Venus is—without a doubt—Earth’s toxic sibling. Although both worlds are similar in size and density, our planetary neighbor has temperatures so high they can melt lead, winds that whip around it some 60 times faster than the planet itself rotates and an atmosphere that slams down with more than 90 times the pressure found on Earth’s atmosphere. But there have been a few tantalizing hints that billions of years ago Venus might have been more akin to Earth’s twin. In addition to their comparable sizes, the worlds also formed close together, which suggests that they are made out of the same bulk of materials. The big difference is their proximity to the sun. Because Venus is roughly 41 million kilometers closer, it receives twice as much sunlight as Earth. But a few billion years ago a slightly fainter sun might have allowed for a relatively cool Venus, one where liquid water could have pooled in vast oceans that were friendly to life.
A new study recently accepted in Geophysical Research Letters suggests that not only was Venus habitable in the distant past, it could have remained habitable for billions of years. Michael Way from NASA’s Goddard Institute for Space Studies and his colleagues applied the first three-dimensional climate model—the same computer simulations used to predict human-caused climate change on Earth—to early Venus. Because previous research only looked at one-dimensional climate models on Venus (which consider incoming and outgoing radiation but do not visualize the complexities, like clouds, within an atmosphere), the results are a huge step forward compared with previous studies, scientists say. “There's a real difference between a back-of-the-envelope calculation and actually plugging it into a more sophisticated model,” says Jason Barnes, an astronomer from the University of Idaho, who was not involved in the study.
The team first simulated how the Venusian climate might have looked 2.9 billion years ago. Such an ancient date required the researchers to make a few educated guesses about the early planet, such as assuming it had a shallow ocean just 10 percent the volume of that on Earth today. But the results were clear—2.9 billion years ago the second rock from the sun could have had a balmy Earth-like temperature that hovered around 11 degrees Celsius. The team then ran the model for a later Venus some 715 million years ago and found that even under the sun’s heightened heat, the planet would have warmed by only 4 degrees Celsius since that earlier time. Such a slight increase in temperature would have allowed the planet’s liquid ocean to persist for billions of years.
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What allowed Venus to stay wet for so long? According to the models, clouds played a key role. They likely piled up on the dayside of the planet, acting as a bright shield that reflects incoming sunlight, and never formed on the nightside, letting heat radiate off into space. “To me the real takeaway message is that Venus could have been habitable for a significant period of time, and time is one of the key ingredients to being able to originate life on a planet,” says Lori Glaze, an astronomer at NASA Goddard Space Flight Center who was not involved in the study. This suggestion adds a new element to the question of habitability: time. “Habitability is not something that's static,” says David Grinspoon, an astronomer from the Planetary Science Institute and a co-author of the paper. “It's not just a question of a point in space, it's a point in space and time and how long a planet could potentially retain oceans, and if that’s long enough to be considered a good candidate to have had an origin and evolution of life.”
Those cool conditions, however, depend on whether Venus looked the same in its youth as it does today—although the researchers added an ocean, they kept Venus’s present-day topography intact—and whether it has always spun as slowly as it does now, taking 243 Earth days to complete a single rotation. Because the answers to both questions are fairly uncertain, the research team also modeled what Venus’s climate would have looked like 2.9 billion years ago if it had an Earth-like topography or spun at a slightly faster pace. The differences were huge. With mountain ranges and ocean basins similar to Earth’s, the temperature was 12 degrees warmer than with Venus’s topography. And if the rotation rate was 16 Earth days, the temperature skyrocketed 45 degrees higher than the level with its current rotation rate. The cloud pattern that kept the climate cool only formed if the planet was rotating slugglishly.
This result has vast implications for the world of exoplanets. “The community should be careful about ignoring worlds that are very close to their stars, like Venus-type worlds,” Way says. If a few key characteristics such as an exoplanet’s topography and rotation rate are just right, then the inner edge of the habitable zone—the region in a solar system where conditions conducive to life can arise—will be closer to the host star than is usually thought. The finding is especially important given that these close-in worlds are much easier to observe and characterize than other types of planets. The much-anticipated James Webb Space Telescope—often referred to as Hubble’s successor—for example, will likely only study worlds that hug their host stars, making observations of planets with wider orbits like Mars or even Earth out of the question. Or as Ravi Kopparapu, an astronomer at The Pennsylvania State University puts it: “The closest to Earth we can get with the James Webb Space Telescope is Venus around cool stars.”
But Glaze could not contain her excitement about the latest study because of the light it sheds on a rock back home. “Venus is the planet next door, the sister next door, and it's so surprising how little we know,” she says. “We know Mars so much better than we do Venus. Those [plus Earth] are our three terrestrial planets in our own backyard. If we don't understand those three planets and what makes them the same and what makes them different, we're going to be hard-pressed to interpret the new planets that we're discovering outside our own solar system.” Luckily, there are two Venus missions currently in competition for potential flight: One is a geophysical mission, which would map the planet in higher resolution than before. The other is one led by Glaze herself that would measure the makeup of the Venusian atmosphere.* Both could shed light on what Venus looked like in the past. “There is still more important data that we need to collect in order to put tighter constraints on these models, and we have the ability to collect those data now. We just need the missions,” Glaze says.
*Editor's Note (8/10/16): This sentence was edited after posting. The original erroneously stated the proposed atmospheric mission would also return samples to Earth.