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