Venus may have stayed a habitable planet much longer than previously thought, according to a recent paper by Michael Way of the NASA Goddard Institute for Space Studies and colleagues. The Venusian surface today, of course, is thought to be inhospitable to life as we know it, with temperatures hotter than an oven on broil. Billions of years ago, however—and perhaps as recently as 715 million years ago—the planet may have had moderate temperatures and liquid water on its surface, based on simulations done by Way’s group. The simulations were sophisticated, and included climate data, radiation outputs from the Sun, the orbital parameters of Venus, and an estimated atmospheric composition for the early planet.
It is well known that Venus once orbited in the habitable zone of our Sun, which would have allowed for liquid water on the surface. But this new estimate for how long Venus might have stayed habitable is astounding. If the authors are correct, Venus could have been habitable until a global volcanic event (thought to have happened about 715 million years ago) covered most of the planet with lava, at around the same time that the first macroscopic life emerged on Earth. This could mean that life evolved in parallel on both of these terrestrial planets.
However, the global outpouring of lava 715 million years ago is difficult to explain as one singular event. Because Venus has no plate tectonics to recycle the heat from its interior, it’s more likely that such cataclysms occur every once in a while, say every 500 million to 1 billion years. If so, it is unclear whether an earlier event might have provided a knock-out punch for life, or whether that only occurred relatively recently, as the authors propose.
Louis Irwin and I postulated more than 10 years ago on the likelihood of life on early Venus, a subject Carl Sagan also had considered. With Venus being so similar to Earth early in its history, life could have originated there independently, or could have transferred from Earth via asteroids that fell in Venusian oceans. That hypothesis would be strengthened significantly if we find evidence for river flows and oceanic processes on Venus from the few areas older than 715 million years.
Assuming Venus once was a microbial abode, would there be a chance that life is still around, after the last volcanic cataclysm? Probably not on the surface, but perhaps in the lower atmosphere, as David Grinspoon, co-author of the present study, myself, and three other authors suggested in a 2004 paper. The lower cloud layer of the Venusian atmosphere has a pressure similar to sea level pressure on Earth and temperatures between 30 and 80 degrees Celsius, and is relatively abundant in water, making it the only place on today’s Venus that can qualify (just barely) as a potential habitat for life—the last refuge on an otherwise hellish planet.