


Hakan Svedhem, of the European Space Agency, and his colleagues say that Earth and Venus probably started out much the same. The Earth's oceans teemed with plants and animals that converted most of its atmospheric carbon dioxide into carbonates and sank to the bottom as they died, to become sedimentary rocks. By contrast, Venus lost most of its liquid water. That is because Venus, being closer to the sun, started to warm up. This generated more water vapour in its atmosphere, further increasing the temperature in a runaway greenhouse effect.

Planetary scientists have also long blamed Venus's sterility on the lack of an internal magnetic field. The sun zaps its planetary neighbours with the “solar wind”, a stream of highly energetic charged particles. Damaging cosmic rays also bombard the planets. The Earth is protected from much of this radiation because it has an internal magnetic field that generates a protective magnetosphere. But Venus has no such protection, possibly because it does not rotate much.

Yet according to Tielong Zhang, of the Space Research Institute at the Austrian Academy of Sciences, and his colleagues, Venus does still manage to avoid much of the bombardment. Dr Zhang says that the solar wind itself carries a magnetic field all the way to Venus where it encounters the planet's electrically charged upper atmosphere, or ionosphere. Venus's ionosphere is highly conductive, and data taken by Venus Express suggest that the solar wind cannot penetrate it at any time. Like a boat trying to cut through the ocean waves, the induced magnetic fields pile up where the solar wind meets Venus's ionosphere. The solar wind fluctuates in strength, but is prevented from entering both when it is strong and when it is weak.

On the dark side of the planet things are much quieter. Shielded from the buffeting of the solar wind, ions quietly leak out of the atmosphere. Venus Express has detected oxygen, hydrogen and helium ions—remnants of early oceans—escaping into space. The two sisters may have started as twins but, as they have grown older, they have grown apart.