AS WELL as safeguarding our atmosphere, the Earth鈥檚 protective shield may be stealing some of it on the sly.
The region in space that contains the Earth鈥檚 magnetic field, known as the magnetosphere, protects us from the charged particles that come streaming from the sun. By acting as a barrier to this solar wind, it is also thought to prevent these particles transferring enough of their energy to gas molecules in the atmosphere for these molecules to escape the Earth鈥檚 gravitational pull.
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This may be only half the story, though. At the poles, the magnetosphere might be aiding loss of the atmosphere, according to Stas Barabash of the in Kiruna, who is principal investigator for the European Space Agency鈥檚 Venus Express mission. Barabash bases his suggestion on measurements of the flow of ions escaping from Venus, Mars and Earth. It is thought that Venus has never had a magnetosphere, whereas Mars did until its magnetic dynamo wound down 3.5 billion years ago.
Taking into account the different masses of the three planets, their atmospheric make-up and their distance from the sun, Barabash compared the rate of loss of oxygen ions from each one. He focused on oxygen ions because these are the most abundant ions in the ionospheres of all three planets. He found that Earth lost oxygen around three times as fast as the other planets.
Barabash points out that a planet鈥檚 magnetosphere will always be far larger than the planet itself or its atmosphere. This, he reasons, means that a planet with a magnetic field will absorb more energy from the solar wind than it would if it didn鈥檛 have one. This extra energy would be funnelled down towards the magnetic poles, so molecules in the ionosphere above these regions could be accelerated enough to escape (see diagram). Barabash presented the results this month at the at Noordwijk in the Netherlands.
The idea is supported by past studies of the magnetosphere, such as the European Space Agency鈥檚 mission, which have shown that ions escape from the Earth鈥檚 poles at twice the rate or more compared with the average for the planet as a whole.
At present we are experiencing low solar activity, but a stronger solar wind when Earth and Mars were young could have played a role in shaping these planets鈥� early atmospheres. Barabash estimates that only some 60,000 tonnes of the Earth鈥檚 thousands of trillions of tonnes of gas are lost every year, so we are in no danger of losing the atmosphere altogether.
of the University of California, Berkeley, who also spoke at the conference, described the idea as 鈥減rovocative鈥�, but remains unconvinced. She says that the energy trapped by the magnetosphere could be used in other ways apart from accelerating ions, like stirring up winds or simply heating the atmosphere.
