Electric fields around Venus stripped its atmosphere of water components

Scientists using data obtained by ESA's Venus Express have discovered a strong electric field around the planet Venus. Venus' electric field is at least 5 – 10 times stronger than Earth's and it seems it played a crucial part in removing water components from the planet's atmosphere.

Venus is in many ways the most like Earth in terms of its size and gravity, and there’s evidence that it once had oceans worth of water in its distant past.

However, with surface temperatures around 460 °C (860 °F), any oceans would have long since boiled away to steam and Venus is uninhabitable today. Yet Venus’ thick atmosphere, about 100 times the pressure of Earth’s, has 10 000 to 100 000 times less water than Earth’s atmosphere. Something had to remove all that steam, and the current thinking is that much of the early steam dissociated to hydrogen and oxygen: the light hydrogen escaped, while the oxygen oxidized rocks over billions of years. Also, the solar wind could have slowly but surely eroded the remainder of an ocean’s worth of oxygen and water from Venus’ upper atmosphere.

“It’s amazing, shocking,” said Glyn Collinson of NASA’s GSFC and a lead author of a paper published by AGU. “We never dreamt an electric wind could be so powerful that it can suck oxygen right out of an atmosphere into space. This is something that has to be on the checklist when we go looking for habitable planets around other stars.”

“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 said.

Just as every planet has a gravity field, it is believed that every planet with an atmosphere is also surrounded by a weak electric field. While the force of gravity is trying to hold the atmosphere on the planet, the electric force can help to push the upper layers of the atmosphere off into space. At Venus, the much faster hydrogen escapes easily, but this electric field is so strong that it can accelerate even the heavier electrically charged component of water – oxygen ions – to speeds fast enough to escape the planet’s gravity. When water molecules rise into the upper atmosphere, sunlight breaks the water into hydrogen and oxygen ions, which are then carried away by the electric field.

The team discovered Venus’ electric field using the electron spectrometer, a component of the ASPERA-4 instrument, aboard the ESA Venus Express. They were monitoring electrons flowing out of the upper atmosphere when it was noticed that these electrons were not escaping at their expected speeds. The team realized that these electrons had been tugged on by Venus’ potent electric field. By measuring the change in speed, the team was able to measure the strength of the field, finding it to be much stronger than anyone had expected, and 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 said, “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.”

Reference:

• "The electric wind of Venus: A global and persistent “polar wind”-like ambipolar electric field sufficient for the direct escape of heavy ionospheric ions" – Glyn A. Collinson et al. – June 20, 2016 – Geophysical Research Letters – ​doi:10.1002/2016GL068327 – This research article will be freely available for 30 days from the date of publication

Featured image: This is an artist's concept of the electric wind at Venus. Rays represent the paths that oxygen and hydrogen ions take as they are pulled out of the upper atmosphere. Credits: NASA/Goddard/Conceptual Image Lab, Krystofer Kim