68 BriefingAstrobiology The EconomistFebruary 13th 2021
2 ried on board Vikings 1and 2 , which landed
successfully on Mars in 1976. These mixed
samples of Martian regolith with organic
compounds of the sort consumed by mi-
crobes on Earth. Those compounds were
labelled with radioactive carbon atoms,
and any gas evolving from the experimen-
tal mixture was tested for radioactivity, to
see if those compounds were being meta-
bolised. The results suggested metabolic
activity in the regolith at both landing
sites, which were over 6,000km apart.
They were discounted at the time as being
the product of some unknown abiotic reac-
tion, because other experiments showed
no sign of organic compounds in the rego-
lith. But they have never been explained.
The other result that has not quite gone
away was the analysis in 1996 of a meteorite
blown off the surface of Mars by an asteroid
impact and collected in Antarctica. This
contained several features, including puta-
tive microfossils, unusual grains of mag-
netite of a type made biologically on Earth,
and organic compounds, that were taken at
the time as possible signs of life. All are
now known to have alternative, and more
widely believed, explanations. But, at the
time, the meteorite’s discovery stimulated
interest in the possibility of life on Mars.Wild rovers
And there has, indeed, been an intensive
programme of investigation of the place
since the 1990s, using orbiters, landers and
wheeled surface vehicles known as rovers.
This has followed up the discovery by the
Vikingmissions of what looked like water-
carved topography, by showing that parts
of Mars are covered with sedimentary
rocks, and that these include clay minerals,
which, on Earth, often form in the presence
of water. This demonstrates, to most peo-
ple’s satisfaction, that the planet did once
play host to bodies of liquid water.
nasa’s next Mars rover, Perseverance,
scheduled to arrive there on February 18th,
will look in some of the rocks it encounters
for fossilised remnants of microbial mats
called stromatolites. It will also, by collect-
ing and bottling for future retrieval the
most interesting samples it finds, be the
first step in a decade-long multi-agency
mission to bring samples of Martian rock
back to Earth. Then, in 2023, Perseverance
will be followed by a rover from esacalled
Rosalind Franklin. This will drill a few me-
tres under the Martian surface to search for
microbes, both fossilised and living.
The idea of life on Mars goes back at
least as far as 1877, when Giovanni Schiapa-
relli, an Italian astronomer, thought he saw
dark lines on the planet, which he inter-
preted as water-carrying channels with ve-
getation growing along their banks. These
turned out to be optical illusions, but the
idea stuck. That there might be life on some
of the moons of Jupiter and Saturn is amore recent suggestion, stimulated by the
discovery by probes sent to orbit those
planets that some of their ice-covered
moons appear to have subsurface oceans of
liquid water.
This thought is taken seriously enough
for nasato be laying plans for missions to
Europa, a moon of Jupiter, and Enceladus, a
moon of Saturn. The idea is to sample water
in the plumes of geysers erupting from
these moons (see picture on previous
page), and test it for chemicals that might
indicate life. nasaalso plans to launch a
drone called Dragonflyon Titan, another
moon of Saturn, to search its surface for
molecules that might be life’s precursors.
If it turns out that somewhere else in
the solar system either has, or had, life, one
further question needs to be answered.
This is, is it the same as life on Earth? One
school of thought, known as panspermia,
suggests that life might not have evolved in
situeverywhere that it is found, but could
instead spread from place to place. This
would be easier within systems of planets
than between them. If organisms live on
other bodies in the solar system it should
be possible to work out from their bio-
chemistry whether they share an ancestor
with those on Earth—and it might even be
possible to do this with fossils, if enough of
their chemical structure is preserved.
Either answer to the panspermia ques-
tion would be interesting. If any life on Ve-
nus, Mars or the Jovian or Saturnian moons
had separate origins from life on Earth, it
would suggest that biology starts up easily.
If it started on only one of them, and then
spread from place to place within the solar
system, that would give a boost to the idea
of it spreading between star systems, too,
since two examples are now known of
rocks from elsewhere entering and then
departing from the solar system, either of
which might have carried bacteria-like or-
ganisms deep within it, in cold storage.The third approach to astrobiology, be-
sides looking for biosignatures and visit-
ing promising planets and moons, is to
scan the cosmos for signs of technology
elsewhere. This is, perhaps, the most flaky
tactic. But it is also the most far reaching. It
is flaky because the history of life on Earth,
at least, suggests that the path from pri-
mordial soup to engineering prowess is a
long and winding one. It is far reaching be-
cause such prowess might manifest itself
in ways such as radio signals that can be de-
tected at greater distances than any natural
sign of life.
People have been listening for radio sig-
nals from etalmost since the invention of
radio telescopes. There have been a couple
of false alarms, but nothing definite. The
use of lasers for communication on Earth
has raised suggestions of looking for sig-
nals from these, too—or even taking the
initiative and beaming messages by laser
towards promising nearby planetary sys-
tems. And real optimists wonder whether
super-advanced civilisations might engage
in engineering projects large enough to
leave a footprint in the spectrum of their
home stars. Again, there have been false
alarms provoked by this thought. But noth-
ing concrete.Hope springs eternal
For the moment, then, sheer weight of
numbers suggests that the most likely
place to find evidence of alien life remains
the Habitable Exoplanets Catalogue or
some future, similar, compilation. Num-
bers increase the chance of a lucky strike.
They also allows the application of statis-
tics to the problem, for it is likely that, rath-
er than looking for yes/no answers to the
question of life elsewhere, researchers will
have to search for probabilities. This could
mean measuring the levels of several gases,
each with a different likelihood of having
come from a biological source, and com-
bining those data with an understanding of
environmental context. The hope would be
that life-bearing anomalies would stick out
like sore thumbs, in the manner of Dr Cock-
ell’s putative oxygen-rich planets.
As to whether astrobiologists actually
believe life exists elsewhere, that is not ex-
actly a scientific question, but it is perti-
nent to their motives. David Grinspoon, a
veteran of the field who is part of the Plane-
tary Sciences Institute, an American re-
search organisation, puts it thus: “I think
there’s widespread belief in extraterrestrial
life now among scientists, even though we
don’t have specific evidence for it.”
Dr Grinspoon observes that astrobiolo-
gy is where exoplanetology was when he
was a student in the 1980s—patiently wait-
ing for the right tools to become available.
Exoplanets made the transition from belief
to reality with aplomb. Whether etwill fol-
low suit remains to be seen. 7