NASA is planning to return to Venus. It’s about time. - The Washington Post

Paul Byrne is an associate professor of planetary science at North Carolina State University.

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. In the years since, American spacecraft have visited Mercury; cruised above Jupiter’s clouds; shot past Pluto on the way to the outer reaches of the solar system; and landed, roved and even flown on Mars. But NASA missions to Venus have remained conspicuously absent.

The prospect of returning to Venus later this decade doesn’t just represent an exciting new chapter in NASA’s exploration of the solar system. Certainly, these missions will dramatically change our view of Venus: VERITAS will characterize the surface with radar like never before, searching for evidence of geological activity today; DAVINCI+ will measure the composition of the atmosphere to establish how the planet formed and evolved (and perhaps whether the planet really contains phosphine, a chemical tentatively detected in Venus’s atmosphere last year with telescopes on Earth, and which some consider a potential “biosignature”).

But Venus is more than simply a curious, little-explored world next door. It holds the key to understanding the past — and present — of our own world.

Venus’s 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’s 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 DAVINCI+ and VERITAS 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 — raising the tantalizing possibility that, for billions of years, there were two large, blue rocky worlds orbiting the sun.

The kinds of volcanic eruptions invoked to explain Venus’s present state have taken place on Earth, too. Those eruptions, forming what we call “large igneous provinces,” are associated with some of the worst extinction events in Earth history. It’s unclear what controls when and where such events take place, but they’re far from unique to Venus.

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’s fate something to be expected for all large, rocky worlds — with Earth simply playing the odds so far?

That’s why NASA’s return to Venus is more than just a chance to get reacquainted with a planetary neighbor. It’s an opportunity to understand the forces that shape Earth-size worlds in general, which will help us make sense of those we’re finding in orbit of other stars — including whether we should expect to find with our telescopes blue skies and oceans ... or suffocating blankets of lethal clouds. As humans continue to alter Earth’s climate, unveiling Venus might end up showing just how precious, and rare, our own planet truly is.

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