Venus and Earth are the roughly the same size, and millions of years ago Venus possibly had Earth-like oceans. But our sunny-side sister is now arid, its Venusian air up to 100,000 times drier than Earth's.
Chemical hints are all that is left of oceans on Venus. One of the strongest clues is atmospheric deuterium, a form of hydrogen found in earthly seas, which remains on Venus like a chalk outline of long-dead bodies of water.
The lack of oxygen and other ions associated with water is puzzling. If Venus once had liquid water, its molecules should linger in the atmosphere. The grip of gravity does not simply let go, no matter how hot a planet is. Scientists previously thought that the sun, blasting Venus with a stream of particles known as solar wind, had stripped the planet of its steam.
But, according to new research from an international team of scientists, the sun is only indirectly a factor. The true culprit, they say, is Venus's super-strong electric wind.
"It's amazing, shocking," Nasa's Glyn Collinson, an expert in atmospheric electric fields and an author of the new study, said in a statement. "We never dreamt an electric wind could be so powerful that it can suck oxygen right out of an atmosphere into space." (This electric wind is no gentle spring breeze, but a charged stream similar to solar wind.)
Earth, too, has an electric field in the atmosphere, though it is weaker and therefore tough to measure. Venus's field was also difficult to measure, as Collinson and his colleagues point out in the journal Geophysical Research Letters, but that is because Venus is a five-month spaceflight away. It is certainly not as weak. At about 10 volts, the Vesuvian electric wind is at least five times stronger than Earth's.
Using an electron spectrometer on the European Space Agency satellite Venus Express, researchers were able to measure the electric potential of the planet's atmosphere. The scientists were stunned by what they found. The electric wind, far from the bit part they had hypothesised, appears to have the starring role drying out Venus. Thanks to the unusually high voltage, the wind would be able to defy gravity, shoving heavy charged particles - including the oxygen ions that once belonged to Venus's water - all the way off-world.
"If you were unfortunate enough to be an oxygen ion in the upper atmosphere of Venus then you have won a terrible, terrible lottery," Collinson said. "You and all your ion friends will be dragged off kicking and screaming into space by an invisible hand, and nothing can save you."
The scientists are not entirely sure why the electric wind is so strong on Venus, but they believe it could be the close proximity to the sun.
This is a crucial finding for the future of space exploration, the researchers argue.
Although Mars gets the lion's share of attention for future human missions, scientists - such as Nasa's Geoffrey Landis - have pointed to the Venusian atmosphere as a candidate for floating colonies. The trip to Venus would take less travel time, and the planet boasts better radiation protection and a more amenable pressure. But the newly-discovered strong electric wind would be a significant obstacle.
Beyond Venus, Collinson and his colleagues believe that exoplanets may also have stronger electric winds than the field on Earth.
Exoplanets - planets found outside our solar system - tend to be closer to their respective suns. (It's easiest for scientists to spot distant planets when they travel in front of stars.) If Venus has a strong electric wind because it is close to our sun, it is possible that exoplanets have been sucked bone-dry in the same way.
"This is something that has to be on the checklist," Collinson said, "when we go looking for habitable planets around other stars."
