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Why is liquid water on Mars necessary for life?

NASA revealed Monday that liquid water is present on Mars. Why is liquid water a prerequisite for life? 

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    Dark narrow streaks, called 'recurring slope lineae,' hypothesized to be formed by flow of briny liquid water, emanate from the walls of Garni Crater on Mars, in this view constructed from observations by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter.
    NASA/JPL-Caltech/Univ. of Arizona
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Yesterday, NASA revealed the “strongest evidence yet that liquid water flows intermittently on present-day Mars.” NASA's Mars Reconnaissance Orbiter (MRO) detected hydrated salts in the recurring slope lineae (RSL), the streaks that appear on the Red Planet’s slopes.

"We can look and see if we can determine if there is some sort of aquifer network that may be supplying these [RSL features]. We don't know that – there are other theories, other ideas – but that is actually the next step," Jim Green, director of NASA's Planetary Science division, said during the press briefing. "So if there is indeed those kind of resources that we can begin to probe, we might be able to answer that question pretty quickly."

Does the discovery of liquid water mean there is life on Mars? Not so fast.

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"Finding evidence for flowing water is not the same as finding life," says National Geographic's Nadia Drake. "Right now, scientists don’t know where this water is coming from, or if the chemistry in these Martian seeps is even life-friendly."

Planetary scientists have long said that where water exists, life may also exist. “Water is essential at the molecular level to moving life beyond its basic building blocks; thus, searches for extraterrestrial life usually involve a search for liquid water,” writes NASA.

But what makes water the necessary liquid for sustaining life?

“Part of the reason is that we've never discovered an organism that's proven otherwise,” says Jonathan Attebery, science writer for “HowStuffWorks.” Water facilitated the beginning of life on Earth, “acting as a medium in which organic compounds could mix with one another.”

As a solvent, water allows vital chemical reactions to occur. “From the list of potentially abundant solvents in the universe, water looks to be the best candidate to support a complex ecosystem,” says Steve Nerlich of Universe Today in an article for io9.

Furthermore, liquid water “conveys vital substances like metabolites and nutrients from one place to another” and is also capable of “bending enzymes,” the proteins that catalyze chemical reactions, says Peter Tyson, editor in chief of NOVA Online.

"Given that life on Earth is so dependent on water, and given that water is so prevalent in the universe, we don't feel that we're going out on a limb to say that life would require liquid water," said Neil deGrasse Tyson, an astrophysicist and director of the Hayden Planetarium at the American Museum of Natural History, in an interview with PBS.

It isn’t impossible that lifeforms exist that don’t require water to survive, but they would not resemble life on Earth. Biochemistries based on other solvents “seem likely to be limited to cold, low energy environments where the rate of development of biological diversity and evolution may be very slow,” says Mr. Nerlich.

According to Mr. Attebery, “we simply don't have enough information to say whether or not life could exist without water. We know with certainty, however, that life on Earth definitely couldn't.”

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