by
The Europlanet Society
September
22, 2019
from
PHYS Website
Italian version
Artist's representation
of
Venus with water.
Credit:
NASA
Venus
may have been a temperate planet hosting liquid water for 2-3
billion years, until a dramatic transformation starting over 700
million years ago resurfaced around 80% of the planet.
A study presented today
at the EPSC-DPS Joint Meeting 2019 by Michael Way of the
Goddard Institute for Space Studies (GISS)
gives a new view of Venus's climatic history and may have
implications for the habitability of
exoplanets in similar orbits.
Forty years ago, NASA's
Pioneer Venus mission found
tantalizing hints that Earth's 'twisted sister' planet may once have
had a shallow ocean's worth of water.
To see if Venus might
ever have had a stable climate capable of supporting liquid water,
Dr. Way and his colleague, Anthony Del Genio, have created a
series of five simulations assuming different levels of water
coverage.
In all five scenarios, they found that Venus was able to maintain
stable temperatures between a maximum of about 50 degrees Celsius
and a minimum of about 20 degrees Celsius for around three
billion years...
A temperate climate might
even have been maintained on Venus today had there not been a series
of events that caused a release, or 'outgassing',
of carbon dioxide stored in the rocks of the planet approximately
700-750 million years ago.
"Our hypothesis is
that Venus may have had a stable climate for billions of years.
It is possible that
the near-global resurfacing event is responsible for its
transformation from an Earth-like climate to the hellish
hot-house we see today," said Way.
Three of the five
scenarios studied by Way and Del Genio assumed the topography of
Venus as we see it today and considered a deep ocean averaging 310
meters, a shallow layer of water averaging 10 meters and a small
amount of water locked in the soil.
For comparison, they also
included a scenario with Earth's topography and a 310-metre ocean
and, finally, a world completely covered by an ocean of 158 meters
depth.
To simulate the environmental conditions at 4.2 billion years ago,
715 million years ago and today, the researchers adapted a 3-D
general circulation model to account for the increase in solar
radiation as our Sun has warmed up over its lifetime, as well as for
changing atmospheric compositions.
Although many researchers believe that Venus is beyond the inner
boundary of our Solar System's habitable zone and is too close to
the Sun to support liquid water, the new study suggests that this
might not be the case.
"Venus currently has
almost twice the solar radiation that we have at Earth.
However, in all the
scenarios we have modeled, we have found that Venus could still
support surface temperatures amenable for liquid water," said
Way.
At 4.2 billion years ago,
soon after its formation, Venus would have completed a period of
rapid cooling and its atmosphere would have been dominated by
carbon-dioxide.
If the planet evolved in
an Earth-like way over the next 3 billion years, the carbon dioxide
would have been drawn down by silicate rocks and locked into the
surface.
By the second epoch
modeled at 715 million years ago, the atmosphere would likely have
been dominated by nitrogen with trace amounts of carbon dioxide and
methane - similar to the Earth's today - and these conditions could
have remained stable up until present times.
The cause of the outgassing that led to the dramatic transformation
of Venus is a mystery, although probably linked to the planet's
volcanic activity.
One possibility is that
large amounts of magma bubbled up, releasing carbon dioxide from
molten rocks into the atmosphere. The magma solidified before
reaching the surface and this created a barrier that meant that the
gas could not be reabsorbed.
The presence of large
amounts of carbon dioxide triggered a runaway
greenhouse effect, which has
resulted in the scorching 462 degree average temperatures found on
Venus today.
"Something happened
on Venus where a huge amount of gas was released into the
atmosphere and couldn't be re-absorbed by the rocks.
On Earth we have some
examples of large-scale outgassing, for instance the creation of
the
Siberian Traps 500 million
years ago which is linked to a mass extinction, but nothing on
this scale.
It completely
transformed Venus," said Way.
There are still two major
unknowns that need to be addressed before the question of whether
Venus might have been habitable can be fully answered.
-
The first relates
to how quickly Venus cooled initially and whether it was
able to condense liquid water on its surface in the first
place.
-
The second
unknown is whether the global resurfacing event was a single
event or simply the latest in a series of events going back
billions of years in Venus's history.
"We need more
missions to study Venus and get a more detailed understanding of
its history and evolution," said Way.
"However, our models
show that there is a real possibility that Venus could have been
habitable and radically different from the Venus we see today.
This opens up all
kinds of implications for exoplanets found in what is called the
'Venus
Zone', which may in fact host liquid water and
temperate climates."
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