Sky & Telescope - April 2025 - 8

NEWS NOTES
INNER PLANETS
Ancient Venus Didn't Have Oceans After All
ASTRONOMERS HAVE CONDUCTED a
long and fierce debate about whether
Venus was always a dry hellscape with
oven-like temperatures. Now, as published
December 2, 2024, in Nature
Astronomy, three researchers from the
University of Cambridge, UK, offer
support for that idea.
Of two rival theories of Venus's
past, one argues that Venus was once
temperate and hosted oceans of water
- enough to cover the surface to a
depth of 500 meters (1,600 feet).
But then, more recently, volcanoes
released vast quantities of carbon
dioxide and sulfur dioxide, greenhouse
gases that trapped the Sun's heat. The
water first evaporated, then escaped
into space. Alternatively, Venus may
always have been too hot and dry to
support liquid water.
Tereza Constantinou and her colleagues
utilized the Venusian atmosphere
- largely the product of volcanic
gases from within the planet - to
probe the interior and examine how
dry the planet really is.
The team calculated the current
rate at which water, carbon dioxide,
and carbonyl sulfide (COS) molecules
are destroyed in the planet's atmosphere.
They also considered chemical
reactions occurring between Venus's
air and surface, weathering that can
cause these molecules to be stored in
the planet's crust.
Constantinou and her team conclude
that Venus's volcanic gases are
at most 6% water. This implies the
planet's interior is too dry to ever have
brought large quantities of water to
the surface.
" This is an intriguing study, " says
James Holmes (Open University, UK),
who was not involved in the research.
However, he adds, " There still remain
uncertainties that are likely to influence
the results, such as the precise
chemical composition of the deep
atmosphere and the current rate of
active volcanism. "
Not everyone is convinced by these
new calculations. " They make two
This computer-generated 3D model of
Venus's surface (with a vertical scale exaggerated
10 times) shows the summit of Maat
Mons, a site of past and perhaps current
volcanic activity.
assumptions that I would question, "
says Richard Ghail (Royal Holloway,
University of London), who was also
not involved in the research. " Their
assumption about Venus surface
weathering is, I suspect, wrong. " If
the team overestimated the amount
of carbonyl sulfide and carbon dioxide
that chemical reactions deposit into
the planet's crust, then there would
be fewer of those molecules relative to
water. Then, to keep the atmosphere
in balance, the amount of water
ejected from the planet's interior
would need to be higher than the
team estimated.
" I think they [also] significantly
underestimate the importance of the
sulfur cycle, " Ghail adds. For example,
as sulfur-rich minerals break
down on the surface, carbonyl sulfide
could be put back in the air, providing
another way to shift the ratio of
that molecule relative to water. The
team admits the story for sulfur is
" more nuanced. "
Perhaps it will take future missions
- including NASA's VERITAS
and DAVINCI, and the European
Space Agency's Envision - to settle the
debate on Venus's past once and for all.
¢COLIN STUART
8
APRIL 2025 * SKY & TELESCOPE
NASA / JPL-CALTECH
Sky & Telescope - April 2025

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