Why has Venus lost its oceans but kept a thick, hot atmosphere? The answer might be lying in the subtle electromagnetic interaction in and around the planet.

Venus has an electric wind in its upper atmosphere that is strong enough to accelerate the heavy ions from water molecules to speeds greater than the planet's escape velocity, launching them into space.

“It’s amazing and shocking,” said Glyn Collinson, previously at UCL Mullard Space Science Laboratory and now a scientist at NASA’s Goddard Space Flight Center, in a statement.

“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 definitely has to be on the checklist when we go looking for habitable planets around other stars.”

Venus is very similar to Earth in term of its size and volume, but it has a much thicker atmosphere and a higher surface temperature of 460°C (860°F). The atmosphere has significantly less water than Earth, between 10,000 and 100,000 times, and as the planet is thought to have once had oceans, astronomers have often wondered where it all went. The solar wind was the likely suspect, but the instruments aboard ESA's Venus Express showed that the planetary electric field has a greater impact on Venus.

Every planet with an atmosphere is surrounded by an electric field, which can help lift charged particles and ions, even to the point that they break free from gravity. The team showed that Venus’ electric field is much stronger than expected, at least five times Earth’s. These results are published in Geophysical Research Letters.

“The electric field is started when electrons are produced by sunlight hitting the atmosphere of Venus,” Professor Andrew Coates, co-author of the study, told IFLScience.

“These electrons can move very quickly along the magnetic field generated by the solar wind and doing so they pull the ions with them, so they create an electrical pull.”

This difference between Venus and Earth could depend on Venus being closer to the Sun, and thus receiving more ultraviolet light than Earth, breaking more water molecules apart. Although Venus loses such a large fraction of gas, its atmosphere remains thick.  

“Venus Express has found possible signs of active volcanism,” added Coates. “So, it could be that the atmosphere is being replenished by these volcanoes but only to then be lost into space.”

Venus is not the only object in the Solar System losing material. Mars has lost most of its atmosphere, and Saturn’s moon Titan loses 7 metric tons (7.7 US tons) per day.

“All these objects in the Solar System, they are losing materials from their atmosphere to space,” said Coates. “And we are trying to understand how this loss occurs. This is an important mechanism of the atmospheric evolution of an object through time.”

Understanding how planets lose their atmosphere is important not only for our Solar System but for exoplanets as well. If planets orbiting other stars are also losing material, this could affect their capacity to host life.

^{The thick Venusian atmosphere as seen in ultraviolet light in 1979 by the Pioneer Venus Orbiter. NSSDC Photo Gallery}