60秒科学:NASA将发火星探测器 揭火星大气层流失之（在线收听）

诸多线索综合起来为我们描绘出一幅画面：那是火星遥远的过去，那时的火星还有着一层厚厚的大气层，令这颗星球温热暖和，从而使水能够在火星表面流淌。但如今，地表的水消失不见，而大气层变得如此之薄，任何水分都会蒸发掉。

Arriving in 2014, the Mars Atmosphere and Volatile Evolution Mission (MAVEN) will see if the solar wind and radiation are behind the loss of the Martian atmosphere. Cynthia Graber reports

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