by Chris McKay, NASA Ames Research Center, Moffett Field CA. (firstname.lastname@example.org)
Recent results from the Cassini mission suggest that hydrogen and acetylene are depleted at the surface of Titan. Both results are still preliminary and the hydrogen loss in particular is the result of a computer calculation, and not a direct measurement. However the findings are interesting for astrobiology. Heather Smith and I, in a paper published 5 years ago (McKay and Smith, 2005) suggested that methane-based (rather than water-based) life -- ie, organisms called methanogens -- on Titan could consume hydrogen, acetylene, and ethane. The key conclusion of that paper (last line of the abstract) was "The results of the recent Huygens probe could indicate the presence of such life by anomalous depletions of acetylene and ethane as well as hydrogen at the surface."
Now there seems to be evidence for all three of these on Titan. Clark et al. (2010, in press in JGR) are reporting depletions of acetylene at the surface. And it has been long appreciated that there is not as much ethane as expected on the surface of Titan. And now Strobel (2010, in press in Icarus) predicts a strong flux of hydrogen into the surface.
This is a still a long way from "evidence of life". However, it is extremely interesting.
Benner et al. (2004) first suggested that the liquid hydrocarbons on Titan could be the basis for life, playing the role that water does for life on Earth. Those researchers pointed out that "... in many senses, hydrocarbon solvents are better than water for managing complex organic chemical reactivity. Two papers in 2005 followed up on this logic by computing the energy available for methanogenic life based on the consumption of both the organics in Titan's atmosphere along with the hydrogen in the atmosphere (McKay and Smith, 2005; Schulze-Makuch and Grinspoon, 2005). Both papers made the case that H2 on Titan would play the role that O2 plays on Earth. On Earth organisms (like humans) can react O2 with organic material to derive energy for life's functions. On Titan organisms could react H2 with organic material to derive energy. The waste product of O2 metabolism on Earth is CO2 and H2O; on Titan the waste product of H2 metabolism would be CH4. As a result of the Cassini mission, there is now abundant evidence for CH4, even in liquid form, on Titan.
Organic molecules on the surface of Titan (such as acetylene, ethane, and solid organics) would release energy if they reacted with hydrogen to form methane. Acetylene gives the most energy. However this reaction will not proceed under ordinary conditions.
This is similar to our experience on Earth. Consider a chocolate bar in a jar full of air. The organics in the chocolate would release energy if they reacted with the oxygen in the air but the reaction does not proceed under normal conditions. There are three ways to make it proceed: heat it to high temperatures (fire), expose it to a suitable metal catalyst that promotes the reaction, or eat it and use biological catalysts to cause the reaction. Biology can thrive in an environment that is rich in chemical energy but requires a catalyst for the chemical energy to be released. Such is the case on Titan.
McKay and Smith (2005) predicted that if there were life on Titan living in liquid methane then that life should be widespread on the surface because liquid methane is widespread on the surface. We have direct evidence that the surface of Titan at the landing site of the Huygens Probe near the equator was moist with methane, and radar and near-infrared imagery from Cassini have revealed extensive polar lakes on Titan, both north and south. Methane-based life would have a lot of environments in which to live.
Again, this is analogous to Earth. Life is widespread on Earth because it uses water and water is widespread on Earth.
Furthermore, because it is widespread, life on Earth, in turn, has a profound effect on the environment. For example, each spring the amount of CO2 in the atmosphere drops as plants consume it to form leaves; each autumn, the amount of CO2 in the atmosphere goes up as these leaves decompose. That is, because of the ubiquity of life, the Earth breathes: one breath in during the spring, one breath out during the autumn. Widespread life has observable effects.
Taking this logic to Titan, McKay and Smith (2005) predicted that Titanian life at the surface would consume near-surface hydrogen and that this might be detectable. The depletion of hydrogen is key because all the chemical methods suggested for life to derive energy from the environment on Titan involve consumption of hydrogen (McKay and Smith 2005; Schulze-Makuch and Grinspoon 2005). Acetylene, ethane, and solid organic material could all be consumed as well. Acetylene yields the most energy, but all give enough energy for microorganisms to live.
