Billions of years ago, when the solar system was young, you might have mistaken Venus for Earth at a glance—both planets were home to life-sustaining water.
Venus lost that water long ago, and scientists generally place the blame on the solar wind. The electric field carried by the solar wind penetrated Venus’s ionosphere, stripping away its ions—including water group species like oxygen and hydrogen.
However, this isn’t the only way that Venus might have dried out. A different mechanism involves how an electric field could be generated from the movement of electrons, even while gravity pulls the ions downward. The field could make it easier for water’s constituent atoms to be lost to space: It could fling hydrogen ions away from the planet at escape velocity, and it might give a considerable boost to the heavier oxygen ions as well.
Now Collinson et al. have made the first definitive measurement of this electric field, and their results show it may indeed have played a larger role than previously thought. The results may even force scientists to downgrade prospects for water on exoplanets thought to be habitable.
The researchers used data from the European Space Agency’s Venus Express probe, which arrived at the planet in 2006 and operated until it ran out of propellant and sank into the atmosphere in 2014. The team chose orbits where the satellite flew over Venus’s North Pole on every orbit. This allowed the probe to measure the strength of the electric field associated with the planet’s magnetic field lines.
The authors were surprised to find just how much potential the electric field had: roughly 10 volts, strong enough to fling not only hydrogen ions but also oxygen atoms into space. For context, that’s at least 5 times stronger than the equivalent field in Earth’s ionosphere. The team suspects this could be because Venus is closer to the Sun and receives more ionizing radiation, and some preliminary modeling supports that conclusion.
If true, the discovery could be bad news for the dozens of potentially habitable exoplanets that NASA’s Kepler satellite has discovered. Although these planets are in the “Goldilocks zone”—situated at the right distance from their star to support water—in Kepler’s sample of systems, that zone tends to be closer to their stars because those planets are easier to detect. If they too have strong electric winds like Venus, they may be more likely to share its bone-dry, lifeless fate. (Geophysical Research Letters, doi:10.1002/2016GL068327, 2016)
—Mark Zastrow, Freelance Writer