THE RED EDGE: DETECTING EXTRATERRESTRIAL PLANTSKatharine Diehl4/21/2011

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 chlorophyll-producing plants

Due to the strong reflectivity/transmittance of the leaf structure and the strong absorption of chlorophyll and Fig 1. Reflection spectrum of a

deciduous leaf

WHY DO PLANTS LOOK GREEN?

Near- IR

Fig 2. Reflectance/Transmittance (%) of a young aspen leaf

Green

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

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

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

Fig 6. Earthshine flux spectra from telescope in Feb 2002

Lots of vegetation

Almost entirely ocean

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 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_Page)

Good Review

BRDF Paper