The surface of Mars is etched (蚀刻) with ancient river valleys and lake basins,Some re searchers think that liquid water once flowed on the Red Planet. Today, Mars is too cold for much, if any,liquid water to exist, And 3.8 billion years ago,when the flowing water formed, the sun was fainter than it is today, making it even harder to imagine a warm Martian climate, That's why many researchers think Mars may have gone in and out of deep freezes,
Some researchers have suggested that early Mars only melt when large asteroid (小行星) impacts or volcanic eruptions temporarily warmed the planet.But Kasting, a geologist at Pennsylvania State University thinks warm windows from such dramatic events would have been too brief to carve the vast valleys that exist on Mars .Now, Kasting and his colleagues have come up with an alternative explanation: They think Mars may have experienced a series of climate cycles caused by changes in the strength of the greenhouse effect.
The idea goes like this: When Mars was cold and frozen volcanoes continued to erupt the greenhouse gases carbon dioxide and hydrogen into the atmosphere.There, the gas blanket trapped heat and warmed the planet up until liquid water began to flow,forming Mars rivers and lakes. However, warm temperatures and abundant water would also have sped up certain chemical reactions that consumed carbon dioxide,mitigating the greenhouse effect and cooling the planet back down again.Then the cycle would repeat.
Kasting shared his theory with other astronomers at the December meeting of the American Geophysical Union in San Francisco, and in the journal Earth and Planetary Science Letters. So far, Kasting's team has only shown that such an explanation is possible.But the researchers say field trips could help test the idea by looking for evidence of multiple warm events, and their duration.
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