NASA announced last week that it has decided to send not one but two spacecraft missions to Venus. It’s about time.
The plan delighted those of us in the planetary science community who have long advocated for a return to the planet. The two missions—named DAVINCI+ and VERITAS—would be the first Venus-bound treks from the United States since NASA’s Magellan radar mapping mission launched in 1989 and ceased operations in 1994.
Venus is more than simply a curious world next door. It holds the key to understanding the past—and present—of our own world.
Venus’ surface is hell made real. The temperature there is that of a self-cleaning oven; the pressure is 90 times that at sea level on Earth. Instead of oceans, Venus has vast seas of frozen lava. Instead of blue skies, Venus has a toxic, carbon dioxide atmosphere under a stifling layer of sulfuric acid clouds that blanket the entire planet. Measurements by NASA’s Pioneer Venus probe in the 1970s found chemical evidence that the planet may once have had an Earth ocean’s worth of water. But whereas modern Earth abounds with life, modern Venus today is one of the least hospitable locales in the entire solar system.
After a golden era of Venus exploration in the 1960s through to the mid-1980s (which saw more than 30 missions from the United States and the Soviet Union dispatched to the so-called second planet), a consensus developed that Venus at some point fell into a “runaway greenhouse” state. This model holds that Venus’ proximity to the sun led to the planet overheating early on, its carbon-dioxide-rich atmosphere retaining more heat than could be reflected back into space—similar to how human-emitted greenhouse gases are warming Earth, albeit at a much greater magnitude. With continued heating, any oceans present evaporated, in turn helping to speed up that runaway greenhouse process. Eventually, the surface became a barren, sweltering wasteland.
But recently, a new view of Venus has emerged that paints quite a different picture of our neighbor’s history. Sophisticated climate modeling suggests the planet might have escaped an early period of overheating, instead remaining clement and hosting oceans for several billion years. Under this scenario, the culprit for its dreadful runaway greenhouse was not the sun, but something intrinsic to Venus: its volcanoes. If enough eruptions occurred close enough in time, the combined volume of carbon dioxide injected into the atmosphere would have overwhelmed the planet’s ability to regulate its climate, putting it on a path to catastrophe.
We don’t know whether this scenario is correct, but the missions will begin to test it. The point here is that, far from being the scorched hellscape we see today, Venus could have been Earth-like for much of our solar system’s history.
Which raises the question: If Venus really was Earth-like for much of its history, was the second planet unlucky? Did geological happenstance kill a once habitable world? Or is Venus’ fate something to be expected for all large, rocky worlds—with Earth simply playing the odds so far?
Paul Byrne is an associate professor of planetary science at North Carolina State University.