Scientific American - February 2019

62 Scientific American, February 2019

NASA, JPL AND USGS

questions follow: Why did plate tectonics not de velop

sooner, and what course will it take now? As Venus

cools more fully over time, the faults that are now

opening may endure, allowing the planet to un der go

the same transition to plate tectonics experienced on

Earth. If we can watch the beginning of plate tectonics

unfold on Venus, then this process and its accompany-

ing atmospheric stabilization may be common on exo-

planets on the path to habitability themselves.

A          

WE HAVE NEVER HAD BETTER REASONS to send a new major

mission to the oft-ignored second planet from the sun.

With high-resolution global imaging and spectra, we

can answer compelling questions about volcanism

and possible plate tectonics at Venus. Is the process

truly occurring now? How do the surface activities

relate to what is happening in the planet’s interior?

Without volcanism, there would be little surface
water and no place for the origins of life. This cycling
of volatiles helps to sustain Earth’s atmosphere, which
was crucial for the emergence of life. Similarly, conti-
nents, which provide a buoyant, stable platform above
sea level for marine life to evolve onto land, are a
product of plate tectonics. For these and many other
reasons, understanding whether Venus has plate tec-
tonics—and why or why not—is key.
On Earth, limited data suggest that plate tectonics
began as early as four billion years ago, leaving little
record. We do not really know how a planet transitions
from a basalt-covered world, possibly with oceans, to
an intricate system of moving plates with complex
features. One leading hypothesis is that blobs of mate-
rial from deep inside Earth called plumes burst onto
the surface, initiating subduction—the act of one
plate sliding under another. The hot plume weakens
the lithosphere (which includes the crust and upper
mantle) and pushes up, causing the surface to crack,
or “rift.” Pressure from the plume head can create vio-
lent volcanism, as observed on both Earth and Venus.
The load on the cracked lithosphere can cause this
layer to sink and prompt subduction, whereby one
layer of the lithosphere slides under another. If this
process happens often enough, the subducting plates
link up, and plate tectonics begins.
This may be happening on present-day Venus. The
lithosphere on Venus now is warm and thin—much
like Earth’s was back when plate tectonics started up.
And some data show compelling similarities between
features on Venus and terrestrial subduction zones.
One example is Artemis Corona, a circular formation
near the equator on Venus that is similar in scale and
shape to the Aleutian trench that lies under the ocean
along the coast of Alaska. Scientists have theorized
that such Venusian features represent spots where
plumes from the mantle are rising up to the surface
and pushing the crust apart.
Furthermore, recent laboratory experiments and
computer simulations suggest that these plumes are
inducing subduction where they crack through the
top layer of crust. In particular, the experiments
explain why subduction seems to take place around
only part of the circle: as the brittle lithosphere rips
apart in the center, it splits into segments, just as
paper rips into different wedges when poked with a
pencil. As the lithosphere sinks, it continues to tear,
forming segments. If these segments were to join,
we would be seeing the initiation of plate tectonics
on Venus.
Existing images of these features are too low in res-
olution for us to know for sure what we are seeing.
But it appears that plate tectonics on Venus is in the
early stages of development. The Magellan observa-
tions show no evidence of interconnected plates—
rather we see isolated spots where subduction is
beginning, in each case around one of these circular
regions where plumes appear to be rising up. Two

GLOBAL MAPS

of Venus from

Magellan and

Venera space-

craft data show

diverse features,

including the

circular Artemis

Corona ( box ),

that could be

indications of

plate tectonics.

© 2019 Scientific American