Biosignatures: Alien’s View of Earth

1. Biosignatures:Alien’s View of Earth ASTR 1420 Lecture : 18 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. Thesigns of life must be a global phenomenon!

4. Galileo’s view of Earth • Galileo spacecraft (launched in 1989), arrived at Jupiter in 1995. • 1st orbiter of Jupiter. Earth & Moon seen from Galileo 8 days after its “departure” from Earth!

5. 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 be to see (and resolve) planet far away!

6. 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

7. What a planet looks like from space depends on many things….. disk-averaged spectrum of a planet can manifest in many different ways due to weather, viewing angle, diurnal/seasonal changes, etc. Let’s study how our Earth will be viewed from space…

8. 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.

9. AIRS’ view of Earth

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

11. Effect of Clouds Typical Clear Sky 100% cloudy

12. Viewing Angle Differences Phase and Seasonal Variations

13. αCentaurian’s view of our world α Centauri is the closest star to Earth : 1.34 pc = 4.37 Ly.

14. Vegetation signature

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

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

17. 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 from beyond the snowline. • 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.

18. ? VENUS X 0.60 CO2 CO2 EARTH-CIRRUS O2 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!

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

20. 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.

21. 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

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

23. 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

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

25. 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 F2 : 6900°K, G2: 5800°K, K2: 4900°K

26. 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.

27. Earth AD Leo planet Earth-like Planets Around M-type Stars O3 AD Leo : M4.5V (3100°K), active flaring star 4.7 pc away. O2 CH4 CH4 O2 CO2 CH4 H2O H2O H2O H2O Segura et al., Astrobiology, 2005.

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

29. Take home message! • Even for the same planet (with abundant life on the surface), detectable biosignatures depend on • viewing angle • temporal variations (diurnal, seasonal, long-term) • host star

30. Can we detect Biosignatures with TPF-C? Simulated spectrum of Earth H2O H2O H2O O2

31. Can we detect Biosignatures with TPF-I? Simulated spectrum of Earth

32. In summary… Important Concepts Important Terms Biomarkers = biosignatures Vegetation red-edge • Disk-averaged spectrum! • Viewing Earth from the space • Recognizing biosignatures • Biosignatures are changing… • Chapter/sections covered in this lecture : Not in the textbook