As
NASA missions explore our solar system and search for new worlds, they are
finding water in surprising places. Water is but one piece of our search for
habitable planets and life beyond Earth, yet it links many seemingly unrelated
worlds in surprising ways.
"NASA science activities have provided a
wave of amazing findings related to water in recent years that inspire us to
continue investigating our origins and the fascinating possibilities for other worlds,
and life, in the universe," said Ellen Stofan, chief scientist for the
agency. "In our lifetime, we may very well finally answer whether we are
alone in the solar system and beyond."
The
chemical elements in water, hydrogen and oxygen, are some of the most abundant
elements in the universe. Astronomers see the signature of water in giant
molecular clouds between the stars, in disks of material that represent newborn
planetary systems, and in the atmospheres of giant planets orbiting other stars.
There
are several worlds thought to possess liquid water beneath their surfaces, and
many more that have water in the form of ice or vapor. Water is found in
primitive bodies like comets and asteroids, and dwarf planets like Ceres. The
atmospheres and interiors of the four giant planets -- Jupiter, Saturn, Uranus
and Neptune -- are thought to contain enormous quantities of the wet stuff, and
their moons and rings have substantial water ice.
Perhaps
the most surprising water worlds are the five icy moons of Jupiter and Saturn
that show strong evidence of oceans beneath their surfaces: Ganymede, Europa
and Callisto at Jupiter, and Enceladus and Titan at Saturn.
Scientists
using NASA's Hubble Space Telescope recently provided powerful evidence that
Ganymede has a saltwater, sub-surface ocean, likely sandwiched between two
layers of ice.
Europa
and Enceladus are thought to have an ocean of liquid water beneath their
surface in contact with mineral-rich rock, and may have the three ingredients
needed for life as we know it: liquid water, essential chemical elements for
biological processes, and sources of energy that could be used by living
things. NASA's Cassini mission has revealed Enceladus as an active world of icy
geysers. Recent research suggests it may have hydrothermal activity on its
ocean floor, an environment potentially suitable for living organisms.
NASA
spacecraft have also found signs of water in permanently shadowed craters on
Mercury and our moon, which hold a record of icy impacts across the ages like
cryogenic keepsakes.
While
our solar system may seem drenched in some places, others seem to have lost
large amounts of water.
On
Mars, NASA spacecraft have found clear evidence that the Red Planet had water
on its surface for long periods in the distant past. NASA's Curiosity Mars
Rover discovered an ancient streambed that existed amidst conditions favorable
for life as we know it.
More
recently, NASA scientists using ground-based telescopes were able to estimate
the amount of water Mars has lost over the eons. They concluded the planet once
had enough liquid water to form an ocean occupying almost half of Mars'
northern hemisphere, in some regions reaching depths greater than a mile (1.6
kilometers). But where did the water go?
It's
clear some of it is in the Martian polar ice caps and below the surface. We
also think much of Mars' early atmosphere was stripped away by the wind of
charged particles that streams from the sun, causing the planet to dry out.
NASA's MAVEN mission is hard at work following this lead from its orbit around
Mars.
The
story of how Mars dried out is intimately connected to how the Red Planet's
atmosphere interacts with the solar wind. Data from the agency's solar missions
-- including STEREO, Solar Dynamics Observatory and the planned Solar Probe
Plus -- are vital to helping us better understand what happened.
Understanding
the distribution of water in our solar system tells us a great deal about how
the planets, moons, comets and other bodies formed 4.5 billion years ago from
the disk of gas and dust that surrounded our sun. The space closer to the sun
was hotter and drier than the space farther from the sun, which was cold enough
for water to condense. The dividing line, called the "frost line,"
sat around Jupiter's present-day orbit. Even today, this is the approximate
distance from the sun at which the ice on most comets begins to melt and become
"active." Their brilliant spray releases water ice, vapor, dust and
other chemicals, which are thought to form the bedrock of most worlds of the
frigid outer solar system.
Scientists
think it was too hot in the solar system's early days for water to condense
into liquid or ice on the inner planets, so it had to be delivered -- possibly
by comets and water-bearing asteroids. NASA's Dawn mission is currently
studying Ceres, which is the largest body in the asteroid belt between Mars and
Jupiter. Researchers think Ceres might have a water-rich composition similar to
some of the bodies that brought water to the three rocky, inner planets,
including Earth.
The
amount of water in the giant planet Jupiter holds a critical missing piece to
the puzzle of our solar system's formation. Jupiter was likely the first planet
to form, and it contains most of the material that wasn't incorporated into the
sun. The leading theories about its formation rest on the amount of water the
planet soaked up. To help solve this mystery, NASA's Juno mission will measure
this important quantity beginning in mid-2016.
Looking
further afield, observing other planetary systems as they form is like getting
a glimpse of our own solar system's baby pictures, and water is a big part of
that story. For example, NASA's Spitzer Space Telescope has observed signs of a
hail of water-rich comets raining down on a young solar system, much like the
bombardment planets in our solar system endured in their youth.
With
the study of exoplanets -- planets that orbit other stars -- we are closer than
ever to finding out if other water-rich worlds like ours exist. In fact, our
basic concept of what makes planets suitable for life is closely tied to water:
Every star has a habitable zone, or a range of distances around it in which
temperatures are neither too hot nor too cold for liquid water to exist. NASA's
planet-hunting Kepler mission was designed with this in mind. Kepler looks for
planets in the habitable zone around many types of stars.
Recently
verifying its thousandth exoplanet, Kepler data confirm that the most common
planet sizes are worlds just slightly larger than Earth. Astronomers think many
of those worlds could be entirely covered by deep oceans. Kepler's successor,
K2, continues to watch for dips in starlight to uncover new worlds.
The
agency's upcoming TESS mission will search nearby, bright stars in the solar
neighborhood for Earth- and super-Earth-sized exoplanets. Some of the planets
TESS discovers may have water, and NASA's next great space observatory, the
James Webb Space Telescope, will examine the atmospheres of those special
worlds in great detail.
It's easy to forget that the story of
Earth's water, from gentle rains to raging rivers, is intimately connected to
the larger story of our solar system and beyond. But our water came from
somewhere -- every world in our solar system got its water from the same shared
source. So it's worth considering that the next glass of water you drink could
easily have been part of a comet, or an ocean moon, or a long-vanished sea on
the surface of Mars. And note that the night sky may be full of exoplanets
formed by similar processes to our home world, where gentle waves wash against
the shores of alien seas.
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