The phenomenon that causes a diamond to sparkle could be
used to find large bodies of water on rocky, Earth-like planets, says Darren M.
Williams, lead author of a paper in Icarus that describes the process.
The trick, he says, is to look for planets when they are in
crescent phase, ideally in orbits that lie at an edge-on angle to Earth. In
that position, the glare bouncing off the water would make the planet seem
unusually bright.
"Crescent phase is where the starlight would be
glancing off the edge of the planet toward our telescopes," says Williams,
an associate professor of physics and astronomy at Penn State Erie. "That
would be when the light is coming at the surface at a very steep angle, and the
specular reflection would be the strongest and most intense."
Williams ran simulations of idealized, cloud-free planets
with three types of surfaces: unfrozen land, snow and ice, and water. His goal
was to see to what extent the presence of water would contribute to the light coming
from a planet in another solar system.
He found that for slightly tilted systems the reflection
from water would be the most powerful signal, particularly if the planet were
observed in crescent phase. When a star appears directly over a body of water,
almost all the light is absorbed. But from a glancing angle, most of the light
is reflected. The shape of the light curve would help distinguish Earth-like
planets with water from those without.
More than meets the eye
At the University of Florida, Dr. Eric Ford has also been
learning about how light can reveal the presence of ice, sand, vegetation and
water. Asymmetry in a planet's light curve over time could indicate major
seasonal transformations or weather patterns caused by the ebb and flow of
clouds.
Warmer, ice-free planets, for example, might experience a
large blooming
event of plants or oceanic algae, changing the way the planet reflected
light. Scientists could detect those changes by looking for certain signatures
in the planet's colors and measuring their intensities. While fluctuations
would signal some type of surface feature, scientists would need a lot more
follow-up before they could say for sure what was causing it.
"It could be water and land, it could be some other
liquids, particularly where it's further out where you might have methane, like on
Titan," says Ford. But, he explains, scientists could rule out many
alternatives using other methods.
"If we have an idea of how far away the planet is from
the star, and how bright the star is, then maybe we could say methane would not
be liquid at those temperatures," for example. Scientists could narrow the
possibilities by comparing their findings and might conclude the only logical
explanation is the presence of water, "particularly if there was
spectroscopic evidence of water vapor in the atmosphere," says Ford.
Experiments from
Interplanetary Space
Looking at Earth-sized planets beyond our own solar system
will require advanced new telescopes using modern technology to its full
potential. Two proposed efforts, NASA's Terrestrial Planet
Finder (TPF) and the European Space Agency's (ESA's) Darwin mission, will both
have instruments capable of picking up the sparkle of an ocean.
The way those instruments are designed and built will be
based, in part, on the results of what Williams sees of Earth from Mars and
Venus.
Williams is using observing time on ESA's Venus Express and
Mars Express orbiters, to look
at Earth from interplanetary space. From Mars, all of Earth's phases can be
seen, but from Venus, only Earth's gibbous (more than half-full) and full
phases are visible. Regardless, Williams can use the results to test his
predictions.
"That's the most exciting application from my
standpoint," he says, "the observation of extreme glint off the Earth's
oceans as seen from Mars
Express."
Looking at terrestrial planets while they are in crescent
phase, when only a small fraction of their disk is illuminated, will be
extremely difficult, perhaps taking two or even three weeks of observing time.
But, as Williams points out, of all the extremely difficult measurements
astronomers plan to do, it might be one of the easiest.
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