Spectroscopic Signatures of Terrestrial Exoplanets

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Spectroscopic Signatures of Terrestrial Exoplanets

• Adam Kraus

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• What are the expected spectral features of
  terrestrial exoplanets?
• What can we learn from them about local
  conditions and chemistry?
• What does this imply about design options for
  future planet-search missions?

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Signature Vibrational Transitions

• Not all molecules (N2, O2) are IR active
• Those of interest
• H2O (Asymmetric Top)
• CH4 (Symmetric Top)
• N2O (Asymmetric Linear)
• CO2 (Symmetric Linear)
• Absorption strength gt Abundances
• Resolution of lines gt P, T
• Presence gt Nonequilibrium chemistry

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Infrared Spectra
http//www.atmos.umd.edu/owen/CHPI/IMAGES/transir
.html
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Signature Photodissociation

• O3 is produced in the upper atmosphere
• O O2 gt O3
• thus it is a tracer gas.
• O3 photodissociates in two bands
• Chappuis band 440-760 nm
• Huggins-Hartley band 200-360
• Shortward of 300 nm, photodissociation of other
  molecules absorbs all light

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Optical Absorption Spectrum
http//daac.gsfc.nasa.gov/CAMPAIGN_DOCS/OCDST/ozon
e.html
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Signature Broadband Colors

• Terrain types like ocean, vegetation, and soil
  have distinctive albedos and colors
• Ocean Albedo 5, blue color
• Soil Albedo 20, red color
• Vegetation Albedo 20, green color
• On a planet with differentiated terrain, rotation
  will lead to variability in both color and
  reflected luminosity

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Optical Reflection Spectra
http//www.geografforlaget.dk/course/ENGLISH/basic
/tele1.htm
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TPF Terrestrial Planet Finder

• Goal Directly observe terrestrial exoplanets
• Scheduled to launch in mid-2010s
• Two technology choices
• Optical coronagraph
• Infrared interferometer

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Technology Choice Coronagraph
http//spaceweather.com/glossary/coronagraph.html
http//planetquest.jpl.nasa.gov/TPF/tpf_sample.cfm
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Technology Choice Interferometer
http//www2.keck.hawaii.edu/geninfo/about.html
http//www.svsinc.com/TPF.html
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Comparative Benefits

• Coronagraph
• Broadband colors (Geography, vegetation)
• Optical/NIR spectra (O3, H2O)
• Interferometer
• NIR/MIR spectra (H2O, CO2, N2O, CH4)
• Decision pending the outcome of precursor
  projects like Kepler and SIM.