5.7 Methane clumped signatures in astrobiology
Our experiments suggest methane produced by abiogenesis on other worlds
may yield apparent
Δ12CH2D2 deficits.
This is because the difference between equation 6 and 9 remains
insensitive to the absolute value of the D/H ratio; only D/Hdifferences between various hydrogen pools matter in causing
combinatorial effects (Rockmann et al., 2016, Taenzer et al., 2020), not
the absolute values of the source pools. Since D/H differences
inevitably follow from lower-T hydrogen additions to carbon, we expect
Δ12CH2D2 deficits to
be common anywhere methane is synthesized from the reduction of CO or
CO2. This means that on other worlds, where the D/H
ratio of molecular hydrogen is unknown and could be extremely different
from Earth, abiotic methane should still exhibit
Δ12CH2D2 deficits
comparable to the values observed here (Fig. 3). As on Earth, if this
methane is entrained in a hydrothermal system with ambient temperatures
approaching 300 °C, this signature may be replaced by
isotopologue equilibrium, a situation that may be uncommon however given
the low temperatures conditions prevailing on most other solar system
bodies.
While biosignatures have been a long-term focus of investigation, both
field and experimental consideration of potential abiosignatures has
been flagged as a critical gap in the search for life beyond Earth
(NASEM, 2022). As such, bond reordering of abiotic methane may be a
useful tool. The in-situ discovery of methane gas in Mars’s atmosphere
by Curiosity (Webster et al., 2015) indicates that methane is produced
on the planet, likely from an on-going source. Methane is also detected
in the plumes of Saturn’s moon Enceladus, together with molecular
hydrogen (Waite et al., 2017). There, the origin of CH4is thought to be abiotic, derived from hydrothermal reactions occurring
at depth in the oceans of Enceladus, but at unconstrained temperatures
(Waite et al., 2017). A Bayesian analysis of the escape rates of
Enceladus has recently challenged a hydrothermal origin of methane, and
suggested it might potentially be biological (Affholder et al., 2021).
It is not clear that conventional isotopic tools can yield unambiguous
results as to the origin of methane on Mars and Enceladus, because sound
interpretations of bulk isotope ratios by conventional means require
contextual information: what is the meaning of methane D/H ratios with
no constraints on the D/H of the co-existing molecular hydrogen? What is
the overall 13C/12C of the carbon
reservoirs from which methane might evolve? This is underscored by the
complex interpretations of 13C/12C
ratios measured on Mars rocks by Curiosity (House et al., 2022). On Mars
and on Enceladus, if methane is synthesized abiotically at high
temperature, we see no physical reason against the appearance of
equilibrium Δ13CH3D and
Δ12CH2D2 signatures
resulting from reordering, as on Earth. Should
Δ13CH3D and
Δ12CH2D2 measurements
ever be done for Enceladus methane, any equilibrium temperatures ≥ 250°C
would rule out a microbial origin of methane.
A complication in developing methane isotopologues as tools for
identifying biogenicity to extraterrestrial methane is that
combinatorial effects are also associated with microbial methane and
under the right conditions, with thermogenic methane (i.e., produced
from thermal decomposition of organic matter). All of these processes
can yield large
Δ12CH2D2 deficits in
the absence of bond reordering because all of them involve multiple
steps of hydrogen addition and draw upon pools of hydrogen with distinct
D/H. Experiments verify this effect for hydrogens with variable D/H
causing Δ12CH2D2deficits in microbial methane (Taenzer et al., 2020). Current evidence
suggests that microbial methane is unique in that it shows
non-equilibrium Δ13CH3D
values, together with markedly negative
Δ12CH2D2 values,
probably as the result of classical kinetics involving carbon. The
release of methane during cracking of organic matter in the laboratory
can result in large
Δ12CH2D2 deficits with
near equilibrium Δ13CH3D (Dong et al.,
2021), and could thus be confused with purely abiotic methane. In any
case, mass-18 methane isotopologues significantly aid us in
differentiating microbial, thermogenic, and abiotic methane on other
planets or icy moons independent of the unknown bulk13C/12C and D/H of sources.