Sky & Telescope - January 2021 - 15

COSMIC RELIEF by David Grinspoon

Venus
Surprise
Will phosphine be
the "Mars rock" of
our sister planet?

phine, a candidate biosignature gas, on
Venus! (See page 10.) Exciting, but what
does this mean?
There are three possible outcomes.
Least likely is that it turns out to be
genuine evidence for life. But we can't
rule this out and indeed must take it
seriously as a viable hypothesis while
we investigate further. That's worthy of
excitement and wonder.
Barring life, phosphine may point to
unknown chemistry in the Venusian
atmosphere. This is important to understand if we hope to use biosignature
gases to infer the possible presence of
life on exoplanets.
Or, the detection could be shown to
be false, as scientists scrutinize, repeat,
and supplement the detection methods
with further, less ambiguous techniques. The team that published this
ﬁnding did careful, rigorous work, but it
was a tricky analysis, and new research
will conﬁrm or reject it.
We've had false alarms before. A
famous one was the "Mars rock" - the
evidence for microfossils in the Martian meteorite ALH84001, reported in
Science in 1996. Though now widely
regarded as a misinterpretation, the
"Mars rock" episode forced us to think
about possible life on early Mars. Many
skeptics suddenly began saying, "Why
not?" Out of this came a massive boost

for both astrobiology research and Red
Planet exploration.
The same could happen for Venus.
Indeed, I believe we'll soon see several
new spacecraft launched to explore the
clouds, atmosphere, and surface, to
better understand the geology, climate,
evolution, and maybe even past or present biology.
There's more than one way to look
at the question of life on other planets.
This is good since we know nothing
about extraterrestrial life. We think
we know that life needs water, organic
matter, and energy. But it's worth noting that Earth, the only living world
we know, has some rare properties that
could also be key to life's emergence.
Our planet is active and self-renewing. Life here relies on two giant heat
engines: the restless, overturning interior, which constantly feeds new minerals and gases to the surface; and the
sunlight-driven churning of our atmosphere, which circulates those gases and
erodes those rocks, ever exposing fresh
surfaces and fueling the geochemical
cycles upon which life feeds.
Venus, too, has a geologically active
surface fed by a restless interior engine
and a thick, dynamic, solar-driven
envelope. In our solar system, arguably
only Venus and Earth share these qualities (although Titan gets an honorable
mention).

S Japan's Akatsuki spacecraft captured this
false-color image of Venus's dayside on July
23, 2016.

This is why I've long pushed the idea
of a possible biosphere in Venus's cool
clouds. It's not just the relatively moderate conditions, potential nutrients, and
energy sources, and not just strange
mysteries such as the "unknown ultraviolet absorber" that acts almost like a
photosynthetic pigment, hungrily sucking up more than half the solar energy
reaching the planet.
It's that rare liveliness of the Venusian atmosphere, which suggests to
me that perhaps something could have
evolved to take advantage of that fertile
environment. Recent model results
showing that Venus was likely habitable
on the surface for most of its history
provide a compelling origin story for
life, which may have ended up as "climate refugees" in the clouds.
If the phosphine holds up, then
clearly we need to go and investigate.
Even if it doesn't, the ﬁnding is shining
a bright spotlight on this underexplored
and mysterious realm. It is forcing scientists to reexamine their assumptions
and to once again ask, "Why not?"
■ Contributing Editor DAVID GRINSPOON is the author of Venus Revealed:
A New Look Below the Clouds of Our
Mysterious Twin Planet.
sk yandtelescope.org * JANUARY 2 021

SCIENTISTS HAVE REPORTED phos-

http://www.skyandtelescope.org

Sky & Telescope - January 2021

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