WITH scorching rocks, downpours of sulphuric acid, and a crushing atmosphere with a pressure 90 times Earth’s, Venus has to be the most hellish planet in the solar system. Yet in one respect it turns out to be surprisingly Earth-like.
A year’s worth of results from the seven experiments aboard the European Space Agency’s Venus Express mission, released this week, have helped to create the most detailed 3D model of Venus’s atmosphere to date – and it paints a surprisingly familiar picture (Nature, vol 450, p 629). “In many respects, Venus is actually pretty much like Earth,” says project scientist Håkan Svedhem of ESA’s European Space Research and Technology Centre in Noordwijk, the Netherlands.
Despite dramatic differences in temperature and chemical make-up, the atmosphere’s basic machinery is similar to Earth’s, Svedhem says. One of the main features is Hadley-like convection cells in the lower atmosphere, moving between the equator and up to around 60 degrees latitude in each hemisphere. Similar features are one of the main drivers of the Earth’s weather.
Advertisement
“Despite temperature and chemical differences, the atmosphere’s basic machinery is similar to Earth’s”
Some aspects of the Venusian atmosphere remain mysterious, but the new data could help explain the mechanics behind them, says Svedhem. For example, nobody knows exactly why the upper atmosphere goes around the planet every 96 hours, whereas Venus itself takes no less than 243 Earth days to spin on its axis. Another mystery is the origin of bizarre double vortices in the atmosphere above both poles, though some scientists think they might be linked to the upper atmosphere’s rapid rotation.
Alongside the new atmospheric data, the Venus Express scientists have presented evidence confirming that Venus has lightning – a previously disputed idea (see “Thunder and lightning”). They have also released the results of an experiment that in effect watched Venusian water escaping into space (see “Gone with the wind”). Venus is similar in size and mass to Earth, and this finding adds weight to the view that it started out as Earth’s twin sister, with oceans to match, before evolving into a bone-dry greenhouse planet as carbon dioxide accumulated in its atmosphere and water was lost.
Gone with the wind
Venus may once have had oceans as vast and deep as Earth’s – so where did all the water go?
Its atmosphere is known to contain an unusually high ratio of deuterium (heavy hydrogen) to ordinary “light” hydrogen. As hydrogen can escape the planet’s gravitational field more easily than deuterium, this suggests that there was once much more of it – and it could well have come from water.
Now the Venus Express spacecraft has observed the remnants of smashed-up water molecules escaping from the planet’s atmosphere. Using the ASPERA-4 experiment (Analyser of Space Plasmas and Energetic Atoms), Stas Barabash of the Swedish Institute of Space Physics and colleagues detected hydrogen and oxygen ions being blown away from the upper atmosphere into interplanetary space by the solar wind. The number of escaping hydrogen atoms was twice the number of oxygen atoms – exactly what you would expect if the atoms are the fragments of water molecules that had been broken apart by ultraviolet radiation from the sun.
This supports the theory that the solar wind – a stream of electrically charged particles from the sun – sweeps the constituent elements of water from the upper atmosphere. Since Venus is closer to the sun and has no deflecting magnetic field, the solar wind’s influence is more pronounced on the planet’s atmosphere than on Earth.
The researchers hope that future measurements will provide an estimate of how much water Venus has lost over the past few billion years. Recombining the hydrogen and oxygen now in the Venusian atmosphere, plus the existing water vapour, would yield only enough water to cover the planet to a depth of 3 millimetres.
Thunder and lightning
Though artists’ impressions often show the apocalyptic landscape of Venus criss-crossed by lightning bolts, the existence of lightning on Venus has been controversial.
Electromagnetic signals possibly generated by lightning were recorded more than 30 years ago by the Soviet Venera probe, but since then, efforts to detect lightning directly have been inconclusive. This led some scientists to question the ability of electric fields to discharge easily in the planet’s dense atmosphere. What’s more, the planet’s high clouds might make cloud-to-ground strikes unlikely.
Now observations from Venus Express have shown the artists were right all along. In May and June 2006, the spacecraft’s magnetometer detected bursts of electromagnetic radiation propagating upwards into the ionosphere – a layer containing electrically charged particles. These so-called “whistler mode” waves can only be produced by lightning discharges in the Venusian clouds, according to a team of scientists led by Chris Russell of the University of California, Los Angeles.
Extrapolating from the observed number of whistler mode waves, the team concludes that there must be at least 50 lightning discharges per second on Venus – about half as many as on Earth. “In six months’ time, we expect to have better information on where lightning strikes,” says Venus Express project scientist Håkan Svedhem.
