Water. It's found everywhere on Earth, from the polar ice caps to steamy geysers. And wherever water flows on this planet, you can be sure to find life.
"When we find water here on Earth — whether it be ice-covered lakes, whether it be deep-sea hydrothermal vents, whether it be arid deserts — if there's any water, we've found microbes that have found a way to make a living there," said Brian Glazer, an oceanographer at the University of Hawaii at Manoa, who has studied astrobiology.
That's why NASA's motto in the hunt for extraterrestrial life has been "follow the water."
Yesterday (Sept. 28), NASA scientists announced they'd found it on Mars: Dark streaks that scientists have spotted seasonally for more than a decade in images of the Red Planet are evidence of flowing water, new research suggests. While the briny flows may be too full of chlorine-based salts to support life, they do raise the odds that Mars could have life right now, the researchers said. [In Photos: Is Water Flowing on Mars?]
But just why is water such a crucial molecule for life? And could there be other ingredients that also provide the perfect recipe for life on other planets?
It turns out that several chemical properties of water make it indispensable for living creatures. Not only can water dissolve nearly anything, but it is also one of only a few materials that can exist as solid, liquid and gas within a relatively narrow range of temperatures.
At heart, all life on Earth uses a membrane that separates the organism from its environment. To stay alive, the organism takes in important materials for making energy, while shuttling out toxic substances such as waste products.
In this regard, water is essential simply because it's a liquid at Earth-like temperatures. Because it flows, water provides an efficient way to transfer substances from a cell to the cell's environment. By contrast, deriving energy from a solid is a much tougher prospect (though there are microbes that eat rock), Glazer said.
But the other part of the equation — that water can carry things into and out of the cell — has to do with water's unique chemical configuration.
The humble water molecule is made up of two hydrogen atoms bonded to an oxygen atom.
"The way they're bonded together makes water this wonderful universal solvent," meaning that almost every substance can dissolve in water, Glazer told Live Science.
That's because the molecule has polarity, meaning the hydrogen atoms tend to bunch on one side of the molecule, creating a positive region, while the oxygen end has a negative charge. The positive hydrogen end tends to attract negative ions (or atoms with an extra electron in the outer shell), while the negative region lures in positive ions (which have had one of their electrons stripped off).
Water, with its amazing dissolving properties, is the perfect medium for transmitting substances, such as phosphates or calcium ions, into and out of a cell.
Phases of water
Another feature of water is that it can act as a solid, liquid and gas within the range of temperatures that occur on Earth. Other molecules that have been identified as good candidates for supporting life tend to be liquid at temperatures or pressures that would be inhospitable for most known life-forms, Glazer said. [5 Mars Myths and Misconceptions]
"Water really is at that sweet spot," Glazer said.
The fact that water can be in all three phases in a relatively tight range of pressures creates many opportunities for life to flourish, he added.
"All three [states of water] available on our planet creates this really neat variety of habitats and microclimates," Glazer said.
For instance, frozen ice can be found in glaciers that carve through mountains, whereas water vapor helps warm the atmosphere, Glazer said.
Watery cradle of life
Water may be more than a fluid to help facilitate life's essential processes — it may also have been the protective cradle that carried the building blocks of life to Earth, said Ralf Kaiser, a physical experimental chemist at the University of Hawaii at Manoa, who has research experience in astrochemistry.
One theory for how life on Earth emerged, called panspermia, posits that icy comets smashed into Earth, bearing tiny organic molecules that formed the precursors to life. But traveling through space is a harsh journey, with punishing levels of radiation that would normally degrade those delicate molecules, Kaiser said.
However, in its solid form, water could have provided a way to shield those molecules from radiation, Kasier speculated.
"One possibility is that because the building blocks are frozen within the water, it has this protective mantle around it that could be delivered," Kaiser told Live Science.
Accept some substitutes
Of course, while water is crucial to life on our home planet, there could be life-forms that don't conform to the Earthling playbook.
Scientists are also looking at other liquids that could play a similar role as universal solvent and transport medium. Some of the top contenders are ammonia and methane, said Chris McKay, an astrobiologist at the NASA Ames Research Center in Moffett Field, California. Ammonia, like water, is a polar molecule that is relatively abundant in the universe, but scientists haven't found any large bodies of ammonia anywhere in the solar system, McKay said.
Methane isn't polar, but it can dissolve many other substances. Unlike water, however, methane becomes liquid only at very cold temperatures — at a frigid minus 296 degrees Fahrenheit (minus 182 degrees Celsius).
"We know that there are large lakes of liquid methane and ethane on Titan," one of the moons of Saturn, McKay told Live Science in an email. "Thus there is keen interest is the question of whether life can use liquid methane/ethane."
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Tia is the managing editor and was previously a senior writer for Live Science. Her work has appeared in Scientific American, Wired.com and other outlets. She holds a master's degree in bioengineering from the University of Washington, a graduate certificate in science writing from UC Santa Cruz and a bachelor's degree in mechanical engineering from the University of Texas at Austin. Tia was part of a team at the Milwaukee Journal Sentinel that published the Empty Cradles series on preterm births, which won multiple awards, including the 2012 Casey Medal for Meritorious Journalism.