When NASA announced last week it would spend $1 billion developing two new missions to Venus—the agency’s first visits in decades to Earth’s hothouse twin—planetary scientists were elated, and not just because a long wait had ended. A dramatic shift in thinking about the planet over the past few years has made a visit even more enticing. Venus was once thought to have boiled off all its water almost as soon as it was born 4.5 billion years ago, turning into the parched, hostile world of today. But many scientists now think Venus might have kept expansive oceans for billions of years—a nearly perfect setting for life.
The missions, to arrive late this decade, are equipped to look for signs of that water—and clues to why Venus ultimately declined into an inferno. If their findings support the new picture, Mars, the longtime hope for discovering signs of ancient extraterrestrial life, will have a rival. “Why look at Mars, which had water for 300 million years, when Venus had water for 3 billion years?” asks Darby Dyar, a planetary scientist at Mount Holyoke College who is deputy principal investigator for one of the new missions, VERITAS (Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy).
Early visits to Venus cemented the picture of a dead, waterless planet when they detected no sign of oxygen in its thick carbon dioxide (CO2) atmosphere, which keeps its surface at a lead-melting 460°C. Venus likely started out with plenty of water, as it formed from roughly the same building blocks as Earth. The thinking was that as the water boiled off, ultraviolet light broke down water vapor molecules. Hydrogen would have escaped to space while oxygen, being heavier, would have lingered in the air. Its absence led scientists to assume Venus lost its water very early on.
In 2014, however, in a computer modeling study of potential exoplanet atmospheres, a team led by Jun Yang, a planetary scientist at Peking University, made a striking observation: Planets that rotate as slowly as Venus, which makes one full turn every 243 Earth days, have a very weak Coriolis effect, a twisting force caused by planetary spin. On Earth, the Coriolis causes air flow in each hemisphere’s lower atmosphere to split into three cells: the tropical, subtropical, and polar circulations. On a slow spinner, however, winds could flow all the way from the equator to the poles. In a moist atmosphere, the updrafts would create a massive cloud deck shrouding the sunny side of the planet.
Soon after, Michael Way, a climate modeler at NASA’s Goddard Institute for Space Studies (GISS), and his colleagues picked up this idea, testing it with a climate model built to simulate early Venus conditions. They found the same result, they reported in 2016: Massive water clouds formed, blocking almost half of the sunlight. In the perpetual twilight, liquid water could have survived for billions of years. On Earth, water lubricates the moving tectonic plates, and it would have boosted the odds that Venus, too, had some sort of plate tectonics—and potentially life, Way says. “I do think this work has changed people’s opinions about the viability of a biosphere.”
How Venus went bad and what happened to the water’s oxygen remained a puzzle. In a paper last year, Way and Anthony Del Genio, also at GISS, suggested Venus could have been struck with multiple continent-spanning eruptions—the kind of catastrophic events that have caused mass extinctions on Earth. The eruptions would have poured CO2 into the atmosphere, causing a runaway greenhouse effect that boiled the planet dry. Then, perhaps half a billion years ago, the oxygen released by the water could have been purged by reactions with magma and ash from subsequent widespread eruptions, also thought to explain Venus’s relatively young surface, which lacks ancient impact craters.
It’s a compelling story, Dyar says. “When I first saw Michael Way’s paper, I got so excited,” she says. Way acknowledges that his model includes many assumptions and little data. But that will soon change, he says. “These missions coming along now are fantastic for testing these models.”
VERITAS, developed by NASA’s Jet Propulsion Laboratory and led by JPL planetary scientist Suzanne Smrekar, will use radar to peer through Venus’s thick clouds of sulfuric acid and trace its topography in 100 times finer detail than Magellan, a NASA mission that ended in 1994. Besides guiding potential future landers, the mapping should show whether blurry features seen in the Magellan data are the signatures of plate tectonics, such as trenches.
The orbiter will also be armed with a spectrometer capable of looking for chemical signatures in the faint light that reflects off the surface and escapes through the clouds. “We have to work with what Venus has given us,” Dyar says. The presence or absence of iron will be critical. Scientists have long thought Venus’s rugged highlands, called tesserae, could be the remains of continental crust. On Earth, the continents are rich in granite, an iron-depleted rock that only forms with the help of water and plate tectonics; widespread detection of granite would indicate Venus had a very Earth-like history.
The second mission, DAVINCI+ (Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging Plus), would address Way’s hypothesis more directly. Developed at NASA’s Goddard Space Flight Center and led by GSFC Chief Scientist James Garvin, DAVINCI+ will drop an instrument-laden sphere, armored against the crushing pressure of the venusian atmosphere. During its hourlong plunge, slowed by a parachute, its instruments will measure noble gases, especially xenon, to see whether the abundances match those on Earth, which could suggest a similar early history of water. It will also be able to refine measurements made decades ago of the ratio between light and heavy isotopes of hydrogen, especially in the dense lower atmosphere, which will help pin down how much water Venus has lost over its history.
The probe’s cameras and spectrometer, meanwhile, will take aim at a highland region called Alpha Regio, searching again for the signal of granite. Toward the end of its descent, the cameras will capture Alpha Regio’s features in superhigh resolution, including a 3D view, complementing the VERITAS maps. “Together we can paint a really wonderful comprehensive picture,” says Giada Arney, a deputy principal investigator for DAVINCI+ at GSFC.
On its way down, DAVINCI+’s instruments might also detect a sign not of past habitability, but present-day life: phosphine. In September 2020, astronomers probing the planet’s atmosphere with telescopes said they had picked up hints of the gas, which on Earth is a sign of microbial life, but the evidence has sparked debate ever since. The DAVINCI+ team is exploring tweaks to a laser spectrometer that could enable it to sniff out trace abundances of the gas, Arney says.
It’s been a long wait for these missions, with proposal after proposal turned down. Dyar still remembers her days as a graduate student at the Massachusetts Institute of Technology in the 1980s, when then-President Ronald Reagan canceled a proposed Venus mission, causing tears in the hallway. The new missions could begin to make amends, she says, and the potential payoff is greater than ever. “These questions have been haunting us for 30 years,” she says. “It’s Venus’s turn.”
sciencemag.org, 9 June 2021