A few notes about liquid methane based life on Titan.
First, while such life would produce CH4 it would not be a net source of CH4 but would be merely recycling C back into CH4 - undoing the photochemistry caused by sunlight in the upper atmosphere. It does not explain the persistence of CH4 on Titan over geological time.
Second, it is impossible to predict any isotopic effect that this life might have on C. On Earth, methanogens produce CH4 from CO2+H2, or from organic material derived from CO2. The net reaction is CO2 + 4H2 => CH4 + 2H2O and thus methanogens on Earth are a net source of CH4 in a world of CO2. The enzymes that mediate these reactions create methane with a large isotopic enrichment of 12C over 13C of ~5%.
On Titan, it has been predicted that methanogens would produce CH4 by C2H2 + 3H2 => 2CH4 (eg. McKay and Smith 2005). This is obviously not a net source of CH4: it merely recycles CH4, thereby undoing the photolysis of CH4 and there is no a priori reason to expect the resulting CH4 to exhibit an isotopic shift from these reactions. The C-C bond in acetylene is strong but this by itself does not imply a strong isotopic selectivity. For example, life on Earth breaks the strong bond between the N atoms in N2 without leaving a clear isotopic effect. Thus, the istopic state of C on Titan is not relevant to the question of the presence of Titanian methanogens..
The data that suggests that there is less ethane on Titan than expected is well established (Lorenz et al. 2008). Photochemical models have predicted that Titan should have a layer of ethane sufficient to cover the entire surface to a thickness of many meters but Cassini has found no such layer. The new results of Clark et al. (2010) find a lack of acetylene on the surface despite its expected production in the atmosphere and subsequent deposition on the ground. There was also no evidence of acetylene in the gases released from the surface after the Huygens Probe landing (Niemann et al. 2005, Lorenz et al. 2006). Thus, the evidence for less ethane and less acetylene than expected seems clear and incontrovertible.
The depletion of ethane and acetylene become significant in the astrobiological sense because of this latest report of a hydrogen flux into the surface This is the key that suggests that these depletions are not just due to a lack of production but are due to some kind of chemical reaction at the surface.
The determination by Strobel (2010) that there is a flux of hydrogen into the surface of Titan is not the result of a direct observation. Rather it is the result of a computer simulation designed to fit measurements of the hydrogen concentration in the lower and upper atmosphere in a self-consistent way. It is not presently clear from Strobel's results how dependent his conclusion of a hydrogen flux into the surface is on the way the computer simulation is constructed or on how accurately it simulates the Titan chemistry.
In conclusion, there are four possibilities for the recently reported findings, listed in order of their likely reality:
1. The determination that there is a strong flux of hydrogen into the surface is mistaken. It will be interesting to see if other researchers, in trying to duplicate Strobel's results, reach the same conclusion.
2. There is a physical process that is transporting H2 from the upper atmosphere into the lower atmosphere. One possibility is adsorption onto the solid organic atmospheric haze particles which eventually fall to the ground. However this would be a flux of H2, and not a net loss of H2.
3. If the loss of hydrogen at the surface is correct, the non-biological explanation requires that there be some sort of surface catalyst, presently unknown, that can mediate the hydrogenation reaction at 95 K, the temperature of the Titan surface. That would be quite interesting and a startling find although not as startling as the presence of life.
4. The depletion of hydrogen, acetylene, and ethane, is due to a new type of liquid-methane based life form as predicted (Benner et al. 2004, McKay and Smith 2005, and Schulze-Makuch and Grinspoon 2005).
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Strobel, D.F. (2010) Molecular hydrogen in Titan's atmosphere: Implications of the measured tropospheric and thermospheric mole fractions. Icarus, in press.