The methane puzzle: Is it possible that a comet hit Mars?

The presence of methane gas in the atmospheres of Mars and Titan may indicate life or unusual geological activity. In any case, this is one of the most challenging mysteries in our solar system

Sushil K. Atria, Scientific American

Among all the planets in the solar system, except Earth, Mars is undoubtedly the place where the chance of finding life, whether signs of life that has already become extinct or life that exists there today, is the highest.
Mars is similar to Earth in many ways. Its formation process, its ancient climatic hysteria, its water reservoirs, its volcanoes and other geological processes create a picture in which microscopic creatures can fit in without difficulty.
Another planetary body in the solar system, Titan, Saturn's largest moon, also routinely comes up on the table of discussions dealing with extraterrestrial biology. In its primordial past, processes occurred on Titan that encouraged the formation of pre-living molecules, and some scientists believe that there was life on it then, and perhaps even today.
Adding to the challenge, astronomers studying these two worlds discovered a gas often associated with living things: methane. On Mars, the gas is found in small, but significant quantities, while Titan is actually flooded with it. A biological source for methane on Mars is as plausible an explanation as a geological source, and perhaps the same is true on Titan.
Each of the two explanations is fascinating on its own. The first implies that we are not alone in the universe. The other explanation reveals the existence of large underground water reservoirs alongside geological activity at an unexpected level, both on Mars and on Titan. Understanding the source of methane and its fate in both places will provide vital clues to the processes that shape the formation, development and life-bearing capacity of Earth-like worlds in our solar system, and perhaps even in other solar systems.
The gas methane (CH4) is common in the giant planets - Jupiter, Saturn, Uranus and Neptune - where it was created in the chemical processes that took place in the initial gas cloud that created the solar system. On the other hand, on Earth methane is a special case.
Of the 1,750 parts per billion (ppbv) that methane occupies in the volume of Earth's atmosphere, 90% to 95% is of biological origin. Herbivorous and ruminant animals, such as cows, goats and yaks, emit a fifth of the world's annual amount of methane as a byproduct of bacterial metabolism in their guts.
Other important sources include termites, rice fields, swamps, natural gas leakage (which is also a product of past life) and photosynthetic plants (see "Methane, Plants and Climate Change" by Frank Kepler and Thomas Rockman, Scientific American Israel, June-July 2007) . Volcanoes contribute less than 0.2% of the planet's global methane balance, and perhaps they too simply emit methane into the air that was produced by living organisms in the past. Compared to this, the emission from non-biological sources, such as industrial processes, is a secondary emission. The detection of methane on another Earth-like body therefore raises the hope that there is life there.

carried in the air
This is exactly what happened on Mars in 2003 and 2004, when three separate groups of scientists announced the discovery of methane in the atmosphere of this planet. Using high-resolution spectrographs located at the Infrared Telescope Facility in Hawaii and the Gemini Southern Telescope in Chile, a team led by Michael Mumma of NASA's Goddard Space Flight Center measured methane concentrations of over 250 parts per billion. Changes in concentration between the different regions of Mars were also observed, and perhaps even changes that occurred over time.
Vittorio Formisano from the Institute of Physics and Interplanetary Science in Rome and his colleagues (including me) analyzed thousands of spectral measurements collected by the Mars Express spacecraft orbiting Mars. We found that methane is much less common: its concentration range is between zero and 35 parts per billion and the planetary average is about 10 parts per billion. And finally, Vladimir Krasnopolsky from the Catholic University of America and his colleagues measured using the Canadian-French telescope in Hawaii a planetary average of about 10 parts per billion. They could not determine changes in methane concentration in different regions of the planet due to low signal and spatial resolutions.
Momma's team is now analyzing the data in their hands to try to determine why the value they determined is outside the range of the other measurements. For now, I will assume that a volumetric concentration of 10 parts per billion is the most likely value. This concentration, in terms of number of molecules per unit volume, is only 40 millionths of the concentration of methane in the Earth's atmosphere. Even so, even this tiny presence of the gas requires an explanation.
Although astronomers discovered methane on Titan as early as 1944, it was the further discovery of nitrogen, 36 years later, that sparked the enormous interest in this cold and distant moon. Nitrogen is a central component of biological molecules such as amino acids and nucleic acids.
A body with an atmosphere containing nitrogen and methane and whose surface pressure is 1.5 times greater than that of our planet, may contain the right ingredients for the formation of pre-life molecules and perhaps, as some scientists speculate, even the formation of actual life.
Methane plays a key controlling role in maintaining Titan's thick nitrogen atmosphere. It is the source of the hydrocarbon haze that absorbs the infrared radiation coming from the sun and raises the temperature of Titan's stratosphere by approximately 100 degrees Celsius. It is also the source of hydrogen, whose molecular collisions heat the troposphere by 20 degrees.
If the methane runs out, the temperatures will drop, the nitrogen will condense into liquid droplets, the atmosphere will collapse and Titan's unique character will be changed forever. The smog and its clouds will disperse. The methane rains, which apparently shaped the surface, will stop falling. Lakes, ponds and streams of liquid methane will evaporate. Then, when the veil covering it is removed, the desolate surface will be revealed to the eyes of telescopes on Earth. Titan will lose its mystery and become just another moon with a thin atmosphere.
Could it be that the source of the methane on Mars and Titan is a biological source, as it is on Earth? Or maybe there is another explanation, such as volcanic activity or impacts from comets and meteorites? Our understanding of geophysical, chemical, and biological processes has helped us narrow down the range of possible sources of methane on Mars, and many of these reasonings apply to Titan as well.

