Sky & Telescope - May 2020 - 12

COSMIC RELIEF by David Grinspoon

Venus Is Dead.
Long Live Venus.
FOR MORE THAN TWO DECADES I've

been arguing that Venus - so often
voted "least likely to succeed" by astrobiologists - might possibly harbor life.
Not on the scorching surface but high
up in the clouds, where temperatures
are mild, nutrients and energy abound,
and droplets consist of water that,
while steeped in sulfuric acid, are less
acidic than some environments where
life is found.
This contrarian position has produced some eye-rolling at conferences,
but nobody has come up with a good
reason why it's wrong. And now a
funny thing has happened: An international workshop on Venusian cloud

t A falsecolor image of
Venus's nightside taken by
the Akatsuki
spacecraft in
2016

12

M AY 2 0 2 0 * S K Y & T E L E S C O P E

life held in Moscow in October 2019,
along with several recent publications,
appear to have moved the idea from
the fringe to near-respectability.
Several results contribute to this
shift. Observations from the Venus
Express and Akatsuki missions have
deepened some of the Venusian mysteries that cloud life might explain,
such as unidentiﬁed airborne material
that absorbs more than half the solar
energy falling on Venus. Exoplanets
in the "Venus zone" have renewed
astronomers' interest in the possible
habitability of close-in planets. And
new calculations point toward Venus
having had water oceans for much of
its lifetime.
So now let me tell you everything
that's wrong with the idea. Here's why
Venus is most likely a dead world:
Maybe life can't live in clouds.
Although microbes thrive in clouds
on Earth, no known species lives its
entire life cycle in this environment.
Maybe the chemistry of Venus's
clouds is just too harsh. While Earth
does have acid-loving microbes, we
don't know the detailed composition of the Venusian cloud particles
or whether they would be compatible
with any form of life.
Maybe life couldn't migrate to
the clouds when the oceans disappeared, or maybe the clouds haven't
been continuously habitable. Evolution on Earth shows that life is
extremely good at ﬁnding habitable
niches and evolving when conditions
change. But we don't know if Venus
has been continuously cloudy since the

surface became uninhabitable, so a
cloud niche may not have been stable
over long time scales.
Maybe an ocean never had the
right conditions. Some scientists think
that for life to start you need oceans
and continents. If early Venus had too
much water and no land/water interface, life might not have arisen. Or a
speciﬁc necessary environment, such
as seaﬂoor hot springs, might have
been absent or chemically different
enough to preclude life's origin.
Maybe an ocean didn't last long.
Calculations showing that an ocean
could have lasted for billions of years
depend on several assumptions about
early Venus. They require that the
planet always rotated slowly, as it does
now; a fast-rotating Venus would have
quickly lost its oceans. But we have no
idea what the early rotation rate was.
Venus may never have had an
ocean. The young Sun may have vaporized any water, with the solar wind
blowing away a steam atmosphere
before it condensed on the surface.
Maybe Earth just lucked out. Life
could be improbable, dependent as
much on some absurd twist of fate
as on speciﬁc planetary conditions,
stages, or events.
So what do you think? I've made
my best attempt to tell you why Venus
should be dead - though I haven't
really convinced myself. In any case, I
think it's worth a look!
■ DAVID GRINSPOON is the author of
Venus Revealed: A New Look Below the
Clouds of Our Mysterious Twin Planet.

© PL A N E T- C PRO JECT TE A M

In which I attempt a contrarian argument against
my own contrarian argument
Sky & Telescope - May 2020

Table of Contents for the Digital Edition of Sky & Telescope - May 2020
