Sky & Telescope - March 2023 - 27

INFRARED TREE: SENG P. MERRILL / WIKIMEDIA COMMONS / CC BY SA;
PHYTOPLANKTON BLOOM: NASA / JSC
space. As a result, two different satellite systems will typically
differ by 0.7 percent in the monthly average albedo. That may
not sound like much, but it's worth about 1.5 degrees in the
global temperature, a non-negligible fraction of the expected
greenhouse warming. Satellites are also expensive - typically
costing hundreds of millions of dollars - and they can break . . .
Koonin noted that, in addition to being very inexpensive,
ground-based earthshine measurements are self-calibrating
and automatically integrate the reflectivity of a large portion
of the planet. Earthshine data also promised to be very valuable
for precisely calibrating satellite instruments.
In 1998 a team of investigators led by Philip Goode of the
New Jersey Institute of Technology began to regularly monitor
the brightness of earthshine at the Big Bear Solar Observatory
in California. The NASA-funded Project Earthshine
acquired almost 1,500 nights of usable data by the end of
2017. As noted by Goode and his collaborators in an article
appearing in the May 2004 edition of Science, these measurements
compare favorably with simultaneous satellite observations
of global cloud cover from the International Satellite
Cloud Climatology Project and indicate that " Earth's reflectance
as measured from earthshine is noticeably variable on
monthly, yearly, and decadal scales, with a net over sixteen
years that is essentially nil. "
The three most recent years of data indicate a very modest
decline in global albedo that may be related to a decrease in
bright, low-lying clouds over the warming waters along the
Pacific coast of North and South America. No correlation
was found between changes in terrestrial albedo and sunspot
number, cosmic ray flux, or other solar-activity indices.
In addition to its usefulness in investigating climate
change, earthshine points to a method of detecting life on
distant worlds. Chlorophylls, the pigments that power photosynthesis,
efficiently absorb light over much of the visible
region of the spectrum but are highly reflective in near-infrared
light. This abrupt increase in reflectivity at a wavelength
of 720 nanometers produces a characteristic spectral signature
known as the " red edge. "
Phytoplankton in the oceans do not produce this signature
because seawater is a strong absorber of red and nearinfrared
light. However, they do show a peak in reflectivity
at 550 nanometers that gives phytoplankton-rich waters a
distinctive greenish hue.
In 2001 both the " red edge " of vegetation-covered land and
the green fingerprint of ocean phytoplankton was detected
in low-resolution spectra of earthshine obtained with the
2.3-meter (90-inch) Bok Telescope on Kitt Peak by University
of Arizona astronomers Neville Woolf and Paul Smith. They
noted that an alien observer would have seen the green phytoplankton
signal appear in the spectrum of Earth about two
billion years ago. The red edge feature would only have been
present after extensive plant cover on land developed about
440 million years ago. Astrobiologists are keen to look for
these biosignatures in the spectra of exoplanets.
SEEING THE INVISIBLE The " red edge " spectral signature of chlorophylls
causes foliage to appear brilliant white in this photograph of
a cypress tree and grasslands taken through a near-infrared filter that
blocks visible light.
LIFE FROM SPACE Fed by nutrient-rich urban and agricultural runoff,
a bloom of phytoplankton appears as bright swirls in this satellite image
showing the waters off the Russian Black Sea port of Novorossiysk in
the spring of 2013.
With the Moon serving as a distant mirror, earthshine
provides a cosmic perspective. " Even the spectrum of Earth by
the Galileo spacecraft, made by Carl Sagan and colleagues in
1993, was just a small region of ocean, " Woolf noted in the
online magazine Wiley Analytical Science. " 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. "
¢ Contributing Editors TOM DOBBINS and BILL SHEEHAN
frequently write about the history of observational
astronomy. Together they authored the widely acclaimed
2001 book, Epic Moon: A History of Lunar Exploration
in the Age of the Telescope.
sk yand tele scope .o r g * MARCH 2023 27
Sky & Telescope - March 2023

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