Sky & Telescope - September 2019 - 14

COSMIC RELIEF by David Grinspoon

Sea Change on Venus
Tidal drag from a putative ocean might have helped cool our sister planet for billions of years.

for two planets so similar in size and
apparent composition. Our planet's
rapid spin sets the 24-hour day-night
rhythm of light and life on Earth, gives
us our cyclonic weather patterns, and
shapes ocean currents. It's a paradise
next to Venus, which a runaway greenhouse desiccated and broiled long ago.
Venus rotates backward, compared to
Earth and most other solar system planets, and so slowly that its yearly orbit
outpaces its daily spin. You can't see the
stars on Venus, but if the sky was ever
clear, their rising and setting wouldn't
be diurnal but annual.
You might expect the planet's
sunward side to be much hotter than
its nightside, but Venus also possesses
continuous planet-wide clouds of sulfu-

S E P TE M B E R 2 019 * SK Y & TELESCOPE

ric acid and an atmosphere 92 times as
thick as Earth's. This redistributes heat
so effectively that no temperature differences exist from day to night or from
equator to pole. At ground level, it's all
a sweltering 460°C (860°F).
As hard as it is to imagine today,
observations suggest that Venus might
once have had water oceans. What
effect might global seas have had on the
planet's unusual rotation rate? Oceanographer Mattias Green (Bangor University, UK), collaborating with planetary scientists Michael Way (NASA
Goddard) and Rory Barnes (University
of Washington), tackled this question in a recent study. They found that
ocean tides could have caused drag
forces on the planet, reining in its rotation by as much as 72 Earth days every
million years. So if Venus started out
with an Earth-like rotation rate and an
ocean, it could have decelerated to
its current day in less than 50
million years.
This would seem to
relate to climate history
as well, but how? In
an illustration of
just how complex
and counterintuitive planetary science can be, several initial news
stories about the
team's result got
it backwards.
As the rotation
slowed, these
stories implied,
the planet's oceans
would have been more
vulnerable to heating
and evaporation by the
young, warming Sun.
In reality, the effect would
likely have been the opposite. When

some colleagues and I, led by Way, modeled the ancient atmosphere of Venus
with a general circulation model of the
kind we use to model climate changes
on Earth, we learned - to our surprise
- that a slowly rotating Venus is more
effective at holding onto an ocean than
a rapidly rotating one.

On a slowly spinning oceanic
Venus, the clouds organize themselves so that the
dayside is always cloudy and
the nightside is always clear.
This is perfect for keeping
the planet cool.
This results from cloud behavior.
On a slowly spinning oceanic Venus,
we discovered that the clouds organize
themselves so that the dayside is always
cloudy and the nightside is always clear.
This is perfect for keeping the planet
cool, because clouds reﬂect sunlight but
clear night skies allow for maximum
cooling. So if Venus had a habitable
ocean that put the brakes on its rotation
speed, it might have helped keep the
planet cool for up to 2 billion years.
Clearly, "habitable zone" is not a
simple question of distance from a
star. Through more missions and more
modeling, we'll need to understand the
complexity of oceans, tides, and atmospheric motions to get a handle on just
where a biosphere could thrive - on a
primordial Venus or elsewhere.
■ Contributing Editor DAVID GRINSPOON is the author of Venus Revealed:
A New Look Below the Clouds of Our
Mysterious Twin Planet.

LE AH TISCIONE / S&T

VENUS AND EARTH are a strange pair

Sky & Telescope - September 2019

Table of Contents for the Digital Edition of Sky & Telescope - September 2019