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　　土卫六的大气与地球很相似

　　Scientists at UCL have observed how a widespread polar wind is driving gas from the atmosphere of Saturn's moon Titan. The team analysed data gathered over seven years by the international Cassini probe, and found that the interactions between Titan's atmosphere, and the solar magnetic field and radiation, create a wind of hydrocarbons2 and nitriles being blown away from its polar regions into space. This is very similar to the wind observed coming from the Earth's polar regions. Titan is a remarkable3 object in the Solar System. Like Earth and Venus, and unlike any other moon, it has a rocky surface and a thick atmosphere. It is the only object in the Solar System aside from the Earth to have rivers, rainfall and seas. It is bigger than the planet Mercury.

　　Thanks to these unique features, Titan has been studied more than any moon other than Earth's, including numerous fly-bys by the Cassini probe, as well as the Huygens lander which touched down in 2004. On board Cassini is an instrument partly designed at UCL, the Cassini Plasma4Spectrometer (CAPS), which was used in this study.

　　"Titan's atmosphere is made up mainly of nitrogen and methane5, with 50% higher pressure at its surface than on Earth," said Andrew Coates (UCL Mullard Space Science Laboratory), who led the study. "Data from CAPS proved a few years ago that the top of Titan's atmosphere is losing about seven tonnes of hydrocarbons and nitriles every day, but didn't explain why this was happening. Our new study provides evidence for why this is happening."

　　Hydrocarbons are a category of molecules6 that includes methane, as well as other familiar substances including petrol, natural gas and bitumen7. Nitriles are molecules with nitrogen and carbon tightly bound together.

　　The new research, published today in the journal Geophysical Research Letters, explains that thisatmospheric8 loss is driven by a polar wind powered by an interaction between sunlight, the solar magnetic field and the molecules present in the upper atmosphere.

　　"Although Titan is ten times further from the Sun than Earth is, its upper atmosphere is still bathed in light," says Coates. "When the light hits molecules in Titan's ionosphere, it ejects negatively charged electrons out of the hydrocarbon1 and nitrile molecules, leaving apositively9 charged particle behind. These electrons, known as photoelectrons, have a very specific energy of 24.1 electronvolts, which means they can be traced by the CAPS instrument, and easily distinguished10 from other electrons, as they propagate through the surrounding magnetic field."