Why Venus is lifeless

For a planet named after the goddess of love, Venus is something of a misfit. Its clouds of sulphuric acid, crushing atmosphere of carbon dioxide and blast-furnace surface temperature of 457°C are anything but lovely. Yet in its youth it was, like its gentler sister Earth, swathed in oceans that provided a suitable breeding ground for life. What went wrong?

Since 1962 more than 30 spacecraft have made the trip to Venus, seeking to understand Earth's nearest neighbour and so-called twin planet. The most recent of these, Venus Express, was launched by the European Space Agency in 2005. In the current issue of Nature, nine papers report what it has found so far.

Earth and Venus are supposed to have 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 Venus does still manage to avoid much of the bombardment. 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, with the solar wind not being able to 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. Oxygen, hydrogen and helium ions - remnants of early oceans - are escaping into space. The two sisters may have started as twins but, as they have grown older, they have grown apart.

The Economist

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