Sister Planet: Mission to Venus reveals watery past

Dense clouds of sulfuric acid blanketing Venus have posed a problem for scientists seeking inside information about Earth’s nearest planetary neighbor.

Now, the Venus Express probe, launched by the European Space Agency in 2005, has ventured beneath those clouds and found evidence that Venus once had more water than it does today. The probe also provided detailed new measurements of the weather on Venus, proof of lightning on the planet, and signs of a formerly unknown hot spot near its south pole.

In nine papers appearing in the Nov. 29 Nature, researchers say these findings could be useful for understanding Earth’s atmosphere too.

“Venus resembles the Earth in many, many ways,” says Andy Ingersoll of the California Institute of Technology in Pasadena. Not only do Venus and Earth orbit the sun at similar distances, but the two planets are similar in size, gravity, and composition. Though Venus’ atmosphere contains much more carbon dioxide than Earth’s, both have water vapor.

Water’s role in Venus’ past, in particular whether there used to be more of it, was one of the biggest questions about the planet, says Ingersoll.

“There’s some water, but where’s the ocean on Venus?” he asks. “Venus Express has addressed that.”

If Venus once had more water, scientists figured, then vast amounts of hydrogen and oxygen must at some point have escaped Venus’ gravity. But some hydrogen comes in a heavy form, deuterium. It is harder for the heavy form to escape gravity, so if lots of hydrogen from water left Venus, the ratio of deuterium to hydrogen left behind would rise.

A team of scientists led by Jean-Loup Bertaux of the Service d’Aéronomie du CNRS in Verrières-le-Buisson, France, showed that the deuterium-to-hydrogen ratio on Venus is, in fact, higher than that on Earth.

If the water vapor in Venus’ atmosphere today were instead an ocean, it would be 3 centimeters deep. Using the deuterium-hydrogen ratio to estimate how much water has been lost, the scientists extrapolated that there once would have been enough water to cover Venus with at least 4.5 meters of water. (If all the water on Earth were spread out, it would be 2.8 kilometers deep.)

Furthermore, in a separate paper, Stanislav Barabash of the Swedish Institute of Space Physics in Kiruna showed that hydrogen and oxygen ions are still escaping from Venus today.

“The surprising discovery was the escape of oxygen atoms and hydrogen atoms keeps the same ratio as in a water molecule,” says Barabash.

Understanding how and why water leaves Venus has important implications on Earth, Ingersoll says. When a climate heats up and oceans evaporate, the increased water vapor in the atmosphere acts as a greenhouse gas and can accelerate the warming of the oceans.

“If this runaway greenhouse effect could happen on Venus, could it happen on Earth too?” asks Ingersoll.

Insights into water on Venus weren’t the only surprise findings from Venus Express. Scientists found a hot spot near the south pole that’s 10°C warmer than the surrounding atmosphere. A hot spot of similar shape and size had previously been discovered near Venus’ north pole.

The probe also improved scientists’ understanding of weather patterns on Venus. Radio signals sent through the clouds recorded a difference in temperature between nighttime and daytime of 40°C, much larger than anticipated. Other instruments showed a lightning rate about half that on Earth.

Håkan Svedhem of the European Space Agency in Noordwijk, the Netherlands, says that the Venus Express findings offer a much-needed baseline for comparison with data from future missions.

“To follow all this and see how it evolves as a function of time will be interesting,” he says.