NASA: in fact, we have not found the water

美國國家航空航天局又一重大發現：火星有水或存在生命！消息一出，震驚全球。可是再仔細解析一下他們的“發現”，怎么還是沒有看到我們翹首盼望的液態水的真面目？原來還是只找到“證據”，只不過這一次號稱是“強有力證據”而已。

Water trickles down gullies and crater walls on the surface of Mars， raising intriguing questions about whether life could exist on the red planet.

The findings are the strongest evidence yet that liquid water darkens the Martian surface today. In fact， weve known for years that water flowed on Mars long ago.

Described Monday in Nature Geoscience， the observations suggest that this water appears seasonally， forming dark lines as it tumbles down steep Martian slopes. But scientists still dont know where the water is coming from， or if the chemistry is even right for supporting life.

For now， the find solves the mystery of dark streaks that appear and disappear seasonally， called recurring slope lineae.

The streaks were first spotted in 2010 by Lujendra Ojha， then an undergraduate at the University of Arizona. Ojha was studying images returned to Earth from the HiRISE camera， aboard NASAs Mars Reconnaissance Orbiter. At the time， he says， he had no clue how important his observation would be.

Is Mars sweating？

A big question swirls around the origin of that water： Where is it coming from？ One possibility is that the seeps are fueled by an aquifer or melting subsurface ice. These scenarios would have Mars essentially sweating， with saltwater seeping from its pores and trickling down slopes as the planet warms.

The water might also be atmospheric in origin， which is the hypothesis the team seems to favor. In this scenario， surface salts absorb water vapor in the Martian atmosphere.

But until now， evidence has been scarce for flowing water at the surface today. What it means， in the bigger picture of planetary exploration and the search for life beyond Earth， is still a mystery.