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Biosignatures : Alien’s View of Earth

Biosignatures : Alien’s View of Earth. ASTR 1420 Lecture : 19 Section: Not from the textbook. Biomarkers (=biosignatures). = feature whose presence or abundance can be attributed to life. Remote Detection of Life Sign. We will not be able to “resolve” the extrasolar planet

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Biosignatures : Alien’s View of Earth

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  1. Biosignatures:Alien’s View of Earth ASTR 1420 Lecture : 19 Section: Not from the textbook

  2. Biomarkers (=biosignatures) = feature whose presence or abundance can be attributed to life

  3. 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

  4. 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: A planet’s atmosphere A planet’s surface A planet’s appearance over time Biosignatures always be identified in the context of the planetary environment e.g. Earth methane and Titan methane Remote Sensing the Sign of Life

  5. What a planet looks like from space depends on many things…..

  6. AIRS scans Earth… ~3million spectra/day at 3.75-15.4 micron with /~1200

  7. AIRS’ view of Earth

  8. Effect of Landscape • Sahara desert • Nile delta • Red sea • high cloud

  9. Effect of Clouds Typical Clear Sky 100% cloudy

  10. Viewing Angle Differences Phase and Seasonal Variations

  11. αCentaurian’s view of our world

  12. Vegetation signature

  13. Surface Biosignatures : Vegetation “Red-Edge” Vegetation Red-Edge

  14. Tim Lenton, Centre for Ecology and Hydrology Atmospheric Biosignatures • Oxygen, of course! • Effect of life in the Earth Atmosphere is prominent!

  15. Origin of the Terrestrial Atmospheres • Terrestrial planets did not capture their own atmospheres • Too small and warm • Our atmospheres are considered “secondary” • enriched with impact delivered 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.

  16. ? VENUS X 0.60 CO2 CO2 EARTH-CIRRUS O2 H2O H2O O2 O3 H2O H2O MARS Iron oxides H2O ice EARTH-OCEAN 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!

  17. Evolution of the Earth’s Atmospheric Composition The Earth

  18. The Archean Atmosphere • Life arose by at least 3.5Gya • Evidence from microfossils and stromatolites. • 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 • Not H2O based as today  no O2 production in the early stage! • Even oxygenic photosynthesis would not have immediately produced an O2-rich atmosphere. • O2 would have been consumed by atmospheric gases or surface materials.

  19. O3 Earth at visible light at various time ARCHEAN PROTEROZOIC MODERN In the visible, the O2 absorption is reduced, but potentially detectable, but CH4 is less detectable for the mid-Proterozoic case. O2 CH4 H2O CO2 CO2 CH4 H2O H2O CH4 H2O O2

  20. Earth’s changing appearance at IR Modern Earth 355ppm CO2

  21. Changing Biosignatures with time Proterozoic 0.1PAL O2 100ppm CH4 15% decrease in ozone column depth Mid-Proterozoic Earth-like atmospheres show strong signatures from both CH4 and O3 Segura, Krelove, Kasting, Sommerlatt,Meadows,Crisp,Cohen

  22. Changing Biosignatures with time Archean N2 99.8% 2000ppm CO2 1000ppm CH4 100ppm H2 Karecha, Kasting, Segura, Meadows, Crisp, Cohen

  23. F2V G2V K2V O3 O3 CO2 O2 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… • 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.). CH4

  24. 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.

  25. Earth AD Leo planet Earth-like Planets Around M Stars (e.g., AD Leo) O3 O2 CH4 CH4 O2 CO2 CH4 H2O H2O H2O H2O Segura et al., Astrobiology, 2005.

  26. Earth N2O AD Leo planet CH4 O3 + CH3Cl H2O CO2 Earth-like Planets Around M Stars Segura et al., Astrobiology, 2005.

  27. Can we detect Biosignatures with TPF-C?

  28. Can we detect Biosignatures with TPF-I? H2O H2O H2O O2

  29. Terrestrial Planet Finders Terrestrial Planet Finder NASA All canceled! Darwin ESA

  30. In summary… Important Concepts Important Terms Biomarkers = biosignatures Vegetation red-edge • 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. • Chapter/sections covered in this lecture : Not in the textbook

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