ASTROBIOLOGY 9Not being able to realistically imagine life
other than as we know it on our planet, as-
tronomers will look for biosignatures iden-
tical to terrestrial ones, having no idea
what biosignatures might be associated
with life forms based on elements other
than carbon or metabolisms different from
those of life on Earth.
The canonical biosignatures of modern
Earth are molecular oxygen (O 2 ) and meth-
ane (CH 4 ). Their simultaneous presence in
an evolved planetary atmosphere is con-
sidered a particularly strong biosignature.
However, their mutual interaction implies
that they have a relatively short lifetime in
Earth-like atmospheres. In fact, stellar UV
rays easily break down oxygen molecules,creating radicals that rapidly destroy meth-
ane molecules (to put it simply). Therefore,
although these two molecules can be pro-
duced by abiotic processes, in the absence
of biological processes capable of con-
stantly replenishing the atmosphere, oxy-
gen and methane could not simultane-
ously be present in quantities detectable
in a planetary spectrum.
On Earth, methanogenesis probably began
in the Hadean, over 3. 5 billion years ago,
thanks to the anaerobic metabolism of the
first bacterial species. Already in that re-
mote aeon, most of the methane present
in the atmosphere was probably biogenic,
despite the fact that the geological pro-
duction of that gas was more prevalantA
n artist’s
concept of a
planet orbiting
in the habitable
zone of a K-type
star. [NASA
Ames/JPL-Caltech/
Tim Pyle]K type star EN_l'Astrofilo 29/08/2019 15:46 Page 9