.
Fourier transform spectrometer (FTS) installation,Cerro Pachón, Chile (Photo: SAO/SRL)
Before getting too carried away by possible parallels to microbe methane production on Earth, a couple of "not-so-fasts" should be considered. First, methane can be and is produced by non-biological mechanisms as well as by "bugs"—in particular, hydrothermal reactions. At mid-oceanic ridges here on Earth, seawater and CO2 react with basaltic magma to produce methane as a by-product in the conversion of the mineral olivine to serpentine, a process called "serpentination." The temperature at which this takes place is relatively modest—100o to 400o Celsius. Methane also is a by-product of asteroid impacts and volcanic activity.
So, although most of Earth’s atmospheric methane comes from biological sources (the methanogens), biology isn’t the only source, particularly when accounting for small amounts. And, on Mars, we are dealing with small amounts. The amount detected on Mars by infrared spectrometer analysis is only 10 parts per billion. Since Mars’ atmosphere is less than 1% of Earths,’ that is a tiny amount. A factor of 100 less than the current production of methane on Earth from non-biological sources.
Because of this, James Kasting and David Catling go so far as to say: "(While) one might conclude . . . that methane is an excellent biomarker (for existence of life) . . . this is not necessarily so."5 Cautious skepticism seems to be in order, especially since the assumption that methanogens are "simple" and "primitive" organisms may be unwarranted. From a biological standpoint, this is not the case. Don’t be fooled by the bad neighborhoods they live in. Yes, it is true methanogens evolved very early on Earth, but they most certainly are not simple and they are not primitive in the sense of being characteristic of an early stage of development. Many other "bugs" using earlier stages of development preceded them. In spite of their name, Archaea are not older than bacteria.
The fact is, methanogens are complex and unique. They are only one phylum of the Domain Archaea. No other living things are methanogenic and they possess metabolic enzymes that are extremely unusual6. In order to live, organisms must fi nd ways to convert external energy into internal energy storage units. The internal energy storage used by all life on Earth is adenosine triphosphate (ATP). Life has two ways of converting external energy into ATP. The simplest, most primitive, and least involved is "substrate level phosphorylation" (SLP) that makes ATP by converting an organic molecule from one form to another. It is used by many bacteria and includes the fermentation process that gives us beer, sour dough bread, cheese, and lactic acid muscle pains. The second way to produce ATP is a more complicated process that came along later called "electron transport phosphorylation" (ETP). It is used by most so-called advanced forms of life who favor it because it is a more effi cient and effective means of producing ATP.
Methanogens are in the advanced camp. They are ETP’ers, not SLP’ers. The methane they produce is a by-product of a very intricate and complicated metabolic process involving the creation of a proton motive force outside their cell membrane (meaning a positively charged electrical fi eld), an electron transport complex within their cell membrane (meaning circuitry for an electrical current), and ATP synthase complexes imbedded as gateways through their cell membrane (meaning electrically powered little ATP factories). Not only are all these parts complicated in and of themselves, they require the manufacture and use of a large supply of complicated chemical cofactors and enzymes—complicated macromolecules that in some instances are unique and whose interactions are not completely understood. Being the chemically complicated little creatures they are, could it be that early evolution of methanogens on Earth was a fl uke? A stochastic happening rather than an inevitability? Could it be an unwarranted assumption that methane on Mars is consistent with evolution on Earth? An experience of a sample of one? Keep in mind that the window for origin of life on Mars was probably short, and during the time it existed conditions were inhospitable. Mars is thought to have formed 4.5 Ga. Stable inventories of liquid water are not thought to have lasted past 3.7 Ga 7. Subsequent to 3.7 Ga, Mars’ internal heat died down, reducing volcanism. And, most signifi cantly, Mars lost its protective magnetic shield and most of its greenhouse warming gases. Mars is only 1/9th the mass of Earth, so its escape velocities are much less (5,000 m/sec. vs. 11,400 m/sec.).
The 4.5 to 3.7 Ga time interval for warm liquid water happens to coincide with the heavy bombardment period for terrestrial planets, called the Noachian period on Mars and the Hadean period on Earth. This was the time when Earth, Mars, and Venus were clobbered by asteroids, some containing enough kinetic energy to completely vaporize and sterilize planetary oceans. On Earth, the last ocean-vaporizing event is thought to have been as late as 4.1 Ga and possibly even 3.8 Ga.8. Similar catastrophes may have occurred on Mars.
Based on geographic features seen today on Mars9, episodic liquid water forming events undoubtedly occurred after the Noachian heavy bombardment period. These would have come from occasional asteroid impacts and volcanic activity. However, it is questionable whether the bodies of water so formed would have lasted long enough or been extensive enough to be a viable site for origin of life. In view of these constraints, would the window of evolutionary opportunity on Mars have remained open long enough for organisms to evolve as sophisticated and complicated as methanogens? I wonder. Fortunately, future robotic expeditions planned for Mars include capabilities for isotopic analysis. If the carbon in the CH4 of Mars turns out to be skewed in favor of C12 vs. C13, that should answer the question, since life forms always favor the lighter isotope of a chemical element. Meanwhile, we wait and wonder.
References:
1, 7. “New Perspectives on Ancient Mars,” S. Solomon, Science ( Vol. 307, 2/25/05, pp. 1216-18.)
2. “Detection of Methane in the Martian Atmosphere: Evidence for Life?”V. Krasnopolski et al, Icarus, (Vol. 172, Issue 2, Dec 2004, pp. 537-547.)
3. “Methane on Mars: A Possible Biomarker?”D. Morrison, NASA Astrobiology Institute.
4. “Astrobiology and the Basaltic Plains of Gusev Crater,” D. J. Des Marais, Lunar & Space Science Conference, (3/14/05.)
5, 8. “Evolution of a Habitable Planet,” J. Kasting & D. Catling, Annual Review of Astronomy & Astrophysics, (Vol. 41, Sep 2003, pp, 441, 435.)
6. Dept. of Bacteriology, University of Wisconsin-Madison, bact.wisc.edu, Microbiology Text maintained on-line by T. Paustian (para. 22-1a)
9. European Space Agency Mars Express :esa.int/SPECIALS/Mars_ Express/