Sky and Telescope - November 2018 - 12

COSMIC RELIEF by David Grinspoon

For both Venus and Earth, life's
last refuge might lie in the clouds.
BEFORE THE SPACE AGE, it was common to regard Venus as a planet stuck
in a state resembling Earth's deep
past - perhaps a carboniferous swamp
world with tree ferns and giant reptiles. Now we know that our nearest
planetary neighbor is actually a vision
of our likely future.
When Venus and Earth were both
young, their surface conditions were
probably very similar, sporting warm,
organics-rich oceans and active geology. They were the kind of places
where we think life could easily have
formed, if our current understanding
of both the origin of life and of Venus
are correct.
Yet the Sun gradually brightens as
it ages, causing the inner edge of the
habitable zone, where surface water can
exist on rocky planets, to slide inexora-

N OV E M B E R 2 018 * SK Y & TELESCOPE

bly outward. Some time in the remote
past it swept past Venus, and her seas
evaporated. Some time in the future
the same thing will happen to Earth.
What became of Venusians (if any
existed) when the oceans vanished
and the surface became too scalding for organic life? Is it possible that
some organisms found sanctuary up
in the atmosphere? For more than
20 years I've promoted the plausibility of a cloud biosphere on Venus. My
mentor Carl Sagan ﬂoated this idea
even earlier, and some colleagues have
elaborated upon it in a recent paper
published in the journal Astrobiology
(see https://is.gd/venusclouds).
Conditions in the cloud deck are
moderate - roughly the same temperature and pressure as at Earth's surface.
Moreover, plenty of nutrients and

■ DAVID GRINSPOON is author of Venus
Revealed: A New Look Below the Clouds
of Our Mysterious Twin Planet (1997), in
which he first hypothesized about microbial life in Venusian clouds.

N ASA

A Nebulous
Existence

p Ultraviolet
image of Venus
taken by the
Pioneer Venus
Orbiter

energy sources exist there, even liquid
water in the form of concentrated
sulfuric acid. Also, there are some
stubborn riddles concerning the clouds
and upper atmosphere that make us
wonder. These include unidentiﬁed
structures inside some of the larger
cloud particles, and unidentiﬁed material that absorbs most of the solar
energy striking the planet, both of
which could conceivably have a biological explanation.
Thanks to the discovery of extremophile organisms on Earth, which thrive
in strong acid, along with the persistence of those cloud-deck mysteries, interest in the life-in-the-clouds
proposal has increased lately. When
NASA or another space agency next
launches a mission to explore Venus's
atmosphere, astrobiological prospecting surely will be among the goals.
One compelling reason to explore
Venus and to reconstruct its past is to
understand the far future of Earth.
This is true about climate: If we
can grasp the mechanism and timing of the runaway greenhouse that
destroyed Venus's surface oceans, we'll
know better what lies ahead for our
world when the Sun inevitably warms
it to the point that Earth can no longer
hold its water (S&T: Oct. 2017, p. 22).
It may be true about geology as well:
Plate tectonics, so vital to the functioning of our home world, seems to
have ceased on Venus (if it ever operated there at all). We're not sure why,
but it might have to do with the drying
out of the interior that accompanied
the loss of surface water.
Finally, Venus may even give us a
glimpse of our planet's old age in terms
of its biology. If life on a dying oceanic
world can migrate to the clouds, as
might have taken place on Venus, then
that might be the long-term fate of
Earth's biosphere. The clouds may be
our planet's ﬁnal habitable zone.

http://is.gd/venusclouds

Sky and Telescope - November 2018

Table of Contents for the Digital Edition of Sky and Telescope - November 2018