Astronomical%20Biosignatures:%20Detecting%20Life%20From%20Space

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Astronomical BiosignaturesDetecting Life From
Space

• V. Meadows

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Remote Detection of Life

• 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

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Characterizing Extrasolar Terrestrial Planets
Mass and Orbital Parameters Solar System
Environmental Characteristics parent star,
placement in solar system, preliminary orbit,
other planets Photometric Characteristics brightn
ess, color Spectra composition, physical
properties.
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A Habitable Planet

• A habitable planet is one that has conditions
  that can support life (in all its extremes).
• A planet that can maintain liquid water on its
  surface
• A Habitable Planet may not be inhabited.
• (but how likely is this?)
• So signs of habitability do not (yet!)
  automatically equate to signs of life

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Recognizing Habitable Planets

• Within the stars habitable zone
• distance from the parent star
• Terrestrial, rocky worlds
• Mass, brightness, color
• .with an atmosphere
• Photometric variability (clouds, possibly
  surface)
• Spectra that show CO2 and H2O vapor
• Spectra that show signs of a UV shield (e.g. O3).
  
• Surface conditions that support liquid water
• Observations of MIR brightness, spectral
  determination of atmospheric composition, esp.
  greenhouse gases

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The Distant Signs 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
• A planets atmosphere
• A planets surface
• A planets appearance
• over time
• Biosignatures always be identified in the context
  of the planetary environment
• e.g. Earth methane and Titan methane

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Atmospheric Biosignatures

• Oxygen
• A reduced gas in the presence of oxygen (e.g. O2
  and CH4)
• Any species that can be determined to be out of
  chemical equilibrium

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Surface Biosignatures
Crisp, Meadows
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Temporal Biosignatures

• Cyclical or seasonal behavior that is not due to
  photochemistry or other abiological source.
• On the Earth, although CH4 and CO2 both breathe
  with the seasons, the amplitudes are extremely
  small.

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Biosignatures in the Earths Visible Spectrum
Data Woolf, Traub and Jucks 2001 Model Tinetti
et al., 2004

• O2 (life) water (habitability) are relatively
  easy to detect.
• Surface biosignatures such as chlorophyll may
  also be detectable.

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Biosignatures in the Earths MIR Spectra
MGS-TES Christensen Pearl, 1997
VPL Earth Model Tinetti et al, 2004 60 cloud
cover
O3
CH4
CO2
H2O
H2O
The MIR is sensitive to atmospheric trace gases
which could indicate habitability or life.
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Terrestrial Planet Spectra Vary Widely in Solar
System
Terrestrial planets in our Solar System offer
diverse spectra which aid in their
characterization.
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Terrestrial Planets in the MIR
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Earth Through Time Biosignatures

• Life may have been easier to detect earlier in
  the Earths history.
• In the MIR, Mid-Proterozoic Earth-like
  atmospheres show strong signatures from both CH4
  and O3
• In the visible, the O2 absorption is reduced, but
  potentially detectable, but CH4 is less
  detectable for the mid-Proterozoic case.

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Earths Reflectivity Through Time
ARCHEAN PROTEROZOIC MODERN
Rayleigh Scattering
O2
CH4
H2O
CO2
CO2
CH4
H2O
H2O
CH4
H2O
O2
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Understanding Earth-like Planets Around Other
Stars

• An Earth-like planet around another star may
  have different spectral characteristics due to
  different photochemistry and atmospheric
  temperature structure.
• 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.).
  

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Earth-like Planets Around M Stars

• Molecular biosignatures may have longer
  atmospheric lifetimes for Earth-like planets
  around M stars, and the simultaneous presence of
  O2/O3 and CH4 may be easier to detect (Segura et
  al., 2005, in press).

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H2O
H2O
H2O
O2
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Earth in the MIR spectral resolution
Tinetti, Meadows, Crisp, Fong, Velusamy, Snively