Decomposition in sunlight
The first step in answering these questions is determining the rate at which the methane must be formed or reach the atmosphere. This rate, in turn, depends on the rate of gas removal from the atmosphere. At heights of 60 km and more above the surface of Mars, the ultraviolet radiation breaks down the methane molecules.
Lower in the atmosphere oxygen atoms and hydroxyl radicals (OH), which are formed when water molecules are broken by photons of ultraviolet radiation, oxidize the methane. Without resupply, the methane will gradually disappear from the atmosphere. The "lifetime" of methane - which is defined as the period of time during which the concentration of the gas drops to one part of the mathematical constant e, or approximately to one third - is 300 to 600 years, depending on the amount of water vapor, which undergoes seasonal changes, and depending on the intensity of solar radiation, which changes according to the solar cycle .
On Earth, similar processes determine the lifetime of methane to 10 years. On Titan, where the ultraviolet radiation from the sun is much weaker, and oxygen-containing molecules are much less common, methane may remain in the atmosphere for 10 million to 100 million years (still a short period of time in geological terms).
Methane's lifetime on Mars is long enough for winds and puffs to disperse it through the atmosphere fairly uniformly. The observed differences in methane levels in different regions of the planet are therefore a surprising phenomenon. They suggest that the gas may be coming from local sources or that it is disappearing at certain sites. One possible disposal site is chemically active soil, which may accelerate the disappearance of methane. If other sinks are operating, even larger sources may also be operating that maintain the observed concentration.
The next step is to consider possible scenarios for methane formation. The Red Planet is a good place to start because the distribution of methane is so low. If a particular mechanism cannot account for even these tiny amounts, it is unlikely that it can account for Titan's much larger amounts. With a lifetime of 600 years, an annual output of 100 tons of methane is enough to maintain a constant global average of 10 parts per billion. Such an output is about a quarter of a millionth of the production rate of methane on Earth.
As on Earth, volcanoes are most likely not responsible for this. The volcanoes of Mars have been dormant for hundreds of millions of years. Moreover, if a volcano was responsible for the methane, it would also emit huge amounts of sulfur dioxide. But the atmosphere of Mars is free of sulfur compounds.
The contribution of extraplanetary sources is also probably minimal. It is estimated that about 2,000 tons of micrometeorite dust fall on the surface of Mars every year. Less than one percent of the mass of these particles contains carbon, and it is also mostly oxidized, so it is a negligible source of methane. Methane makes up about 1% of the weight of comets, but they only hit Mars once every 60 million years on average. Therefore, the amount of methane they supply is one ton per year, or less than 1% of the required amount.
Is it possible that a comet hit Mars in the recent past? It would have provided a large amount of methane, and over time its large amount in the atmosphere would have decreased to its current value. An impact by a comet with a diameter of 200 meters 100 years ago, or a comet with a diameter of 500 meters 2,000 years ago, could have provided the amount of methane that explains a global average of 10 parts per billion as observed today. But this idea encountered a difficulty: the distribution of methane is not uniform. It doesn't take more than a few months to spread the methane evenly both vertically and horizontally. A cometary source would therefore produce a uniform distribution of methane over the surface of Mars, contrary to observations.

