The planet had stable temperatures — between a maximum of about 50°C and a minimum of about 20°C — and water worth a shallow ocean
Venus was likely a habitable planet once, with stable temperatures and water worth a shallow ocean for nearly three billion years, according to a new study
However, a global resurfacing event starting over 700 million years ago drove it into a runaway greenhouse state, according to a study by researchers from the United States National Aeronautics and Space Administration's (Nasa) The Goddard Institute for Space Science, The Europlanet Society reported.
The findings were presented on September 23, 2019, at the European Science Planetary Congress 2019.
Venus maintained temperate climate — between a maximum of about 50 degrees Celsius and a minimum of about 20°C — for around three billion years. A series of events released carbon dioxide (CO2) stored in the rocks of the planet approximately 700-750 million years ago changed the planet, the report noted.
“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 Michael Way from the Goddard Institute, in a release.
Venus is assumed to be beyond the inner boundary of our Solar System’s habitable zone and too close to the Sun to support liquid water. But the study suggests the opposite.
“Venus currently has almost twice the solar radiation that we have at Earth. However, in all the scenarios we have modelled, we have found that Venus could still support surface temperatures amenable for liquid water,” Way said.
The presence of liquid water on Venus, was hinted nearly 40 years ago by scientists at Nasa’s Pioneer Venus mission. To verify, the team created a series of simulations assuming different levels of water coverage.
Four simulations were run at 3 different epochs: 4.2 giga-years ago (Gya), 715 gya and present day:
To simulate these time periods, the researchers adapted a three-dimensional general circulation model to account for changes in solar radiation and atmospheric compositions over the years.
The team found that “Venus' atmosphere consisted of 90 per cent CO2 and 10 per cent Nitrogen (N2) 4.2 billion years ago, while at 715 million years ago and present day it had a modern Earth-like N2 dominated atmosphere with 400 parts per million by volume (ppmv) CO2 and 1 ppmv methane (CH4)”.
The researchers explained that at 4.2 billion years ago, soon after its formation, Venus would have completed a period of rapid cooling and then CO2 dominated its atmosphere. “If the planet evolved in an Earth-like way over the next 3 billion years, the CO2 would have been drawn down by silicate rocks and locked into the surface,” they said.
The N2-dominated second epoch (715 million years ago) could have remained stable up until present times had the "outgassing event" not occured.
While the cause behind the outgassing remains unknown, the researchers linked it to the planet’s volcanic activity. When large amounts of magma bubbled up, it released CO2 from molten rocks into the atmosphere. But, this CO2 could not be re-absorbed by the rocks, as the magma solidified, Way explained.
The planet currently has scorching average temperatures of 462°C. This could be the result of high CO2 levels, the researchers said. They linked the outgassing with the creation of the Siberian Traps 500 million years ago.
But the magnitude differed, as the outgassing event on Venus completely transformed the planet, said Way.
But, before concluding that Venus might have been habitable, there are further questions that needs to be answered: how quickly Venus cooled initially and whether it was able to condense liquid water on its surface; whether the global resurfacing event was a single event or simply the latest in a series of events.
“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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