'Electric wind' stripped Venus of oxygen
An unexpectedly powerful force helped suck even large molecules from the planet's atmosphere. Bill Condie reports.
An “electric wind”, so powerful that it sucked oxygen out into space, helped strip Venus of its water, a new study has found.
The study, based on data from the European Space Agency’s Venus Express mission, has forced a re-think on how other planets might have been affected.
“It’s amazing, shocking. This is something that has to be on the checklist when we go looking for habitable planets around other stars,” says Glyn Collinson, a scientist at NASA’s Goddard Space Flight Centre in Maryland, and lead author of a paper published in the journal Geophysical Research Letters.
Venus is in many ways similar to Earth. It is of similar size and gravity and once contained oceans of water.
Most of that water has boiled away, thanks to surface temperatures of around 460 °C. But the steam has also disappeared, probably after it split into its constituent hydrogen and oxygen.
Scientists believe the light hydrogen escaped, while the oxygen vanished into oxidised rocks, or was eroded from the upper atmosphere by the solar wind — a million-mile-per-hour stream of charged particles blowing from the sun.
The new study suggests a third phenomenon in play – the electric wind.
It is believed that every planet with an atmosphere is surrounded by a weak electric field that can push the upper layers of the atmosphere off into space.
“We found that the electric wind, which people thought was just one small cog in a big machine, is in fact this big monster that’s capable of sucking the water from Venus by itself,” Collinson says.
In the case of Venus, the electric field is so strong that it accelerated even the heavier electrically charged component of water — oxygen ions — to speeds fast enough to escape the planet’s gravity.
The researchers discovered Venus’ electric field using the electron spectrometer aboard the Venus Express.
They were monitoring electrons flowing out of the upper atmosphere when it was noticed that these electrons were escaping faster than anticipated – they were being dragged along by Venus’ electric field.
This field, they calculated, was stronger than expected – at least five times more powerful than at Earth.
“We don’t really know why it is so much stronger at Venus than Earth,” Collinson says, “but we think it might have something to do with Venus being closer to the sun, and the ultraviolet sunlight being twice as bright.
"It’s a challenging thing to measure and even at Earth to date all we have are upper limits on how strong it might be.”
Related reading: Venus Express unveils surprising polar atmosphere