ausblenden:
Schlagwörter:
-
Zusammenfassung:
It has long been observed that Earth’s atmosphere
is uniquely far from its thermochemical equilibrium state in
terms of its chemical composition. Studying this state of disequilibrium
is important both for understanding the role that
life plays in the Earth system, and for its potential role in the
detection of life on exoplanets. Here we present a methodology
for assessing the strength of the biogeochemical cycling
processes that drive disequilibrium in planetary atmospheres.
We apply it to the simultaneous presence of CH4 and O2 in
Earth’s atmosphere, which has long been suggested as a sign
of life that could be detected remotely. Using a simplified
model, we identify that the most important property to quantify
is not the distance from equilibrium, but the power required
to drive it. A weak driving force can maintain a high
degree of disequilibrium if the residence times of the compounds
involved are long; but if the disequilibrium is high
and the kinetics fast, we can conclude that the disequilibrium
must be driven by a substantial source of energy. Applying
this to Earth’s atmosphere, we show that the biotically generated
portion of the power required to maintain the methane–
oxygen disequilibrium is around 0.67TW, although the uncertainty
in this figure is about 10% due to uncertainty in
the global CH4 production. Compared to the chemical energy
generated by the biota by photosynthesis, 0.67TW represents
only a very small fraction and, perhaps surprisingly, is
of a comparable magnitude to abiotically driven geochemical
processes at the Earth’s surface. We discuss the implications
of this new approach, both in terms of enhancing our understanding
of the Earth system, and in terms of its impact on the possible detection of distant photosynthetic biospheres.
