Sky & Telescope - April 2022 - 4

SPECTRUM by Peter Tyson
Water Worlds
WHEN WE LOOK UP at the night sky, we don't tend to think of
oceans. Deep, blue seas exist down here, not up there. Yet if what
we suspect about certain moons in our solar system is true, then
oceanic worlds might be ubiquitous, not just in our planetary system
but in many others.
Astronomers think that at least six moons in the outer solar system conceal
water oceans beneath their frozen crusts. These include Jupiter's moons Europa,
Ganymede, and Callisto - the subject of Ben Skuse's cover story on page 14
- as well as Saturn's moons Titan and Enceladus, and Neptune's Triton. Ariel
and Miranda, two moons of Uranus, are among other bodies beyond Mars that
might shelter subsurface seas, too. But we haven't collected enough data on
those satellites yet to feel as confident about them as we are about the other six.
Could one or more of these watery orbs harbor life? For our entire history
as a species, we've not known whether we're alone in the universe. It's a simple
yes-or-no question, but so far we've been unable to answer it. That's what makes
the two upcoming missions to Jupiter's moons that Skuse discusses in his feature
so rousing: They might bring us tantalizingly
close to answering that age-old query.
Any living things in those shadowed oceans
wouldn't be able to breathe fresh, sunlit air on a
spring morning, say, or float beneath a star-spangled
night sky - there's neither air nor sky under a thick
blanket of ice. But they could thrive nevertheless,
perhaps thanks to hydrothermal vents like those
that sustain a wide array of creatures in the pitch
black thousands of feet down in our own oceans.
It's unlikely that the two missions Skuse
pMight the nearest aliens
inhabit the icy underworld of
Jupiter's moon Europa?
describes - NASA's Europa Clipper and the ESA's
Jupiter Icy Moons Explorer (JUICE) - will directly
determine whether any Jovian moons host organisms.
But the spacecrafts' observations will help us
consider that and other compelling mysteries: If someday we find alien life, will
it use the same biochemistry as life on Earth does, or something different? If
the same, would we be able to tell if it represented a wholly independent origin
of life? How does non-life become life? When conditions are right, does life
arise as a matter of course, or is it an exquisitely rare phenomenon?
Such questions fire up astrobiologists, who will be waiting breathlessly for
results from the Europa Clipper and JUICE missions.
Answering one or more of them would truly be, in a manner
of speaking, Earth-shaking.
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Sky & Telescope - April 2022

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