Dr. David Crisp Dr. David Crisp (Jet Propulsion Laboratory/California Institute of (Jet Propulsion Laboratory/California Institute of Technology Dr. Victoria Meadows Technology Dr. Victoria Meadows (California Institute of Technology) (California Institute of Technology)

Understanding the Remote-Sensing Signatures of Life in Disk-averaged

Planetary Spectra: 2

Rationale• Understanding the origin and evolution of

terrestrial planets, and their plausible diversity, will help inform the search and characterization of extrasolar terrestrial planets. – The emphasis is not only on understanding the likely

planetary environments, but • Understanding their appearance to astronomical

instrumentation• Understanding whether they are able to support life

– As we search for habitable worlds, superEarths• Are likely to be the first extrasolar terrestrial planets that are

characterized • represent a class of terrestrial planet that may also support

life

– And this could all happen in our lifetimes!!

?

Planetary Environmental Characteristics

• Is it a terrestrial planet? (Mass, brightness, color)• Is it in the Habitable Zone? (global energy balance?)

– Stellar Type - luminosity, spectrum

– Orbit radius, eccentricity, obliquity, rotation rate

• In general, moderate rotation rate, low obliquity and a near circular orbit stabilizes climate.

– Bolometric albedo – fraction of stellar flux absorbed

• Does it have an atmosphere?– Photometric variability (clouds, possibly surface)

– Greenhouse gases: CO2,H2O vapor present?

– UV shield (e.g. O3)?

– Surface pressure

– Clouds/aerosols

• What are its surface properties?– Presence of liquid water on the surface

• Surface pressure > 10 mbar, T> 273 K

– Land surface cover• Interior: What is the geothermal energy budget?

Exploring Terrestrial Planet Environments

• Modern Earth– Observational and ground-measurement data

• Planets in our Solar System– Astronomical and robotic in situ data

• The Evolution of Earth– Geological record, models

• Extrasolar Terrestrial Planets– Models, validation against Solar System

planets including Earth.

The Planet We Know and Love

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Habitability Markers and Biosignatures in the MIR

•CO2 – atmosphere, greenhouse gas, vertical T structure, secondary indicator of possible UV shield. •H2O•SO2, OCS, H2S –volcanism, lack of surface water

Selsis et al., 2002; Tinetti, et al., 2005.

Potential Biosignatures: O3,CH4, N2O,SO2, DMS, CH3Cl, NH3, H2S

The Earth at TPF Resolution

Biomarkers at Visible Wavelengths

O2

H2OO4

O3

Changes in disk-averaged reflectivities with phase are due to clouds

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The Photosynthetic Red Edge

Life Changes a Planet’s Surface

Harry LehtoHarry Lehto

~40%

Vegetation in the diurnal cycleVegetation in the diurnal cycle

Earth, clear sky case Earth with cloudsEarth, clear sky case Earth with clouds

NDVI 0.045

Tinetti et al., 2005c

NDVI at Dichotomy

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The red-edge could be potentially observed even on a cloudy planet using filters. - but the “red” edge may shift for different plants and star types!

Would need to be at significantly higher concentration than modern Earth

Biosignatures for Ocean Life

Tinetti et al., 2005b

Exploring Terrestrial Planet Environments

• Modern Earth– Observational and ground-measurement data

• Planets in our Solar System– Astronomical and robotic in situ data

• The Evolution of Earth– Geological record, models

• Extrasolar Terrestrial Planets– Models, validation against Solar System

planets including Earth.

Solar System Planets

Origin of the Terrestrial Atmospheres

• Terrestrial planets did not capture their own atmospheres– Too small and warm– Our atmospheres are considered “secondary”

• Instead, terrestrials were 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, probably started with a similar inventory of volatiles.

Terrestrial Planet Atmospheres

Nitrogen, N2

Oxygen, O2

Argon, Ar

Water Vapor, H2O

Carbon Dioxide, CO2

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Carbon Dioxide, CO2

Nitrogen, N2

Argon, Ar

Water Vapor, H2O

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1.6 and 7x10-3

0.06 and 0.01

Earth – 1bar % Composition

Mars and Venus ~ 0.01 and 100 bars

Thermal IR Spectra of Terrestrial Planets

crisp

Venus’ Climate History

• Although Venus and Earth are believed to have started with the same amount of volatiles, they followed very different evolutionary paths.

• The early Venus may have been habitable with water oceans– Evidence of loss of water seen in the present

day D/H ratio• This water was most probably lost to

space via a “runaway greenhouse effect”– Venus’ closer proximity to the Sun increased

the amount of water vapor in its atmosphere, which enhanced the greenhouse effect in a positive feedback loop

– The water vapor was photolyzed, and the H lost to space

– Over billions of years, Venus may have lost an ocean of water this way (lower limit is a global ocean several meters deep).

Mars’ Climate History• Mars may have had a much warmer

climate in its past– Geological evidence from erosion patterns

suggest that liquid water was stable on the surface. (picture)

• Warming was probably due to an enhanced greenhouse effect. – A CO2 atmosphere at 400 times present

density would work for the present Sun• Volcanism may have been a source of CO2

– However, the faint young Sun would require that Mars had an extra means of warming the surface.

• CH4 has been postulated as the missing greenhouse gas

• Source of CH4 for early Mars?

Modeling Solar System Planets

Solar System planets offer diverse spectra for characterization.

Solar System Planets at R~70

Earth

Venus

Titan

Neptune

H2O H2OCO2 CO2

H2O H2OH2O

CH4

IAUC200: Fortney and Marley, Tuesday, Session V

Temporal Variability- Seasonal Changes

Seasonal changes are visible in the disk-averaged spectra

- As either changes in intensity or spectral shape

The ice cap is most detectable for : 10-13.5m, due to wavelength dependent emissivity of CO2 ice.

Tinetti, Meadows, Crisp, Fong, Velusamy, Snively, Astrobiology, 2005

Modern Mars

Frozen Mars

Haze is thought toform from photolysis(and charged particleirradiation) of CH4

(Picture fromVoyager 2)

Titan’s Organic Haze Layer

Titan Anti-greenhouse Effect

Pavlov et al., JGR (2001)

Conclusions

• Our Solar System planets are a good starting point, but– terrestrial planets may be larger in the sample that

TPF finds. – terrestrial planets may exist in planetary systems very

unlike our own

• Modeling will be required to interpret the data returned from TPF-C, TPF-I and Darwin– To explore a wider diversity of planets than those in

our Solar System– To help interpret and constrain first order

characterization data