火星的大氣層哪兒去了？

The clues are adding up to give a picture of a distant Martian past, when the planet had a thick atmosphere that was warm enough for water to stream on the surface. But now the water’s gone, and the atmosphere is so thin that any water would boil away. In fact, the vestigial atmosphere is still slowly dissipating into space. Why? Well, researchers believe that solar wind and radiation is behind the theft, and they’re planning a mission to find out, called MAVEN: Mars Atmosphere and Volatile Evolution Mission The solar wind is a stream of electrically-charged particles. It continuously flows from the sun out into space. We on Earth have protection in the form of a magnetic shield. But that shield is dead on Mars, gone for billions of years. Instead, the solar wind and the sun’s UV radiation give an electric charge to atoms and particles in Mars outer atmosphere. Then electric fields generated by the solar wind sweep away the charged particles. So the atmosphere becomes even thinner. Maven is scheduled to reach the red planet in 2014. Which will allow researchers to measure how much water and atmosphere are gone with the solar wind. —Cynthia Graber