Sky & Telescope - October 2021 - 17

some other efficient means of drawing
carbon out of the atmosphere and
returning it to the interior, a sudden,
massive influx of this greenhouse
gas would overwhelm that system
and quickly start to raise the surface
temperature. Increasing temperatures
would then lead to more evaporation
of liquid water on the surface, which
would trap more radiation, which
would cause more evaporation . . . you
get the picture.
A Matter of Luck?
The implication of this line of thinking
is that as the Sun ages, it does not
have as quick an inimical effect on its
nearby planets as we might have feared.
Sure, eventually a swelling red giant Sun
will roast Earth (S&T: Oct. 2017, p. 22),
but what was once seen as perhaps a
climatic inevitability - that, on geological
time scales, Earth would soon
endure the same climate catastrophe as
Venus - is now a more distant destiny.
That's the good news.
The bad news is that enormous
volcanic eruptions are geologically
common on our planet. In addition to
the flows in India and Russia, scientists
know of about 200 other examples of
vast expanses of basaltic rock, both on
land and under the sea. Termed large
igneous provinces by geologists, these
are regions that within only a few
million years erupted enough lava to
either cover an area of 100,000 km2
- equivalent to a region the size of
Iceland - or that occupy a volume of
100,000 km3 (about four times the volume
of Lake Baikal, the largest freshwater
lake on Earth). The potential
climate impact such events can have is
profound: The Siberian eruptions may
have been responsible for the Permian
extinction event, the largest die-off of
living creatures in Earth's history.
We don't fully understand what
controls when or where a large igneous
province will form; these phenomena
are likely linked to the mantle plumes
deep in the planet's interior. But it
seems Earth has been lucky: Multiple
such major eruptions do not seem to
have taken place at the same time. Is
U.S. AND EUROPE
RETURN TO VENUS
In June, NASA announced that it
would send two Discovery-class
missions to Venus at the end of
this decade. These are the Venus
Emissivity, Radio Science, InSAR,
Topography, and Spectroscopy orbiter
(VERITAS) and the Deep Atmosphere
Venus Investigation of Noble
Gases, Chemistry, and Imaging
(DAVINCI+) probe and spacecraft.
The VERITAS orbiter will map Venus's
surface in 3D, distinguishing
global features as small as 30 meters
across and, for a quarter of the
surface, down to 15 m - about five
times better than Magellan's radar
maps. Height accuracy for topographic
features will be 5 m. With
instruments to probe the planet's
gravity and surface composition,
the mission will explore the planet's
geologic history, including the nature
of the mysterious tesserae.
DAVINCI+ will essentially be a
chemistry lab dropped into the
Venusian clouds. On its plunge to a
(hopefully survivable) landing in the
rugged tessera region Alpha Regio,
it will take images and sample the
surrounding gases, providing clues
about how the planet has evolved
and whether it lost an early ocean.
The team is also considering modifying
their instrument payload to enable
them to check for phosphine,
a potential biosignature that might
exist in Venus's cloud deck (S&T:
Mar. 2021, p. 9). The probe's relay
spacecraft will later enter orbit.
About a week after NASA's
announcement, the European
Space Agency declared it, too, is
Venus-bound: The EnVision orbiter
will launch no sooner than 2031
to study the planet's landscape,
surface composition, and internal
structure. It will also look for
hotspots from active volcanism.
-CAMILLE M. CARLISLE
that because two cannot form at the
same time? Or are these events essentially
random in space and time and
it is simply a coincidence that Earth has
not - yet - suffered the same fate as
Venus? This is one of the biggest questions
in planetary science we have yet
to answer.
And answer it we can. Measurements
of noble gas abundances in
the middle Venusian atmosphere, for
example, would give us a much better
handle on how much gas has erupted
from the planet's interior. Such measurements
could also tell us whether
the planet started off with enough
water to host oceans or if, indeed, it
was always a barren world. And what
of those strange layers within some
tessera exposures? Determining what
these ancient rocks are made of, and
how they formed, will help us piece
together a more complete picture of
Venus's geological history - at least,
for that history still accessible on the
surface. Recently selected missions will
take us a long way toward answering
the question of what happened (see
sidebar at left).
If we one day establish whether
Venus really ever was like Earth, and
then why, and when, the paths of the
two planets diverged, we will know
if the relatively stable climate of our
own planet has been either a foregone
conclusion or simply good luck. We
may also better know whether to expect
Earth-like, or Venus-like, surface conditions
on similarly sized worlds in orbit
about other stars - even if we might
not be able to say whether we're alone
in the universe for quite some time.
And, whatever the reason for
Venus's demise, it will give us a new
appreciation for living on a world that
has largely remained habitable for billions
of years.
¢ PAUL BYRNE is a planetary scientist
at Washington University in St. Louis.
With a combination of spacecraft data,
computer and numerical modeling, and
fieldwork at analog sites on Earth, he
seeks to understand why planets look
the way they do.
sk yand tele scope .o r g * OCTOBER 2021 17
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Sky & Telescope - October 2021

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