Caption:: This image is centered near the location of the Pathfinder landing site. Dark sand dunes that surround the polar cap merge into a large, dark region called Acidalia. This area, as shown by images from the Hubble telescope and other spacecraft, is composed of dark, sand-sized grains of pulverized volcanic rock. Below and to the left of Acidalia are the massive Martian canyon systems of Valles Marineris, some of which form long linear markings that were once thought by some to be canals. Early morning clouds can be seen along the left limb of the planet, and a large cyclonic storm composed of water ice is churning near the polar cap.
- y; _) b' o( r" s9 k天文家园欢迎您！天文学家利用哈勃在火星上捕捉了罕见的巨大气旋(cyclone)，它跟地球上的台风一样，同样由水冰云(water ice cloud)所组成，跟火星上常见的沙尘暴(dust storm)不同，它比火星的北极冰冠(polar ice cap)还要大。根据推断这类气旋出现在火星北半球的盛夏时节，北极冠上的季节性二氧化碳干冰全部升华(sublimate)后出现，并且只在高纬度的极区附近出现，来去匆匆，生命周期只有数天。这类气旋可能是衍生自火星上不稳的锋面系统(frontal system)，再通过强烈的温差效应帮助而形成。
( U2 N& o# s! y! `$ D8 Q天文家园欢迎您！+ f8 F  @" }+ r$ E, S$ ~" Y/ W
Credit:: Steve Lee (University of Colorado), Jim Bell (Cornell University), Mike Wolff (Space Science Institute), and NASA

哈勃就是不一样,这火星!