60 Scientific American, February 2019boundaries of the habitable zone move outward with
time as the sun’s luminosity increases with age. Venus
is now outside this range and occupies what we call
the “Venus zone,” where surface conditions are so hot
that a planet is likely to have a runaway greenhouse
atmosphere that would boil its oceans away.
Venus and Earth formed under very similar condi-
tions—including those that gave Earth its oceans.
Comet impacts probably brought ice to the surface of
both planets. The solar wind (charged particles gush-
ing off the sun) most likely implanted a thin layer of
hydrogen ions on the surfaces of both. And when
Venus and Earth were protoplanets building up from
the primordial dust disk that circled the sun, both col-
lected hydrogen and other volatiles, chemicals thatcan easily boil away. Simulations of early Venus show
that the planet’s surface may have had liquid water
earlier than Earth and that water might have been
there until about a billion years ago.
The fact remains, though, that Venus is now for-
biddingly inhospitable. What happened? Does Venus
represent the end state for all habitable planets, or is
it merely one of many ways that planets of this size
can turn out? These are some of the major questions
we want to go back to Venus to answer.S S A
OUR KNOWLEDGE OF VENUS is limited in part by the
immense difficulty of seeing through the planet’s
thick, noxious atmosphere. High up, clouds of sulfuric
acid shroud the world. On the ground, the air pres-
sure is comparable to the water pressure 3,000 feet
below the surface of Earth’s oceans. The atmosphere
there is so dense that its main constituent, carbon
dioxide, acts as a supercritical fluid, with properties
midway between a gas and a liquid.
Scientists think this atmosphere was once Earth-
like. Unlike our world, though, Venus now lacks a
magnetic field to repel the solar wind. We think that
over the eons, the solar wind eliminated the planet’swater by dissociating it into hydrogen and oxygen ions
and carrying them off into space. Without surface
water to dissolve the carbon dioxide and other gases
constantly escaping from the interior, these chemicals
accumulated in the atmosphere. Because of the green-
house effect of this atmosphere, surface temperatures
on Venus are nearly 800 degrees Fahrenheit higher
than on Earth—hot enough to make rocks glow.
The only data we have from the surface of Venus
were collected by the four Soviet Venera landers that
touched down in the 1970s and 1980s. These probes
survived for only a few minutes on the planet’s brutal
surface, but in that brief time they gathered and sent
back rough measurements of the chemical composi-
tion there. Beyond those readings, our knowledge of
the surface mineralogy rests solely on controversial
interpretations of radar measurements made by
Magellan and our limited knowledge of probable
chemical reactions between the planet’s rocks and
atmospheric gases under Venusian conditions.
Recently, though, researchers found that it is pos-
sible to map the minerals on Venus from orbit by
looking through several “windows” in the electromag-
netic spectrum where visible light escapes absorption
by carbon dioxide in the atmosphere. Serendipitously,
these windows coincide with critical regions for iden-
tifying the typical planetary minerals olivine and
pyroxene, offering hope that we could finally deter-
mine the basic ingredients of Venus. Europe’s Venus
Express spacecraft, which orbited Venus from 2006 to
2014, used one of these windows to produce the first
map of heat radiating from the planet’s surface over
much of the southern hemisphere. This map includes
spectral features—peaks and dips in light and heat—
that can identify minerals on the ground.
The map also identifies many hotspots—areas emit-
ting so much heat that the most likely explanation is
recent volcanism. This is an exciting find be cause it
shows that unlike the moon, which has long been
silent, and Mars, where modern volcanism has been
isolated at best, Venus is still active—and that discov-
ery has implications for the planet’s suitability for life. A S
ON EARTH, volcanism is usually associated with plate
tectonics—the shifting and sliding of large pieces of
crust responsible for most of the geologic features on
our planet. Plate tectonics is also behind the long-
term climate cycles, occurring over periods of around
100 million years, that enabled life to arise on Earth.
Plate tectonics formed new crust at Earth’s mid-ocean
ridges and allowed layers of its crust to sink into the
mantle—two processes that enabled our planet to lose
its internal heat and cool to a point where life could
arise. Tectonics also released volatile chemicals such
as water, carbon dioxide and sulfur dioxide from deep
within Earth out into the atmosphere and cycled vola-
tiles back into the mantle when plates slipped under-
neath other plates.We have never had
better reasons to
send a new major
mission to the oft-
ignored second
planet from the sun.
© 2019 Scientific American