Hints: Mars dried-up the Viking Space Probe tributary er（注意语气词...）

The conditions there are way too extreme. It's really just too cold. We really need liquid water for life. What we want to find on Mars is some place where we can find water in the more temperate regions. What we ultimately need to find is liquid water. But the first indications were that if life needed liquid water, then Mars was out of luck. Conditions all over the planet seemed too cold, there appeared to be no prospect of water anywhere. These photographs changed everything. Taken by the Viking Space Probe in 1976, they showed what looked like dried-up river valleys. You can see one here. You can see there's a, a valley through here, you can see it branches, the tributaries. Here's one branch going off here with tributaries. So this looks very much like, er, a terrestrial river system. If these were dried-up riverbeds, it meant that Mars must once have had the perfect conditions for life.

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