Sky and Telescope - September 2018 - 15

t PANCAKE DOMES These seven circular hills in Alpha Regio, on average 25 km wide and 750 m high, appear to be thick lava flows that welled
up onto level ground, which allowed them to flow out in an even pattern.

BACKG ROUND: USGS ASTROG EOLOGY SCIENCE CEN TER; CLOUDTOPS: ESA / MPS / DLR / IDA , M. PÉRE ZAY ÚCA R & C. WILSON; PA NCA K E DO MES & TICK: N ASA / JPL; M AG ELL A N TE A M: N ASA / JPL, S&T ARCHIVES

p VENUSIAN TICK This bizarre volcanic construct is a tick, a caldera
surrounded by radiating ridges and valleys. Lava flows breaching the rim
created the "head." Ticks sometimes appear near the deformed terrain
regions called tesserae, but it's unclear if they're related.

Earth, as we know, transformed into a paradise ﬁt for life,
while Venus morphed into a hellscape. It boasts clouds of sulfuric acid and an atmosphere that slams down on the surface
with 90 times the pressure found in Earth's atmosphere. That
surface averages a blistering 460°C (860°F) - hot enough to
melt lead. Smooth, gently rolling plains cover about 70% of
its surface - the result of past volcanic ﬂows, some of which
travel for thousands of kilometers before fanning outward.
Recent research even suggests that volcanism continues
today. Needless to say: Venus is hot, stiﬂing, and dynamic.
Magellan and its predecessors also revealed terrain studded with mountains, plains, high plateaus, canyons, and
ridges. The planet even boasts highlands akin to Earth's
continents. The two largest are the sprawling Aphrodite Terra
along the equator, nearly as large as Europe and Asia combined, and Ishtar Terra in the northern hemisphere, roughly
the size of Antarctica. Those two highlands stand a few kilometers above the plains, roughly similar to the rise of Earth's
continents above its seaﬂoor, and they're marred by more
than 1,000 large volcanoes and 100 mountains - the tallest
of which tower above the landscape at a height greater than
that of Mount Everest in the Himalayas.
But that range of peaks, Maxwell Montes, built up in a different way than the Himalayas, which were created when two
tectonic plates - those large slabs of rock that divide Earth's
crust and jostle about - rammed into each other. In fact,
Venus does not appear to host plate tectonics at all. That much
can be seen from the distribution of volcanoes, which do not

create long chains along the boundaries of tectonic plates like
they do on Earth, but dot the surface haphazardly. It's a mystery whose answer might explain Venus's young landscape, its
stiﬂing atmosphere, and even phenomena on our own world.

Shedding Light on Earth
On Earth, jostling crustal plates help regulate our planet's
temperature over tens to hundreds of millions of years as
carbon dioxide shifts between the atmosphere and Earth's
mantle. So it could be that Venus once had plate tectonics, but over time those plates became thicker and harder to
break apart or subduct. As a result, there was no way to pull
carbon dioxide out of the atmosphere and into the mantle,
forcing the planet down a path that led to extreme temperatures and atmospheric pressures.
u MAGELLAN TEAM
Members of the
Magellan team pore
over images from
the orbiting spacecraft. The map being
studied is assembled
from the long strips of
data from Magellan's
pole-to-pole passes.
From left to right: Nick
Stacy, Ellen Stofan,
Barry Parsons (rear),
and Don Campbell.
sk yandtelescope.com * S E P TE M B E R 2 018

http://www.skyandtelescope.com

Sky and Telescope - September 2018

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