Venus once possibly habitable, study suggests

Venus might have once been prime real estate. New computer simulations suggest that the hellish planet next door could have been habitable in the not-too-distant past, with moderate temperatures, plenty of seaside locales and even a few spots for skiing.

Modern Venus is harsh: sulfuric acid rain, crushing atmospheric pressure and a surface temperature around 460° Celsius. But if Venus maintained its glacial rotation rate for much of its history — one day lasts roughly 116 Earth days — then the average temperature could have been around 15° C as recently as 715 million years ago. The findings were published online August 11 in Geophysical Research Letters.

“This is a very speculative, hypothetical paper,” says Mark Bullock, a planetary scientist at Southwest Research Institute in Boulder, Colo. It doesn’t prove that Venus was habitable, he says, but the researchers do show that conditions existed under which Venus could have maintained oceans and a temperate climate for billions of years.

“Rotation rate is really key,” says lead author Michael Way, an astrophysicist at the NASA Goddard Institute for Space Studies in New York City. Venus’ odd rotation — not only is it poky but it’s backwards relative to most other planets in the solar system — has long been an enigma. One idea is that it once spun more swiftly, but gravitational interactions between its atmosphere and the sun slowed it down. Way and colleagues combined climate simulations with Venus topography data from NASA’s Magellan spacecraft, which orbited Venus from 1990 to 1994, to test how rotation rates might have affected the Venusian climate. They find that if Venus has held on to its current spin speed, it could have been more Earth-like for billions of years. And as long as it spun no faster than roughly once every 16 Earth days, and had a shallow global ocean to help regulate temperatures, the planet could have been toasty but habitable.

On Earth, the relatively brisk rotation of the planet breaks the atmosphere into distinct churning cells that help keep the climate in check. Venus doesn’t turn fast enough to whip up its atmosphere. Instead, a long-lasting cloud formation arises over the sun-facing side of the planet, the researchers find. The cloud reflects enough sunlight to keep the surface balmy for a couple of billion years before temperatures escalate, leading to the current suffocating conditions. Some locations could even have seen occasional snow, and the northern highlands of Ishtar Terra could have maintained a year-round snowpack roughly 5 meters deep, the researchers report.

But these conclusions depend on assumptions that might not be reasonable, says Fred Taylor, a physicist at the University of Oxford. The researchers assume, for example, that Venus once had a predominantly nitrogen atmosphere with a pressure similar to Earth’s — a choice that’s not likely, says Taylor. “Venus probably has never been habitable,” he says.

Future missions to Venus can test some of these assumptions. An orbiter or lander could look for evidence of granite, for example, which requires water. “That would really be the smoking gun for this hypothesis that Venus held on to oceans for a long time,” Bullock says. The Japanese Akatsuki probe is currently studying the planet’s climate (SN: 1/9/16, p. 14), and NASA is considering two ideas for sending a spacecraft as early as 2020.