The Venus of today might be best described as "a hellish hot house" with average temperatures of 462 degrees Celsius. But research suggests its climate may have been very different—and far more temperate—in the distant past. Indeed, up until a point around 700 million years ago, Venus may have held liquid water and if not for a string of unfortunate events, could still contain water today.
The first hints that Venus may have, at one point, managed to sustain liquid water were detected during NASA's Pioneer Venus mission, beginning in 1978. Data collected during the mission suggested that there had been a time when the second planet from the sun had a shallow ocean's worth of water.
However, the view of many researchers today remains that Venus is too close to the sun to support liquid water, and outside the boundaries of the solar system's habitable zone. A new study challenges that assumption.
Researchers from The Europlanet Society presented a study on Venus' climate history at the EPSC-DPS Joint Meeting 2019 on Sunday. Their results suggested that the planet may once have had a habitable climate suitable for life.
Michael Way of the Goddard Institute for Space Science and colleague Anthony Del Genio created five climate simulations, each based on different levels of water coverage. Three of the five used the topography of today's Venus—one with a deep ocean averaging 1,017 feet, another with a shallow layer of water averaging 33 feet deep, and a third with a small volume of water locked in the soil. The other two used Earth's topography—one with a 1,017 feet ocean and one completely covered by a 518-foot ocean. The model accounted for changes in atmospheric composition and solar radiation.
All five suggest that Venus could have sustained a temperature maximum of 50 degrees Celsius and temperature minimum of 20 degrees Celsius for a period lasting three billion years. To put that into perspective, three billion years into Earth's evolution, single-celled organisms had emerged. It was another 600 million years before the first multi-cellular organisms developed.
"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," Way said in a statement.
"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."
According to the researchers, Venus experienced a period of rapid cooling shortly after its formation 4.2 billion years ago, when its atmosphere would have been heavy with carbon dioxide. Provided it evolved in a way similar to Earth, this gas would have been drawn down (and locked in) by the planet's silicate rocks, so that by a time 715 million years ago, nitrogen would have dominated the atmosphere.
CO2 and methane would have been present in trace amounts—just as with Earth.
Way and Genio believe this temperate climate may still exist today if not for a sequence of events that triggered the release of CO2 stored in rocks, a process—called outgassing—that took place 700 to 750 million years ago.
What caused this outgassing is unknown but scientists suspect it lies in the planet's volcanic activity. One hypothesis is that large volumes of CO2 were released as vast amounts of magma bubbled up and reached the surface, releasing gas from the molten rocks. The solidification of magma before it hit the surface could have prevented the carbon dioxide from reabsorbing, resulting in a runaway greenhouse effect.
Nothing of its scale has taken place on planet Earth, says Way. But something similar did occur during the creation of the Siberian Traps 500 million years ago—a huge region of volcanic rock released during an eruption that lasted over a million years. Outgassing at this time triggered a mass extinction.
It is also a concern today, with greenhouse gas emissions from human activity raising levels of CO2 in the atmosphere and leaving the potential of a runaway greenhouse effect.
The surface of Venus, as seen in 1991 by the Magellan spacecraft. In our solar system, Venus is the only planet that can compare with our own in terms of size and distance from the sun. Nevertheless, we know shockingly little about our neighbor. NASA/JPL
The question of Venus's habitability rests on two as-yet unknowns. Researchers do not know the speed at which Venus cooled—or if it could even condense liquid water. Nor do they know if the global resurfacing process was the product a single event or a sequence of events that took place over billions of years. However, the results of the study do open up opportunities for exoplanets that exist in the "Venus zone."
Way says more missions are needed to study Venus and improve our knowledge of its evolution—adding that models show there is a strong possibility that it could have been "habitable and radically different from the Venus we see today."
He said: "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."