Hints: Mars Earth Mars Global Surveyor high-resolution carbon dioxide 本文有一个破折号：-- a winding

For Mars to have rivers, it must once have had streams, rain, clouds and an atmosphere, a planet just like the Earth today,if these were rivers. They had to be sure. The answers would come in 1998 with the launch of Mars Global Surveyor. It was equipped with the latest high-resolution electronic cameras. Sections of the valleys were revealed in fantastic detail. Some valleys had been eroded and filled in with sand. It was impossible to say how they'd been formed. They could have been carved by water, or soil, or carbon dioxide. Then, after they'd searched through thousands of images they found this-- a winding valley, two kilometers wide, and at a bend in the canyon, a tiny channel, the unmistakable trace of an ancient river. This is the best evidence we have that climate in the past was different from what it is today.

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