Sky & Telescope - November 2021 - 12

COSMIC RELIEF by David Grinspoon
Saturn's moon Enceladus, captured here
after a close pass by the Cassini spacecraft
in 2008, is a prime target in our search for
beyond-Earth life.
Life
Beyond
Earth?
By century's end, the
author thinks we'll
have an answer to this
question. But what kind
of answer?
OVER THE 80 YEARS in which Sky &
Telescope has been informing us about
the universe and those who explore
it, scientific and popular beliefs about
extraterrestrial life have fluctuated
between grim and optimistic.
Before the Space Age, many scientists
were naively confident about the
prospects for life nearby. With only
telescopic data to constrain our wishful
thinking, it was possible, and popular,
to imagine vegetation forming the
seasonal " wave of darkening " on Mars
or soaking up the water we thought was
raining down on Venus.
In 1962, NASA's Mariner 2 began
the work - which continues today - of
replacing conjecture with observation.
With the first successful spacecraft
visit beyond the Earth/Moon system,
it showed conclusively that the surface
of Venus was too hot for liquid water or
organic life. Three years later, Mariner
4 revealed a cratered " moonscape " on
Mars that dampened hopes of finding
the Red Planet awash in green.
Since then, we've been through several
rounds of raised and then dashed
hopes of finding nearby company.
Orbiting Mars, Mariner 9 spied dried12
NOVEMBER 2021 * SKY & TELESCOPE
up riverbeds. Carl Sagan and a number
of colleagues convinced themselves that
Martian soil organisms might be merely
awaiting a splash of " chicken soup, " but
their 1976 Viking life experiments came
up short.
NASA soured on Martian life, but
only temporarily. Expectations that
life on Mars lies just under the next
rock have repeatedly risen and fallen.
A " discovery " (later proven false) of
chlorophyll, a hint of possible fossils in
2's reality check on our dreams of an
Earth-like neighbor, a habitat may yet
lurk in the clouds. So even if, as some
of us suspect, Mars is completely dead,
multiple places remain in the solar
system where life-optimists can invest
their hopes - if only because we haven't
really explored them yet.
But I believe that in another 80
years we'll have the answer to " Are
we alone? " The discovery could come
in one of these unexplored corners of
our own system. But given the sheer
number of exoplanets, the odds seem
good that, after we've carefully observed
thousands of them, we'll have found
unmistakable biosignatures (S&T: May
2021, p. 34) or even technosignatures.
By the time the next century arrives, I
bet we'll have identified several classes
of inhabited worlds and be well on our
way to a mature science of comparative
astrobiology that can investigate the
cosmic distribution of life.
But what if we haven't? It's hard
to prove a negative. Yet what if we've
plumbed the ice-moon oceans, sifted
through the Venusian clouds, and
examined scores of high-resolution exoplanet
spectra and found not a trace of
anything swimming or breathing? Then
we will be pretty far down the path of
concluding that there really is someBy
the time the next century arrives, I bet we'll have identified
several classes of inhabited worlds. But what if we haven't?
a meteorite, and perennial announcements
of water and whiffs of methane
all have revived hopes, at least for a
little while.
Meanwhile, in the outer solar system
we've discovered worlds that hold their
oceans on the inside. Europa, Enceladus,
Titan, and several other moons,
even the dwarf planet Pluto - all are
possible homes for aqueous life. And
don't count out Venus. As I've written
here (S&T: Jan. 2021, p. 15), there are
reasons to suspect that, despite Mariner
thing very peculiar and unique about
our planet and its denizens after all. If
that's the case, then in some strange
sense the universe is our responsibility.
Either way, life is precious. Let's work
toward a sustainable presence here on
Earth so we can keep exploring and find
out how lively our universe is.
¢ DAVID GRINSPOON wrote his first
Cosmic Relief column - about how the
field of astrobiology is looking up - for
the January 2009 issue.
NASA / JPL / SPACE SCIENCE INSTITUTE
Sky & Telescope - November 2021

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