1. The Red Edge: Detecting Extraterrestrial Plants
Katharine Diehl
4/21/2011

2. 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

3. What is the red edge? A strong rise in reflectivity around 700 nm
Characteristic of land-based chlorophyll- producing 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

4. Why do plants look green?
Fig 2. Reflectance/Transmittance (%) of a young aspen leaf
Green
Near- IR

5. 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

6. 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

7. Why does the red edge exist?
Fig 5. Vegetation reflects/transmits almost all incident radiation in this near IR region

8. 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

9. 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 nm
Galileo Orbiter

10. Using Earthshine to study the red edge
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 4- 10% around 700 nm
After removal of atmospheric absorption bands of O2 and H2O
Light directly from sun
Earthshine

11. Factors affecting detection of the red edge by a space telescope
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

12. Analyzing Earthshine to detect the red edge
Lots of vegetation
Almost entirely ocean
Fig 6. Earthshine flux spectra from telescope in Feb 2002

13. 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!

14. 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

15. 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)

16. 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

17. References Doughty and Wolf. Astrobiology (2010), 10 (9), 869-79.
BRDF Paper
Doughty and Wolf. Astrobiology (2010), 10 (9),
Fujii et al. The Astrophysical Journal (2010), 715,
Seager et al. Astrobiology (2005), 5 (3),
Shafri et al. Am. J. Appl. Sci. (2006), 3 (6),
Wikipedia: ( age)
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