smoke in the water
We are therefore left with two possible sources: a hydro-geochemical source or a bacterial source. Each of them is fascinating. Hydrothermal vents, also called smokehouses, were first discovered on Earth in 1977 in the Galápagos geological fault. Since then, oceanographers have found similar chimneys along most mid-ocean ridges.
Laboratory experiments have shown that under the conditions prevailing in these chimneys, a chemical reaction, known as serpentinization, may occur, in which hydrogen gas will be produced from ultramafic silicates, rich in iron or magnesium, such as olivine and pyroxene. This hydrogen may in turn react with carbon grains, with carbon dioxide, with carbon monoxide or with carbonaceous minerals to produce methane.
The key factors for this process are hydrogen, carbon, metals (used as catalysts), heat and pressure. All of them are also found on Mars. The serpentinization process can occur at high temperatures (350 to 400 degrees Celsius) or at more moderate temperatures (30 to 90 degrees). Temperatures in the low range should prevail in hypothesized aquifers on Mars.
Although low-temperature serpentinization may be capable of producing the methane on Mars, the biological process still remains a possibility that should not be taken lightly. On Earth, methane is produced by microscopic creatures called methanogens as a byproduct in the processes of consuming hydrogen, carbon dioxide or carbon monoxide. If such creatures lived on Mars, they would find a constant supply of materials necessary for their survival: hydrogen (either as a product of the serpentinization process or by falling to the ground from the atmosphere) and also carbon dioxide and carbon monoxide (in rocks or from the atmosphere).
Once methane is formed, whether by the serpentinization process or by bacteria, it may be stored in stable compounds of clathrate hydrate, a chemical structure that traps methane molecules as if they were caged animals, and then released into the atmosphere. The release may occur perhaps by gradual emission through cracks and fissures or by accidental eruptions caused by volcanic activity. No one knows for sure how efficiently the cage compounds are formed or how easy it is to destabilize them.
Observations made by the Mars Express spacecraft suggest higher methane concentrations above regions containing subsurface water ice. Both processes, geological or biological, explain this adaptation. Aquifers located under the ice are able to provide a habitat for living organisms or a place where the hydrogeochemical production process can occur. Without further data, the biological and geological possibilities seem to remain equally plausible.

A titanic ocean
At first glance, one might think that it might be easier to explain Titan's methane: the moon formed inside Saturn's subcloud, whose atmosphere contains huge amounts of the gas. However, the data points to methane production on Titan and not to the transfer of the gas to this moon. The Huygens lander, part of the Cassini-Huygens mission shared by NASA and the European Space Agency, found no xenon or krypton in Titan's atmosphere. If the small bodies that formed Titan had brought methane with them, they would have also brought these heavy noble gases. The absence of such gases indicates that most of the methane probably formed on Titan.
Therefore, the presence of methane on Titan is no less mysterious than its presence on Mars, and in some ways even more mysterious due to its quantity (5% of the volume of the atmosphere). One likely source, as on Mars, is the serpentinization process at relatively low temperatures. Christophe Sutin of the University of Nantes in France and his colleagues claim that Titan may have a subsurface ocean of liquid water. Ammonia dissolved in water will act as an antifreeze and help keep it in a liquid state. According to their model, the ocean is 100 kilometers below Titan's surface and 300 to 400 kilometers deep. In the past, the heat emitted from the decay of radioactive elements and the heat left over from Titan's creation melted almost all the ice on this moon, and the ocean may have reached its rocky core.
Under such conditions, chemical reactions between the water and the rocks would have released hydrogen gas, which in turn could have reacted with carbon dioxide, carbon monoxide, carbon grains, or other carbon-containing materials to produce methane. I believe that this process could explain the abundance of methane observed on Titan. Once formed, the methane was possibly stored in stable clathrate hydrate compounds and released gradually, in volcanic processes, or in eruptions caused by the impact of bodies from space.
An intriguing clue comes from the argon 40 gas observed by the Huygens spacecraft as it plunged through Titan's atmosphere. This isotope is created from the radioactive decay of potassium 40, trapped deep in Titan's rocky core. Since the half-life of potassium 40 is 1.3 billion years, the small amount of argon 40 in the atmosphere indicates a slow release of gases from the interior. Also, photographs and radar images of the surface show signs of cryovolcanic activity - geyser-like eruptions of ammonia-water ice - suggesting again that materials may rise from the depths to the surface. According to the hypothesis, the rate of material exit from the depths to the surface is fast enough to provide methane to replace the photochemical loss. The role that methane plays on the surface of Titan is similar to the role of water on Earth. It maintains a full-scale methane cycle: liquid reservoirs on the surface, clouds and rain. There is therefore a solid body of evidence, even more so than for Mars, that the methane stored at depth will have no difficulty coming out and reaching the surface and eventually evaporating into the atmosphere.
Can biology also play a role in the creation of methane on Titan? Christopher McKay of NASA's Ames Research Center and Heather Smith of the International Space University in Strasbourg, France, as well as Dirk Schultz-Mekosh of Washington State University and David Greenspon of the Denver Museum of Nature and Science, suggested that acetylene and hydrogen could serve as nutrients for methanogenic bacteria , even in the intense cold that prevails on the surface of Titan (179 degrees Celsius below zero). This biogenic process differs from the process carried out by the methanogens on Earth and their relatives, if any, on Mars, because no water is needed to carry it out. And instead, the liquid hydrocarbons found on Titan's surface act as a medium.
However, this hypothesis has a drawback. The data collected by the Huygens spacecraft ruled out the possibility of an underground source of acetylene. Therefore, this compound must be formed in the atmosphere from methane. The argument is therefore a circular argument: to produce methane (by bacteria) you need methane. Moreover, given the vast amounts of methane on Titan, the methanogens would have had to work overtime to produce it, consuming all available food sources.
Given these setbacks, a biological explanation for methane on Titan seems less likely than on Mars. However, the hypothesis that Titan might support life is worthy of study. Some scientists claim that this moon was able to support life in the past, and maybe even now. It receives enough solar radiation to turn nitrogen and methane into molecules used as the raw materials of biology. An underground brine of ammonia water, which also contains some methane and other hydrocarbons, may serve as a friendly environment for the existence of complex molecules and possibly even living organisms. In the distant past, when young Titan was still cooling, liquid water may have flowed even on the surface.

