According to computer modelling, the planet Venus could have once been habitable, hosting a shallow-water ocean, and surface temperatures hospitable to life. The simulations are based on present-day observations of Venus, paired with data harvested by previous NASA missions that visited the enigmatic planet.
Often referred to as Earth's twin, current-day Venus is anything but. The tortured world's atmosphere, which is believed to be roughly 90 times as thick as that of our own planet, has led to a runaway greenhouse effect, resulting in surface temperatures of 864 ºF (462 ºC).
However, it is possible that, in its ancient past, this hellish world could have sincerely deserved the moniker of Earth's twin. In the 1980s NASA's Pioneer spacecraft observed clues that a water ocean may once have existed on Venus. Now, a team of scientists from NASA's New York-based Goddard Institute for Space Studies has run computer simulations designed to model Venus' ancient atmosphere, and come up with some surprising results.
The computer model employed for the simulations is similar in nature to those used to forecast the effects of climate change on Earth. For the purpose of the simulations, the researchers had Venus rotate at the same speed that it does today.
Digital Venus was given an atmosphere similar to that of present day Earth, and furnished with topographical features based on data harvested by NASA's Magellan mission that visited the planet in the 1990s. The researchers then filled the low-lying areas of the planet with a shallow ocean, leaving only the highlands exposed.
According to the team, landmasses on ancient Venus would have been much dryer than their Earthly equivalent. This fairly mundane characteristic could have played a vital part in keeping the planet's greenhouse gas problem at bay, as less water vapour would escape from the ground via evaporation.
It was discovered that Venus' slow rotational period of 117 Earth days, in conjunction with the ancient analogue of our Sun used in the study, combined to create a hospitable surface temperature only a few degrees cooler than the temperature on present-day Earth.
The slow spin of the planet, which would expose areas of the surface to the glare of the Sun for months at a time, allowed the heat from our star to evaporate enough water to create a kind of cloud shield. This barrier mitigated some of our Sun's heating influence, which, whilst younger and less hot in the simulations than it is today, would still have bathed the planet in 40 percent more sunlight than that received by present-day Earth.
It is estimated that this Earth-like Venus could have remained habitable in this way for around 2 billion years.
However, eventually, the sheer quantity of sunlight striking the planet worked to evaporate the oceans of ancient Venus. Subsequently, ultraviolet light broke down the resulting water vapor particles into their constituent elements. Once the majority of the hydrogen had escaped into space, all that remained was a dense, carbon dioxide-dominated atmosphere shrouding the lifeless planet we see today.
The evidence for the habitability of Venus-like planets could help inform the search for exoplanets with the potential to harbor life. These exoplanets may now present a more attractive prospect for the next generation of ground-based and orbital observatories, including the much anticipated James Webb Space Telescope.