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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.
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
Biomarkers at Visible Wavelengths
O2
H2OO4
O3
Changes in disk-averaged reflectivities with phase are due to clouds
Dat
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~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
Tin
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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.
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
78
21
0.9
0.00001-4
0.036
Carbon Dioxide, CO2
Nitrogen, N2
Argon, Ar
Water Vapor, H2O
97
3
1.6 and 7x10-3
0.06 and 0.01
Earth – 1bar % Composition
Mars and Venus ~ 0.01 and 100 bars
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?
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
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