“Our models show that there is a real possibility that Venus could have been habitable and radically different from the Venus we see today,” said Michael Way of The Goddard Institute for Space Science about Earth’s ‘twisted sister’ planet. “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.”
The Venus Zone
“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 in a 2018 paper.
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 at the EPSC-DPS Joint Meeting 2019 by Michael Way gives a new view of Venus’s climatic history and may have implications for the habitability of exoplanets in similar orbits.
The Water Hypotheses
“We do not know with any certainty whether Venus ever condensed water on its surface or not,” Michael Way wrote in an email to The Daily Galaxy. “All that we know is that the deuterium to hydrogen ratio is much higher (~150) than that on Earth (Donahue et al. 1982), which may indicate that Venus once had a larger water inventory than it has today. Whether that was steam or condensed water we do now know.
“Research published in Nature in 2013 () demonstrated that either scenario was possible: water condensed out early on, or it never condensed out,” Way continued. “Recent work by Turbet et al. 2021 weighted the scales a bit toward the never condensed out scenario, but in fact their results are really just a beginning. We have a lot more work to do before we can properly model the magma ocean atmosphere and quantify its lifetime. We may have to wait until the DAVINCI probe returns its in-situ results of the heavy noble gasses before we can say anything with certainty.”
A Shallow or Deep Ocean?
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, 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.
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-meter ocean and, finally, a world completely covered by an ocean of 158 meters depth.
3-D Circulation Model
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.
Beyond the Habitable Zone?
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 on 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 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.
Runaway Greenhouse Effect
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.
The Last Word
“As for what ‘could’ have transformed Venus from temperate to its present hot-house climate,” Way concluded in his email to The Daily Galaxy, “our hypothesis is that simultaneous Large Igneous Provinces (LIP) like that of the Siberian or Deccan traps could have released so much CO2 into the atmosphere that the climate system would be overwhelmed and unable to re-absorb this additional CO2, effectively ending the CO2 cycle. We have recently submitted a paper that studies the frequency of such LIP events through Earth’s history, and using Monte Carlo methods have come up with an estimate of the likelihood of such overlapping LIPs in alternative Earth histories. We conclude that the probability is high for overlapping events, and that Earth may have been fortunate that no overlapping large LIP event has taken place in Earth’s history thus far.”
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.
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