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| These dark, narrow, 100 meter-long streaks (called recurring slope lineae) are flowing downhill on Mars, and are inferred to have been formed by contemporary flowing water. Recently, planetary scientists detected hydrated salts on these slopes at Hale crater, corroborating their original hypothesis that the streaks are indeed formed by liquid water. The blue color seen upslope of the dark streaks are thought not to be related to their formation, but instead are from the presence of the mineral pyroxene. |
Why
Is Water So Essential for Life?
By Tia
Ghose - Assistant Managing
Editor
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.
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.
Flowing life
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.
"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."
Follow Tia Ghose on Twitterand Google+. Follow Live Science @livescience, Facebook & Google+. Original article on Live Science.
Tia
Ghose
Assistant
Managing Editor
Tia has
interned at Science News, Wired.com, and the Milwaukee Journal Sentinel and has
written for the Center for Investigative Reporting, Scientific American, and
ScienceNow. She has a master's degree in bioengineering from the University of
Washington and a graduate certificate in science writing from the University of
California Santa Cruz.
