72 Science & technology The Economist May 28th 2022
complex structures reminiscent of dna.
Whatever its building blocks, though,
life will need a solvent in which to func
tion. On Earth, that solvent is water.
Water is a good solvent because it is a
“polar” molecule, meaning its electrical
charge is unevenly distributed. In a mole
cule of H 2 O the oxygen has a slightly nega
tive charge and the two hydrogen atoms
are, by way of counterbalance, slightly pos
itive. This polarity causes water molecules
to stick to similarly polar molecules, mak
ing them good at dissolving other chemi
cals—which, in turn, once thus in sol
ution, can interact with each other. That
enables water to support the myriad func
tions of life, and no other abundant chem
ical on Earth matches this versatility.
Other chemicals can, however, fulfil
some of the roles water plays. Life else
where might, perhaps, have found a way to
employ ammonia. This, like water, is polar,
and therefore good at dissolving things. It
is not quite as good at doing so as water,
though, and it also stays liquid (at terrestri
al atmospheric pressures, at least) only be
tween 78°C and 33°C. But that would
make it available in liquid form in frigid
places such as Europa, a moon of Jupiter,
and Titan and Enceladus, moons of Saturn,
where water itself would be frozen.Possible solutions
Titan in particular is believed to host vast
ammoniarich underground lakes which
might act as cradles for chemically exotic
life. But other possibilities exist there, too.
Dr Grefenstette says astrobiologists are al
so intrigued by the lakes of liquid methane
that cover Titan’s surface (the average tem
perature of which is 179°C). Methane ex
ists on the surface of Titan in much the
same way that water does on Earth—in liq
uid, gaseous and solid forms.
Methane is not a perfect solvent for life.
It is not polar and therefore not as versatile
in that regard as water. And it remains liq
uid (again, at terrestrial atmospheric pres
sures) only between 182°C and 161°C.
Since chemical reactions proceed more
rapidly at higher temperatures, on Titan’s
surface they would be pretty slow. But as
trobiologists hypothesise that life com
posed of different materials to those on
Earth—smaller hydrocarbons and nitro
gen, for example—could feasibly eke out
an existence there.
Perhaps the most promising general
purpose alternative to water is formamide,
a colourless organic liquid composed of
carbon, hydrogen, oxygen and nitrogen (all
elements common in the universe) that
can dissolve many of the same chemicals
as water—including proteins and dna. It
can also stay liquid at up to 210°C, making
possible a large range of chemical reac
tions on planets with more extreme sur
face temperatures than Earth’s. Formamide is such an intriguing alternative to
water that some astrobiologists even argue
that it might have been the main solvent
used by the earliest forms of terrestrial life.
This chemical has been located in vast
clouds at the edge of the solar system and
also in more distant nebulae where stars
are forming, according to Claudio Codella,
an astronomer at the Arcetri Astrophysical
Observatory in Florence, Italy. Finding it
definitively on another world would surely
pique interest among those searching for
exotic forms of life.
The units of life on Earth—cells—are
contained within lipid membranes. These
keep the chemical reactions which sustain
life concentrated inside a cell, and the ex
terior world outside it. Such membranes
would not be stable in a medium such as
liquid methane. But exotic lifeforms on Ti
tan might instead build membranes from
structures called azotozomes. These are
molecules, currently hypothetical, made
from nitrogenrich organic compounds,
according to Paulette Clancy, a chemist at
Cornell University who came up with the
idea. They would, she thinks, be capable of
operating in the ultralow temperatures of
a place like Titan.
Or perhaps there could be life without
any membranes at all. Lifelike chemical re
actions have been shown to occur on the
surfaces of certain minerals, including py
rites and various clays. These often con
tain networks of pores and cavities that
could serve the compartmentalising func
tion of lipidbased cells. Or biological reac
tions might be contained within drops of
liquid floating in planetary atmospheres.
Finally, life needs to store information
about itself and pass that information on
to its offspring. Terrestrial organisms do
this using molecules called nucleic acids.
These employ four different molecular un
its known as nucleotides to carry a code of
instructions that can build 20 differentamino acids, which then link up in various
combinations to form proteins. But labora
tory experiments and samples from mete
orites show that many more nucleotides
and amino acids than these exist. Though
they have not been incorporated into life
on Earth, they could form the basis of alter
native systems of genetic information.
Identifying exotic life forms made from
different materials is thus a matter of wid
ening the search from Earthly biosigna
tures—oxygen, methane and so on—to in
clude chemicals that might be made by va
rious imagined biochemical systems. One
tool for this search is the mass spectrome
ter, a device that ionises samples and then
filters those ions by mass. Mass action
Mass spectrometers have been the eyes
and ears of decades of space exploration,
said Luoth Chou, an astrobiologist at
Georgetown University. Successive genera
tions of these devices, flown into space,
have permitted researchers to characterise
chemicals everywhere from the surface of
Mars, via the atmospheres of Venus and Ti
tan, to the plumes of water ejected from
geysers on Enceladus.
The next generation of mass spectrom
eters, though, will be smaller and yet more
powerful. And they will be carried aboard a
range of missions far and wide into the so
lar system. Dragonflywill hop around the
surface of Titan in the mid2030s and take
a closeup look at the molecules there. da-
vinciwill orbit Venus in 2031. The Jupiter
Icy Moons Explorer will explore the Jovian
satellite system, starting in the early 2030s.
And Europa Clipper’s mass spectrometer
will provide a highresolution characteri
sation of that body, beginning at the end of
this decade.
If exotic life does exist, however, it
could use chemistry that goes way beyond
anything astrobiologists can currently
imagine. To get around that means think
ing of biosignatures which depend not on
chemistry but rather on the patterns of be
haviour associated with life.
There is no universal definition of life.
But astrobiologists often default to nasa’s
operational definition of “a selfsustaining
chemical system capable of Darwinian
evolution”. Living things selfreplicate and
make large amounts of specific complex
molecules (for example, proteins or dna).
They also draw energy and consume re
sources from their environments to fuel
their metabolisms. Based on these ideas,
socalled agnostic biosignatures could in
clude looking for excesses in certain ele
ments or isotopes in an environment, or
for specific patterns within groups of
chemicals that cannot be explained by abi
otic processes alone. Peter Girguis, an evo
lutionary biologist at Harvard University,
told the AbSciCon meeting that this new