organic food
One important measurement, which may help and decide what is the source of methane on Mars and Titan, is the determination of the isotope ratio of carbon. The development of life on Earth resulted in a preference for carbon 12, whose binding energy is weaker than that of carbon 13. When amino acids bind together to form proteins, the proteins show a marked deficiency in the liver isotope. Living things on Earth contain 92 to 97 times more carbon-12 than carbon-13, while in inorganic matter the standard ratio is 89.4.
However, on Titan the Huygens spacecraft measured a ratio of 82.3 in methane molecules, which is a ratio smaller, not larger, than that of the inorganic standard on Earth. This finding serves as important evidence against the existence of life as we know it. Keep in mind, however, that some scientists argue that life on Titan could have evolved differently than on Earth or that the inorganic isotope ratio there is different than here.
The isotope ratio of carbon on Mars has not yet been determined. This measurement is much more difficult when the gas concentration is so low (a billionth of the concentration on Titan). The "Mars Science Laboratory" (MSL), NASA's spacecraft destined to reach Mars in 2010, should be able to accurately determine the isotope ratios of carbon in methane, and possibly other organic compounds as well. The lab will also examine solid and gaseous samples to look for additional chemical signs of past or present life, such as an especially high ratio between methane and heavier hydrocarbons (ethane, propane, butane) and chirality (a preference for one molecular structure, over its mirror image in certain organic compounds) .
A related question is the question "Why are organic compounds absent from the surface of Mars?" Even in the absence of life, meteorites, comets, and interplanetary dust particles would have brought organic matter with them over the past four and a half billion years. The answer lies, perhaps, in the puffs of dust and the sandstorms of Mars and the jumps and shakes that the wind makes the grains of dust.
These processes create strong fields of static electricity, which may stimulate the chemical synthesis of hydrogen peroxide. Hydrogen peroxide is a strong oxidizing agent, and they are able to quickly sterilize the surface and remove organic compounds from it. The oxidizing ability of hydrogen peroxide may also accelerate the loss of methane from certain regions of the atmosphere. This process means that even larger sources of methane are needed on Mars to account for the observed amount in its atmosphere.
In summary, methane acts as the glue that holds Titan's system together in mysterious ways. The presence of the gas on Mars is no less fascinating, especially because it evokes visions of life on this planet.
Further study of these two celestial bodies will aim to determine if life ever existed on them. Although life as we know it is capable of producing methane, its presence does not necessarily indicate the existence of life. Planetary scientists must therefore thoroughly investigate the sources of the gas, the processes that remove it, and its isotopic composition. They must also examine other organic molecules and traces of additional components in gaseous and solid samples. Even if it turns out that there is no connection between methane and life, the research will reveal some basic aspects of the formation of Mars and Titan and their climatic history, geology and evolution.