Biosignatures: Alien’s View of Earth ASTR 1420 Lecture : 18 Section: Not from the textbook.
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Transcript of Biosignatures: Alien’s View of Earth ASTR 1420 Lecture : 18 Section: Not from the textbook.
Biosignatures:Alien’s View of Earth
ASTR 1420
Lecture : 18
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 signs of life must be a global phenomenon!
Galileo’s view of Earth
• Galileo spacecraft (launched in 1989), arrived at Jupiter in 1995. • 1st orbiter of Jupiter.
Earth & Moon seen from Galileo8 days after its “departure” from Earth!
Earth seen from Voyager
“Pale Blue Dot” taken by Voyager 1 in 1990 from 4 billion miles away!
4 billion miles = 43 AU = 0.000680 lightyears
Can you find the Earth in this image?
Imagine that how difficult it will beto see (and resolve) planet far away!
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…..
disk-averaged spectrum of a planet can manifest in many different waysdue to weather, viewing angle, diurnal/seasonal changes, etc.
Let’s study how our Earth will be viewed from space…
AIRS scans Earth…~3million spectra/day at 3.75-15.4 micron with /~1200
AIRS: Atmospheric IR Sounder (NOAA) mission. : instantaneous footprint is a square of ~40km side.
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 V
ariations
Viewing Angle Differences
α Centaurian’s view of our world
α Centauri is the closest star to Earth : 1.34 pc = 4.37 Ly.
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 from beyond the snowline. o these volatiles (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
CO2
EARTH-CIRRUS
VENUSX 0.60
MARS
EARTH-OCEAN
H2O H2O
H2O ice
?
O3O2
Evolution of the Earth’s Atmospheric Composition
Prebiotic 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
F2 : 6900°K, G2: 5800°K, K2: 4900°K
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-type Stars
Earth
AD Leo planet
O3
CH4 CH4CH4
O2
O2
CO2
H2OH2O
H2O
H2O
Segura et al., Astrobiology, 2005.
AD Leo : M4.5V (3100°K), active flaring star 4.7 pc away.
CO2
CH3Cl
CH4
O3
+
N2O
H2O
Earth
AD Leo planet
Earth-like Planets Around M Stars
Segura et al., Astrobiology, 2005.
Take home message!
• Even for the same planet (with abundant life on the surface), detectable biosignatures depend on o viewing angleo temporal variations (diurnal, seasonal, long-term)o host star
Can we detect Biosignatures with TPF-C?
Simulated spectrum of Earth
O2
H2O H2OH2O
Can we detect Biosignatures with TPF-I?
Simulated spectrum of Earth
In summary…
Important Concepts• Disk-averaged spectrum!• Viewing Earth from the space• Recognizing biosignatures• Biosignatures are changing…
Important Terms• Biomarkers = biosignatures• Vegetation red-edge
Chapter/sections covered in this lecture : Not in the textbook