The Universe In 101 Words : Why is water on Mars so important?

An artists impression of a pool of hypersaline water on Mars.
Because everyone wants to find ET. 

Where is there's water, on Earth, there's life. 
On Mars the temperature, air pressure, and soil chemistry conspire to make an environment on the tipping point, where liquid water goes from being just possible to truly impossible. 

But our robots have found that things there were more friendly to water billions of years ago - ancient Mars was much more inviting. 

So Mars could let us test an important idea: Does water really always equal life?

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