Solar System’s planet Venus has an ‘electric wind’ strong enough to remove the components of water from its upper atmosphere. This action may have played a significant role in stripping Venus of its oceans, according to new research results from ESA’s Venus Express mission.
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. Image credit: NASA / Goddard / Krystofer Kim, Conceptual Image Lab.
Venus is often called Earth’s twin, since the second planet from the Sun is only slightly smaller than our own.
But the Venusian atmosphere is quite different, consisting mainly of carbon dioxide, with a little nitrogen and trace amounts of sulfur dioxide and other gases.
It is much thicker than Earth’s, reaching pressures of over 90 times that of Earth at sea level, and incredibly dry, with a relative abundance of water about 100 times lower than in Earth’s gaseous shroud.
In addition, Venus now has a runaway greenhouse effect and surfaces temperatures of around 860 degrees Fahrenheit (460 degrees Celsius) — high enough to melt lead. Also, unlike Earth, it has no significant magnetic field of its own.
Planetary researchers think Venus did once host large amounts of water on its surface over 4 billion years ago.
But as it heated up, much of this water evaporated into the atmosphere, where it could then be ripped apart by sunlight and subsequently lost to space.
The solar wind – a powerful stream of charged subatomic particles blowing from the Sun – is one of the culprits, stripping hydrogen ions and oxygen ions from the planet’s atmosphere and so depriving it of the raw materials that make water.
Now, an international team of scientists has identified another difference between the two planets: Venus has a substantial electric field, with a potential around 10 V — at least five times higher than expected.
Previous observations in search of electric fields at Earth and Mars have failed to make a decisive detection, but they indicate that, if one exists, it is less than 2 V.
This graphic compares surface temperatures and gravity on Earth and Venus. The two planets are similar sizes and have similar gravity – but Venus is bone dry, and its average surface temperature in Fahrenheit is more than 10 times higher than Earth’s. Image credit: NASA / Goddard / Brian Monroe, Conceptual Image Lab.
“We think that all planets with atmospheres have a weak electric field, but this is the first time we have actually been able to detect one,” said team member Dr. Glyn Collinson from NASA’s Goddard Space Flight Center, lead author of the study in the journal Geophysical Research Letters.
According to Dr. Collinson and his colleagues, the Venusian electric field is so strong that it can accelerate the heavy electrically charged component of water – oxygen – to speeds fast enough to escape the planet’s gravity.
When water molecules rise into the upper atmosphere, sunlight breaks the water into hydrogen ions which are fast and escape easily, and heavier oxygen ions which are carried away by the electric field.
“It’s amazing and shocking. We never dreamt an electric wind could be so powerful that it can suck oxygen right out of an atmosphere into space,” Dr. Collinson said.
“This is something that definitely has to be on the checklist when we go looking for habitable planets around other stars.”
“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,” added study co-author Prof. Andrew Coates, from the Mullard Space Science Laboratory at the University College London, UK.
“We found that over 100 metric tons per year escapes from Venus by this mechanism – significant over billions of years.”
“The new result here is that the electric field powering this escape is surprisingly strong at Venus compared to the other objects.”
“This will help us understand how this universal process works.”
The researchers discovered Venus’ electric field using the NASA-SwRI-UCL electron spectrometer on ESA’s Venus Express spacecraft.
When monitoring electrons flowing out of the upper atmosphere, they noticed the electrons were not escaping at their expected speeds because they were being tugged on by Venus’ potent electric field.
By measuring the change in speed, the team found the strength of the field to be much stronger than expected, and at least five times more powerful than at Earth.
This graphic compares the atmospheric composition and electric field strength on Earth and Venus. New research suggests that the electric field around Venus may be a key factor in shaping what molecules exist in the Venusian atmosphere – including its lack of the molecules needed to make water. Image credit: NASA / Goddard / Brian Monroe, Conceptual Image Lab.
Another planet where the electric wind may play an important role is Mars.
“With ESA’s Mars Express, we have already caught this process in action at Mars, and NASA’s MAVEN spacecraft can now determine its relative importance,” Prof. Coates said.
“With NASA’s Cassini spacecraft we found that Titan loses 7 metric tons per day this way.”
Understanding the role played by planet’s electric winds will help astronomers improve estimates of the size and location of habitable zones around other stars.
“Even a weak electric wind could still play a role in water and atmospheric loss at any planet. It could act like a conveyor belt, moving ions higher in the ionosphere where other effects from the solar wind could carry them away,” said study co-authors Dr. Alex Glocer, also from NASA’s Goddard Space Flight Center.
“Water has a key role for life as we know it on Earth and possibly elsewhere in the Universe,” said Dr. Hakan Svedhem, Venus Express project scientist at ESA.
“By suggesting a mechanism able to deprive a planet close to its parent star of most of its water, this discovery calls for a rethink of how we define a ‘habitable’ planet, not only in our Solar System, but also in the context of exoplanets.”
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Glyn A. Collinson et al. The electric wind of Venus: A global and persistent ‘polar wind’-like ambipolar electric field sufficient for the direct escape of heavy ionospheric ions. Geophysical Research Letters, published online June 20, 2016; doi: 10.1002/2016GL068327
