Sky and Telescope - September 2018 - 14

THE CASE FOR VENUS

by Shannon Hall

DESTINATION :

Venus?
Our toxic twin might be a scientific treasure-trove,
but infrequent visits are hindering studies, causing
many planetary scientists to grow frustrated.

I

t was a little after 3 a.m. when Ellen Stofan jumped into
her car and raced toward the Jet Propulsion Laboratory.
She had just learned that the first radar image of Venus
had arrived from the Magellan spacecraft, and she literally
could not wait to see it.
Stofan, the mission's deputy project scientist at the
time, was one of the first people to peer beneath the clouds
using Magellan and inspect our sister planet in unprecedented detail. And that first black-and-white image in
August 1990 did not disappoint. It revealed a vast volcanic
plain scarred by a giant impact crater. Unlike the Moon,
Mars, and Mercury, Venus appeared to have been geologically active in the recent past.
But it was only a hint of what was to come. By the end of
its four-year mapping effort, the spacecraft revealed 98% of
the planet's surface at a higher resolution than ever before.
"We uncovered this amazing, confusing planet," Stofan says.
The most confusing - and compelling - aspect, Stofan
says, was that so many images looked like the first. Not
only is Venus covered in lava flows, but it also lacks a lot
of craters (which build up over time), suggesting that the
planet resurfaced much of itself only a few hundred million years ago. And although scientists aren't sure what
can cause such a startling global change, they do think
the answer might help explain how the planet's runaway
greenhouse began - a twist of fate that caused the oncehabitable world to turn into a toxic one. Needless to say,
Stofan and her colleagues were eager to send more missions to the world in search of an answer.
But while the European and Japanese space agencies
have both mounted successful orbital missions to Venus,
NASA has not returned there. And the geologic questions
that Magellan raised remain a mystery today - nearly 30
years later.

14

S E P TE M B E R 2 018 * SK Y & TELESCOPE

p CLOUDTOPS Bright and dark bands caused by an unknown chemical absorber mark this ultraviolet view of Venus from the European
Venus Express orbiter. The patterns are about 70 km above the surface.

It's a tragedy to many U.S. planetary scientists, some
of whom are now up in arms over NASA's shift away from
Venus exploration. In the years since Magellan, planning
teams have proposed more than 25 new Venus missions.
Every single one has been shot down. The latest saga
ended in December when Venus researchers received a
double dose of bad news: Yet another two mission proposals were rejected.
"We jump-started our understanding of Venus with
Magellan, and then it was just like we got cut off at the
knees," Stofan says.
Yet not only does Venus likely hold the key to understanding open questions about Earth, but it also can
address habitability in general - an issue that's crucial now
that astronomers have discovered thousands of exoplanets, many of which likely resemble Venus. For that reason,
Venus scientists are trudging onward, hopeful that the
tides will soon turn and that NASA will one day return to
the evening star.

Volcanic World
There's no doubt that Venus is Earth's deadly sibling. The
two planets are near-twins in size, density, gravity, and,
likely, chemical makeup. Although today Venus doesn't
reside in our Sun's life-favoring "Goldilocks zone," it, along
with Mars, might have been habitable billions of years
ago. (Yes, some astronomers argue that Venus once hosted
global oceans and moderate temperatures.) But despite
these similarities, something caused the two planets to
wander down two very different evolutionary paths.



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