Venus is remarkably Earth-like, with a similar size and gravity to our own planet. But the second planet from the sun is missing a key element to be a twin to our blue planet: water.
Scientists say there were once oceans on Venus's surface, but with surface temperatures topping 860 degrees Fahrenheit, it's no surprise the surface of Venus today is bone-dry.
But where did that water disappear to?
The water should have boiled off the surface of Venus into the planet's atmosphere. But Venus's thick atmosphere is mysteriously dry.
Venus's atmosphere has about 100 times the pressure of Earth's, and yet it has 10,000 to 100,000 times less water than our own atmosphere.
"So where did the steam go?" Glyn Collinson, a scientist at NASA's Goddard Space Flight Center, asks in a NASA video.
The surprisingly strong electrical field wrapped around Venus could be the culprit, according to new research reported in a paper published Monday in the journal Geophysical Research Letters.
Venus's electric field is "a monster of a force," Dr. Collinson, who is lead author on the paper, says. And it's powerful enough to drive the loss of Venus's atmospheric water to space.
Here's how it may have happened:
The water on the surface of Venus evaporates into steam. When those water molecules reach the upper atmosphere, the sun breaks the H2O molecules into their components: hydrogen and oxygen ions.
Hydrogen ions can escape the planet's gravity, which is holding the atmosphere down around the planet, easily. But oxygen ions are heavier and can't escape into space as easily.
Scientists say that every planet that has an atmosphere is surrounded by a weak electric field as well as the known gravity field. And while gravity is holding the atmosphere down onto the planet, the electric force is pushing the upper atmosphere away.
But in order for the heavy oxygen ions to escape gravity, that electric force has to be quite strong. And it is, according to this new study.
"The new result here is that the electric field powering this escape is surprisingly strong at Venus compared to the other objects," Andrew Coates, leader of the electron spectrometer team at University College London and a co-author of the paper, said in a press release.
"It turns out that Venus's electric field is at least 5 to 10 times stronger than on Earth," Collinson says in the video. "It can rip heavy things, like oxygen, straight out of the upper atmosphere and send them kicking and screaming off into space."
The team used an electron spectrometer installed in an instrument on the ESA Venus Express. While monitoring the flow of electrons out of the upper atmosphere of Venus, the researchers noticed that the electrons weren't escaping as fast as expected. That's when they realized Venus's electric field must be strong enough to tug at the escaping electrons.
"We’ve been studying the electrons flowing away from Titan and Mars as well as from Venus, and the ions they drag away to space to be lost forever. We found that over 100 metric tons per year escapes from Venus by this mechanism – significant over billions of years," Dr. Coates said. "This will help us understand how this universal process works."
This could have implications in the search for other worlds outside our solar system too.
"We never dreamt an electric wind could be so powerful that it can suck oxygen right out of an atmosphere into space," Collinson said in the press release. "This is something that definitely has to be on the checklist when we go looking for habitable planets around other stars."
"This really changes the way we have to think about planets," Collinson says in the video, "it turns out that planets can lose heavy things, like oxygen, to space, entirely through electrical forces in their ionospheres." When hunting for habitable planets orbiting other stars, he says, "it's no good having conditions perfect for an ocean and an atmosphere that you might want to breathe if some invisible force is going to come along and rip it all of into space."