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Earth:
The Lone Pale Blue Dot?
11.02.06
A
recent photo from the Cassini spacecraft shows the mighty planet
Saturn, and if you look very closely between its wing-like rings,
a faint pinprick of light. That tiny dot is Earth bustling with
life as we know it. The image is the second ever taken of our
world from deep space. The first, captured by the Voyager
spacecraft in 1990, stunned many people, including the famous
astronomer Carl Sagan who called our seemingly minuscule planet a
"pale blue dot" and "the only home we've ever
known."
Hi-Res
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Earth,
seen as a pale blue dot from Saturn. Image Credit:
NASA/JPL/Space Science Institute
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Last September, a portion
of Cassini's picture showing Earth was unveiled to an auditorium
full of scientists attending the third Pale Blue Dot workshop at
the Adler Planetarium in Chicago. The workshop brought together
scientists from across the country and beyond to talk about how
to find life elsewhere in our universe, a central theme in the
interdisciplinary field of astrobiology. When the workshop
participants were presented with the picture, they spontaneously
began to clap. One of their fundamental goals is to capture a
portrait like Cassini's showing another "pale blue dot"
like Earth in a planetary system beyond our own.
If
scientists do ever acquire such a photo, how will they figure out
whether anybody is home? Workshop participant Dr. Wes Traub of
NASA's Jet Propulsion Laboratory in Pasadena says a photo of a
light-blue extrasolar planet - a planet outside our solar system
- might indicate the planet has an atmosphere, a tantalizing sign
that it could support life. "In our solar system, we are the
only small light-blue planet," he said. Earth appears blue
because its atmosphere scatters blue light around, literally
filling the sky with the color blue.
But colors are only
clues. A planet with an atmosphere might be blue, or not. Mars
has an atmosphere, albeit a thin one, and it's red. The blue
light in the Martian skies is soaked up by iron-containing
molecules on the surface that radiate red.
Ultimately,
astrobiologists will need additional data to determine whether an
extrasolar planet is habitable, and if so, whether it harbors
life. Future planet-finding missions like NASA's Terrestrial
Planet Finder are expected to return both valuable "spectral"
data and imagery of planets looking small enough to fit on a pin.
Spectra are surveys of an object's rainbow-like array of
different wavelengths of light. They reveal the presence of
specific molecules.
To determine if a planet is livable,
scientists will look for carbon dioxide and water vapor,
signposts that a planet has an atmosphere and oceans,
respectively. Atmospheres not only provide air to breathe, but
also act like blankets to keep a planet warm and help buffer
potential residents from damaging ultraviolet and cosmic rays.
Oceans help regulate a planet's temperature and provide liquid
water, an essential ingredient for life on Earth.
Other
molecules enveloping a planet, such as oxygen, ozone and methane,
can suggest that life itself has taken root. On Earth, oxygen is
"breathed out" by plants, and methane by
micro-creatures living in swamps and animals. These chemicals
don't stick around for long on their own, so if they are hanging
around an extrasolar planet, then something must be pumping the
stuff out. That something could be life, but this isn't always
the case. Saturn's moon's Titan is shrouded in an atmosphere
containing lots of methane not produced by life.
A
view of Earth from Voyager 1, at a distance of more than 4
billion miles. Earth is the dot in the middle of the bright
streak. Image credit: NASA/JPL
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Scientists say that oxygen
is a more reliable sign of life than methane, but if they found
large quantities of both, they'd be more convinced. "Finding
two of these molecules together would be much better than one.
The more, the better," said Dr. Victoria Meadows of NASA's
Spitzer Science Center, Pasadena, who served as chair of the
third Pale Blue Dot conference. "For example, if we found
carbon dioxide, oxygen and water vapor, in addition to methane,
then we'd be pretty convinced that we were looking at an
environment like our own."
Not all molecules are a
good sign. For example, abundant amounts of sulfur dioxide
suggest a dead, dry planet. This chemical would dissolve into a
planet's ocean if it had one, so its presence means it is
unlikely there's much water around. Venus is one such parched
planet, with a thick atmosphere containing sulfur and carbon
dioxide.
There are still other signs of life that
astrobiologists will seek out. At the workshop, scientists talked
about looking for pigments from planets coated in slime, as the
young Earth might have been, and the systematic search for
technological signs of civilizations. They discussed observing
global changes in a planet over time - a mark of weather,
continents and possibly life. And they talked about the colors of
vegetation, especially the infrared light that plants shine in
brilliantly.
When it comes to exotic planets, scientists
say that we should be prepared for the unexpected. "One of
the messages to come out of the workshop is that planets are a
complicated result of what they are made of, and their histories.
Planets are potentially as diverse as people," said Meadows.
"In only the last few years, we've found evidence that
extrasolar planets may not be anything like those in our solar
system. That's what's so exciting. Not just the possibility of
finding another planet like the Earth, but the chance to also
look for truly alien worlds."
Source
/ Credit: NASA / JPL
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