New computer models suggest that Venus may have been habitable as recently as 700 million years ago. Based on a series of possible scenarios, Michael Way and Anthony Del Genio of the Goddard Institute for Space Science concluded that the planet may have been able to maintain liquid water for two or three billion years before the atmosphere underwent a radical transformation.
Venus today is a pretty good stand-in for Hell. It isn't bad enough that its opaque atmosphere is 96.5 percent carbon dioxide or that it rains sulfuric acid. It also has an average surface temperature of 462° C (864° F), which is hot enough to melt lead. This means that there is no chance of Venus having any liquid water and it's as far from being habitable as one could imagine.
However, detailed radar maps made by a series of space probes over the past 40 years indicate that Venus could have had a shallow ocean. If that's the case, then the planet must have had a radically different environment that was cool enough to allow liquid water.
To see if this was possible, Way and Del Genio ran a series of five simulations based upon different levels of water coverage on Venus. These included one with an ocean 310 m (1,000 ft) deep, a shallower one with a depth of 10 m (33 ft), one where the water was locked in the soil, a deep ocean with an Earth-like topography, and one where the planet was covered entirely with 158 m (580 ft) of water.
To simulate the Venus of 4.2 billion years ago and 715 million years ago, the pair ran the scenarios through a 3D general circulation model that took into account a changing atmosphere and the increase in solar radiation over time.
What they found was that in each case Venus remained at a stable range of temperatures between 20° C (68 F) to 50° C (122° F) – not exactly comfortable by Earth standards, but cool enough for liquid water to exist. More importantly, the planet could have maintained this temperature for three billion years. If these models hold up, they show that Venus, despite receiving twice the solar radiation of Earth, is still inside the solar system's habitable zone.
The reason for this is that 4.2 billion years ago when Venus first formed it cooled rapidly and the carbon dioxide in the atmosphere was absorbed into the silicates in the crust by the same mechanism as happened on Earth three billion years ago. This would have left an Earth-like atmosphere dominated by nitrogen with traces of carbon dioxide and methane.
But what happened? According to the team, between 715 and 700 million years ago there was a sudden outgassing of carbon dioxide – possibly due to volcanic activity. As the carbon dioxide bubbled out of the molten magma, the latter cooled to form a cap, preventing the gas from being reabsorbed. This triggered a runaway greenhouse effect as the atmosphere became increasingly dense, sending the temperature to its present levels.
"Something happened on Venus where a huge amount of gas was released into the atmosphere and couldn’t be reabsorbed by the rocks," says Way. "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."
The team stresses that there are still major questions to answer, including whether water was able to condense on Venus in the distant past, and whether the outgassing was a single event or a series that occurred over billions of years.
"We need more missions to study Venus and get a more detailed understanding of its history and evolution," says 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."
The research was present at the EPSC-DPS Joint Meeting 2019 in Geneva.
Source: Europlanet Society
