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 We will not be able to “resolve” the extrasolar planet Everything we learn about the planet will be obtained from disk-averaged data. 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 Galileo spacecraft (launched in 1989), arrived at Jupiter in st orbiter of Jupiter. 1 st orbiter of Jupiter. Earth & Moon seen from Galileo 8 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 = lightyears = lightyears Can you find the Earth in this image? Imagine that how difficult it will be to see (and resolve) planet far away!

Remote Sensing the Sign of Life Astronomical Biosignatures are photometric, spectral, or temporal features indicative 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. These biosignatures must be global-scale to enable detection in a disk-averaged spectrum. Life can provide global-scale modification of: Life can provide global-scale modification of: o A planet’s atmosphere o A planet’s surface o A planet’s appearance over time Biosignatures always be identified in the context of the planetary environment Biosignatures always be identified in the context of the planetary environment o 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 ways due 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 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 Sahara desert Nile delta Nile delta Red sea Red sea high cloud high cloud

Effect of Clouds Clear Sky 100% cloudy Typical

Phase and Seasonal Variations 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! Oxygen, of course! Effect of life in the Earth Atmosphere is prominent! Effect of life in the Earth Atmosphere is prominent! Tim Lenton, Centre for Ecology and Hydrology

Origin of the Terrestrial Atmospheres Terrestrial planets did not capture their own atmospheres Terrestrial planets did not capture their own atmospheres o Too small and warm o Our atmospheres are considered “secondary” enriched with impact delivered volatiles from beyond the snowline. 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. 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! O2O2 Iron oxides CO 2 H2OH2O EARTH-CIRRUS VENUS X 0.60 MARS EARTH-OCEAN H2OH2O H2OH2O H 2 O ice ? O3O3 O2O2

Evolution of the Earth’s Atmospheric Composition Prebiotic Atmosphere > 3.5Gya Archean Atmosphere Gya Modern Atmosphere <2.3Gya Surface Pressure N 2 O 2 CO 2 CH 4 H 2 CO 1-10 bars 10-80% ~ % ppm ppm 1-2 bars 50-80% ~ % ppm 1 bar 78% 21% 0.036% 1.6ppm 0.5ppm ppm The Earth

The Archean Atmosphere Life arose by at least 3.5Gya Life arose by at least 3.5Gya o Evidence from microfossils and stromatolites. o Possible evidence for life at 3.8Gya from 13 C depletion The Earth was inhabited - but the atmosphere was anoxic (no O 2 ) prior to ~2.3 Gya The Earth was inhabited - but the atmosphere was anoxic (no O 2 ) prior to ~2.3 Gya Photosynthesis may have been started, but originally used H 2 S (or H 2 ) to reduce CO 2 Photosynthesis may have been started, but originally used H 2 S (or H 2 ) to reduce CO 2 o Not H 2 O based as today  no O 2 production in the early stage! Even oxygenic photosynthesis would not have immediately produced an O 2 - rich atmosphere. Even oxygenic photosynthesis would not have immediately produced an O 2 - rich atmosphere. o O 2 would have been consumed by atmospheric gases or surface materials.

O3O3 Earth at visible light at various time CH 4 H2OH2O H2OH2O CO 2 O2O2 CH 4 ARCHEAN PROTEROZOIC MODERN O2O2 CO 2 H2OH2O H2OH2O In the visible, the O 2 absorption is reduced, but potentially detectable, but CH 4 is less detectable for the mid- Proterozoic case.

Modern Earth 355ppm CO 2 Earth’s changing appearance at IR

Proterozoic 0.1PAL O 2 100ppm CH 4 15% decrease in ozone column depth Segura, Krelove, Kasting, Sommerlatt,Meadows,Crisp,Cohen Changing Biosignatures with time Mid-Proterozoic Earth-like atmospheres show strong signatures from both CH 4 and O 3

Archean N % 2000ppm CO ppm CH 4 100ppm H 2 Karecha, Kasting, Segura, Meadows, Crisp, Cohen Changing Biosignatures with time

O3O3 CO 2 CH 4 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… 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, ). O2O2 F2V G2V K2V O3O3 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 They are the most abundant type of stars in the Universe low mass (10-20% of Solar mass) low mass (10-20% of Solar mass) surface temperature of 2500 – 3000K surface temperature of 2500 – 3000K About 100,000 times more abundant About 100,000 times more abundant More active than Sun More active than Sun Segura et al., Astrobiology, 2005.

Earth-like Planets Around M-type Stars Earth AD Leo planet O3O3 CH 4 O2O2 O2O2 CO 2 H2OH2O H2OH2O H2OH2O H2OH2O Segura et al., Astrobiology, AD Leo : M4.5V (3100°K), active flaring star 4.7 pc away.

CO 2 CH 3 Cl CH 4 O 3 + N2ON2O H2OH2O 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 Even for the same planet (with abundant life on the surface), detectable biosignatures depend on o viewing angle o temporal variations (diurnal, seasonal, long-term) o host star

Can we detect Biosignatures with TPF-C? Simulated spectrum of Earth O2O2 H2OH2O H2OH2O 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