Astronomical Detection of Biosignatures From Extrasolar Planets

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Understanding the Remote-Sensing Signatures of
Life in Disk-averaged Planetary Spectra 2
Dr. David Crisp (Jet Propulsion
Laboratory/California Institute of Technology Dr.
Victoria Meadows (California Institute of
Technology)
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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!!

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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 gt 10 mbar, Tgt 273 K
• Land surface cover
• Interior What is the geothermal energy budget?

?
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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.

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The Planet We Know and Love
G. Chin GSFC
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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

Potential Biosignatures O3,CH4, N2O,SO2, DMS,
CH3Cl, NH3, H2S
Selsis et al., 2002 Tinetti, et al., 2005.
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The Earth at TPF Resolution
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Biomarkers at Visible Wavelengths
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The Photosynthetic Red Edge
Life Changes a Planets Surface
Harry Lehto
Harry Lehto
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Vegetation in the diurnal cycle
Earth,
clear sky case Earth
with clouds
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NDVI 0.045
Tinetti et al., 2005c
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NDVI at Dichotomy
Tinetti et al., submitted, 2005
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!
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Biosignatures for Ocean Life
Tinetti et al., 2005b
Would need to be at significantly higher
concentration than modern Earth
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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.

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• Solar System Planets

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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 Earths interior rock
• Venus and Earth, forming relatively close
  together in the solar nebula, probably started
  with a similar inventory of volatiles.

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Terrestrial Planet Atmospheres
Earth 1bar
Composition
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
Mars and Venus 0.01 and 100 bars
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Thermal IR Spectra of Terrestrial Planets
crisp
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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).

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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?

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Modeling Solar System Planets
Solar System planets offer diverse spectra for
characterization.
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Solar System Planets at R70
IAUC200 Fortney and Marley, Tuesday, Session V
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Temporal Variability- Seasonal Changes
Seasonal changes are visible in the disk-averaged
spectra - As either changes in intensity or
spectral shape
Modern Mars
Frozen Mars
The ice cap is most detectable for ? 10-13.5?m,
due to wavelength dependent emissivity of CO2
ice.
Tinetti, Meadows, Crisp, Fong, Velusamy, Snively,
Astrobiology, 2005
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Titans Organic Haze Layer
Haze is thought to form from photolysis (and
charged particle irradiation) of CH4
(Picture from Voyager 2)
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Titan Anti-greenhouse Effect
Pavlov et al., JGR (2001)
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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