Download - Biosignatures : Alien’s View of Earth
Biosignatures:Alien’s View of Earth
ASTR 1420
Lecture : 19
Section: Not from the textbook
= feature whose presence or abundance can be attributed to life
Biomarkers (=biosignatures)
Remote Detection of Life Sign
• We will not be able to “resolve” the extrasolar planet
• Everything we learn about the planet will be obtained from disk-averaged data.
• The sign of life must be a global phenomenon
Remote Sensing the Sign of Life
• Astronomical Biosignatures are photometric, spectral or temporal features indicative of life.
• These biosignatures must be global-scale to enable detection in a disk-averaged spectrum.
• Life can provide global-scale modification of:o A planet’s atmosphereo A planet’s surfaceo A planet’s appearance over time
• Biosignatures always be identified in the context of the planetary environmento e.g. Earth methane and Titan methane
What a planet looks like from space depends on many things…..
AIRS scans Earth…~3million spectra/day at 3.75-15.4 micron with /~1200
AIRS’ view of Earth
Effect of Landscape
• Sahara desert• Nile delta• Red sea• high cloud
Effect of Clouds
Clear Sky
100% cloudy
Typical
Phase and Seasonal Varia
tions
Viewing Angle Differences
α Centaurian’s view of our world
Vegetation signature
Surface Biosignatures : Vegetation “Red-Edge”
Vegetation Red-Edge
Atmospheric Biosignatures• Oxygen, of course!• Effect of life in the Earth Atmosphere is prominent!
Tim Lenton, Centre forEcology and Hydrology
Origin of the Terrestrial Atmospheres• Terrestrial planets did not capture their own atmosphereso Too small and warmo Our atmospheres are considered “secondary”
• enriched with impact delivered volatiles. o Water, methane, carbon dioxide and other gases were trapped in the
Earth’s interior rock
• Venus and Earth, forming relatively close together in the solar nebula, must have started with a similar inventory of volatiles.
Spectra of Terrestrial Planet in Solar System
Terrestrial planets in our Solar System offer diverse spectra that will be a set of nice references to exoplanet!
O2
Iron oxides
CO2
H2O H2O
CO2
EARTH-CIRRUS
VENUSX 0.60
MARS
EARTH-OCEAN
H2O H2O
H2O ice
?
O3O2
Evolution of the Earth’s Atmospheric CompositionPrebiotic Atmosphere> 3.5Gya
Archean Atmosphere4.0-2.3Gya
Modern Atmosphere<2.3Gya
Surface PressureN2
O2
CO2
CH4
H2
CO
1-10 bars10-80% ~030-90%10-100ppm100-1000ppm100-1000ppm
1-2 bars50-80%~010-20%1000-10000ppm
1 bar78%21%0.036%1.6ppm0.5ppm0.1-0.2ppm
The Earth
The Archean Atmosphere• Life arose by at least 3.5Gya o Evidence from microfossils and stromatolites.o Possible evidence for life at 3.8Gya from 13C depletion
• The Earth was inhabited - but the atmosphere was anoxic (no O2) prior to ~2.3 Gya
• Photosynthesis may have been started, but originally used H2S (or H2) to reduce CO2
o Not H2O based as today no O2 production in the early stage!
• Even oxygenic photosynthesis would not have immediately produced an O2-rich atmosphere. o O2 would have been consumed by atmospheric gases or surface materials.
O3
Earth at visible light at various time
CH4
H2O
H2O
CH4
CO2
O2
CH4
ARCHEANPROTEROZOICMODERN
O2
CO2
H2OH2O
In the visible, the O2 absorption is reduced, but potentially detectable, but CH4 is less detectable for the mid-Proterozoic case.
Modern Earth
355ppm CO2
Earth’s changing appearance at IR
Proterozoic
0.1PAL O2
100ppm CH4
15% decrease in ozone
column depth
Changing Biosignatures with time
Mid-Proterozoic Earth-like atmospheres show strong signatures from both CH4 and O3
Archean
N2 99.8%2000ppm CO2
1000ppm CH4
100ppm H2
Changing Biosignatures with time
O3
CO2
CH4
Understanding Earth-like Planets Around Other Stars• An Earth-like planet around another star may have different spectral
characteristics due to different incident Sun-light…o Synthetic spectra derived via a coupled climate-photochemical model for Earth-
like planets around stars of different spectral type (Segura et al., Astrobiology, 2003, 3, 689-708.).
O2
F2VG2VK2V
O3
Earth-like Planets around M-type Stars…• They are the most abundant type of stars in the Universe• low mass (10-20% of Solar mass)• surface temperature of 2500 – 3000K• About 100,000 times more abundant• More active than Sun
Segura et al., Astrobiology, 2005.
Earth-like Planets Around M Stars (e.g., AD Leo)
EarthAD Leo planet
O3
CH4 CH4CH4
O2
O2
CO2
H2OH2O
H2O
H2O
Segura et al., Astrobiology, 2005.
CO2
CH3Cl
CH4
O3
+
N2O
H2O
EarthAD Leo planet
Earth-like Planets Around M Stars
Segura et al., Astrobiology, 2005.
Can we detect Biosignatures with TPF-C?
Can we detect Biosignatures with TPF-I?
O2
H2O H2OH2O
Terrestrial Planet Finders
All canceled!
Terrestrial Planet Finder NASA
Darwin ESA
In summary…
Important Concepts• Viewing Earth from the space• Recognizable biosignatures of
Earth• Biosignatures are changing over
time!• In about a decade, we will be able
to characterize extrasolar terrestrial planets.
Important Terms• Biomarkers = biosignatures• Vegetation red-edge
Chapter/sections covered in this lecture : Not in the textbook