THE RED EDGE: DETECTING
EXTRATERRESTRIAL PLANTS
Katharine Diehl
4/21/2011
SPECTROSCOPIC BIOSIGNATURES

Atmospheric biosignature:


H2O, O2, O3, CH4
Potential surface
biosignatures:


the red edge of Earth’s
vegetation
distinct bidirectional
reflectance distribution
function (BRDF) of trees

BRDF = how light is reflected
off opaque surfaces
WHAT IS THE RED EDGE?



A strong rise in
reflectivity around
700 nm
Characteristic of
land-based
chlorophyllproducing plants
Due to the strong
reflectivity/transmitt
ance of the leaf
structure and the
strong absorption of
chlorophyll and
Fig 1. Reflection spectrum of a
deciduous leaf
WHY DO PLANTS LOOK GREEN?
Fig 2.
Reflectance/Transmittance
(%) of a young aspen leaf
Green
Near- IR
USING THE RED EDGE ON EARTH
Analyze vegetation
type and age/health of
crops and forests
 Plants with high
chlorophyll and high
Leaf Area Index (LAI):
red edge shifts toward
longer wavelengths
 Plants with low
chlorophyll and low
LAI: red edge shifts
toward shorter
wavelengths

EXAMPLE: STUDY OF FOREST IN EAST ANGLIA
Fig 3. Scatter plot of vegetation species
using the Linear Interpolation Method to
calculated red edge position values
Fig 4. Scatter plot of vegetation based
on mean red edge position values for
the Linear method
WHY DOES THE RED EDGE EXIST?
Fig 5. Vegetation reflects/transmits
almost all incident radiation in this near
IR region
WHY DOES THE RED EDGE EXIST?

Two Theories
Plants would overheat (Thermal regulation argument of
Gates and Benedict)
 The large spaces in leaves aid in gas exchange and
increase absorption at photosynthetically active
wavelengths (Konrad and DeLucia)


No one knows for sure, but the first theory is
unlikely based on our beliefs about evolution
EARTH’S RED EDGE- FIRST DETECTION





Sagan and coworkers
Galileo spacecraft was
used during a fly-by of
Earth in 1990
Low-res spectra of
several different areas
of Earth
Found atmospheric
biosignature and the red
edge surface
biosignature
Vegetated areas:
increase in reflectance
by a factor of 2.5
between 670-760 nm
Galileo Orbiter
USING EARTHSHINE TO STUDY THE RED EDGE




Light
directly from
sun
Earthshine
Earthshine = sunlight
scattered by Earth toward
the Moon and then back to
Earth
Allows viewing of Earth’s
spatially integrated
spectrum
Woolf and coworkers
detected a red edge of 410% around 700 nm
After removal of
atmospheric absorption
bands of O2 and H2O
FACTORS AFFECTING DETECTION OF THE
RED EDGE BY A SPACE TELESCOPE

Factors







Atmospheric composition
Large vegetation-less areas
Cloud cover
Anisotropic scattering by the leaves
Soil characteristics
Resolution of spectral data available
One solution: Take a time series of data at different
spectral bands
ANALYZING EARTHSHINE TO DETECT THE
RED EDGE
Lots of
vegetation
Almost
entirely
ocean
Fig 6. Earthshine flux spectra from telescope in Feb 2002
CAN THE RED EDGE BE USED TO DETECT
LIFE ON EXTRASOLAR PLANETS?

Would light-harvesting
organisms on other planets
use photosynthesis as we
know it?


Likely that some form of it
would exist, but also likely
that it wouldn’t be
identical to what we know
Would such organisms on
other planets have similar
spectral properties to
vegetation?

Sharp spectral features
but at different
wavelengths than Earth
plants

Would we be able to see
the spectral features?
Aquatic microorganisms
and high opacity of H2O
 Other factors discussed
previously


If we saw the such spectral
features, how would we
know they came from
plants and not minerals

Next slide!
FALSE-POSITIVE MINERAL REFLECTANCE



Semiconductor crystals:
scatter photons with
insufficient energy to jump
band gap
Steep reflectance in visible
and near-IR
Shape and size of crystals
affect the semiconducting
properties  unlikely to
find large crystals on the
surface of a planet like
those in our solar system

Fig 7. Reflectance of
sulfur and cinnabar
FALSE-POSITIVE MINERAL REFLECTANCE
Difficult to identify
mineral with
reflectance
 Near and mid-IR
absorption features
 Atmospheric
composition (e.g.
presence of O2)
 The red edge
wavelength doesn’t
correspond to that of
any known mineral

Cinnabar (HgS)
CONCLUSIONS
Weak red edge
signature has been
identified in
Earthshine
measurements
 Time series spectra of
an exoplanet would
likely be necessary
 Looking for a “red
edge” at other
wavelengths

Could be difficult to
prove that plant life is
present with red edge
alone
 Evaluation of
reflectance data in
light of other spectral
data obtained
 Other methods of
detecting tree-like
vegetation

REFERENCES
BRDF
Paper
Good
Review
Doughty and Wolf. Astrobiology
(2010), 10 (9), 869-79.
 Fujii et al. The Astrophysical
Journal (2010), 715, 866-80
 Seager et al. Astrobiology (2005), 5
(3), 372-90.
 Shafri et al. Am. J. Appl. Sci.
(2006), 3 (6), 1864-71.
 Wikipedia:
(http://en.wikipedia.org/wiki/Main_P
age)
