From Lori Stiles, University of Arizona News Services
Astronomers have used "earthshine" to get a spectrum of our planet that will
be useful in their future searches for Earth-like planets around other
stars.
Nick Woolf of the University of Arizona Steward Observatory and his
colleagues are reporting the work today at the 199th national meeting of the
American Astronomical Society in Washington, D.C. Wes Traub and Ken Jucks of
the Harvard-Smithsonian Center for Astrophysics and Paul Smith of Steward
Observatory collaborated in the research.
"The phenomenon of ‘the old moon in the young moon’s arms’ is well known, "
Woolf said. "For a few days before or after the new moon, we see a thin
illuminated crescent of the moon, and a faint illumination of the entire
remaining disk of the moon. The fainter part of this illumination comes from
the Earth, with a substantial part of the day-lit Earth illuminating dawn or
dusk of the moon.
"As a result, it is possible to use the moon to integrate light from the
Earth and to determine what the spectrum of the Earth would be like if it
were seen from far away as a planet. We need this information to prepare to
observe Earth-like planets around other stars."
If astronomers didn’t have earthshine, they’d need a special spacecraft to
get the spectrum of our living planet, Woolf said. And while spacecraft are
good at getting detailed views of Earth from space, they don’t get a distant
overview, they "can’t see the forest for the trees," he said. "Even the
spectrum of Earth by the Galileo spacecraft, made by Carl Sagan and
colleagues in 1993, was just a small region of ocean. "
Philip Goode at Big Bear Solar Observatory at Big Bear Lake, Calif., has
measured the total amount of earthshine as an indicator of global change.
Eric Ford, Sara Seeger and Ed Turner at Princeton analyzed theoretically how
Earth’s radiation could expectably vary as the Earth revolved. And Wes Traub
and Ken Jucks at the Center for Astrophysics modeled the theoretical Earth
spectrum.
But, until recently, there have been no observations of the spectrum.
In June 2001, Woolf and Smith observed the earthshine spectrum using the
Steward Observatory 90-inch telescope at Kitt Peak, Ariz. A simulated image
of how the moon "saw" the Earth at the time is online at
< http://graucho.opi.arizona.edu/graphix/images/earthseenbymoon.jpg >
Woolf and Smith corrected the spectrum for lunar reflection, solar radiation
and an extra pass of light through Earth’s atmosphere by dividing the
earthshine spectrum by the spectrum of the sunlit moon they also observed
using Steward’s 90-inch Kitt Peak telescope. They collaborated with Traub
and Jucks in analyzing the data. The result showed how Earth’s reflectivity
varies with wavelength.
"A number of features were apparent from the observations," Woolf said.
"Although there was very little cloud cover at the time of observation, the
clouds contributed quite substantially to the observed radiation because
clouds are so bright, and sea is so dark," Woolf said. "We also saw the
absorption features produced by Earth’s atmosphere as they would have been
seen from space. We saw many features produced by water vapor, three
features produced by molecular hydrogen, and a feature produced by ozone in
the upper atmosphere. "
The astronomers have produced a graphic that shows the earthshine spectrum
and a line representing a simple theoretical model constructed to
approximately match the observations. It is online at
< http://graucho.opi.arizona.edu/graphix/images/shinespectrum.jpg >
"There were also broad features. We saw the blue of the sky, caused by the
molecules of air scattering sunlight. This measures the amount of atmosphere
on Earth. Also, there is an abrupt rise in the spectrum in the far red,
where vegetation is strongly reflective. This was quite surprising to us
because only 17 percent of the illuminated area of Earth was of land. Sea
vegetation is not able to produce this signature because seawater strongly
absorbs light of this color.
"An extraterrestrial observing Earth would have noticed that about 400 – 500
million years ago, vegetation took root on land. And since land vegetation
requires different parts — roots and leaves, for example — it would
indicate that life had taken hold strongly on our planet. Spectral features
of oxygen and ozone would indicate photosynthesis by living organisms,
further confirming the evidence. Any advanced intelligence that cared to
inquire would know that life has been present on Earth for a very long
time," Woolf said.
"The visible region of the spectrum for an extrasolar planet would be very
interesting to observe, and would provide different information than would
be obtained by measuring the heat-glow (infrared) spectrum of Earth."
The big difficulty of observing Earth is that it is so much fainter than the
sun and would appear so close to it in the sky.
Astronomers plan to overcome this difficulty in two possible ways. One is to
have a space mission search for Earth-like planets. The other is to search
using a supergiant telescope on the ground, a telescope about as large as a
giant radio telescope and equipped with a highly developed adaptive optics
system. Adaptive optics will sharpen images and separate the faint glow of a
planet from the brilliant light of its relatively close-by star.
Both NASA and the European Space Agency (ESA) propose space missions to look
for Earth-like planets in the infrared. NASA is developing the Terrestrial
Planet Finder (TPF) project, part of the Jet Propulsion Laboratory Navigator
Program, and ESA is developing its DARWIN project. The European Southern
Observatory is exploring the possibility of ground-based searches using the
future Overwhelmingly Large Telescope (OWL) project.
The work Woolf is reporting today is supported as part of the Terrestrial
Planet Finder project and by the NASA Institute for Advanced Projects.
