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
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
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).
Benner, S.A., A. Ricardo and M.A. Carrigan (2004) Is there a common
chemical model for life in the universe? Current Opinion in Chemical
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Clark, R. N., J. M. Curchin, J. W. Barnes, R. Jaumann, L. Soderblom, D.
P. Cruikshank, R. H. Brown, S. Rodriguez, J. Lunine, K. Stephan, T. M.
Hoefen, S. Le Mouelic, C. Sotin, K. H. Baines, B. J. Buratti, and P. D.
Nicholson (2010) Detection and Mapping of Hydrocarbon Deposits on Titan.
J. Geophys. Res., doi:10.1029/2009JE003369, in press.
Lorenz, L.D., H.B. Niemann, D.N. Harpold, S.H. Way, and J.C. Zarnecki
(2006) Titan's damp ground: Constraints on Titan surface thermal
properties from the temperature evolution of the Huygens GCMS inlet.
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R.M. Lopes, B. Stiles, S. Ostro, G. Mitri, and E.R. Stofan (2008)
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Niemann H. B., Atreya S. K., Bauer S. J., Carignan G. R., Demick J.E.,
Frost R. L., Gautier D., Haberman J. A., Harpold D. N., Hunten D. M.,
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Raulin F., Raaen E., and Way S. H. (2005) The abundances of constituents
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Schulze-Makuch, D., and D.H. Grinspoon (2005) Biologically enhanced energy and carbon cycling on Titan? Astrobiology 5, 560–564.
Strobel, D.F. (2010) Molecular hydrogen in Titan's atmosphere:
Implications of the measured tropospheric and thermospheric mole
fractions. Icarus, in press.