Polarization as a Tool for Remote Sensing of Planetary Atmospheres

1
Polarization as a Tool for Remote Sensing of
Planetary Atmospheres
Vijay Natraj (Caltech) EGU General Assembly,
Vienna, Austria April 22, 2009
2
Outline

• Introduction to Polarization
• Rainbows
• Applications
• Venus Clouds
• Earth Tropospheric Ozone
• Circular Polarization
• Conclusions

3
Introduction to Polarization

• Light is a transverse wave
• Amplitude and phase of electric field determine
  polarization

4
Stokes Parameters

• Polarization state represented by 4 parameters
• Called Stokes parameters (Stokes, 1852)
• Represent intensity, linear and circular
  polarization

5
Ray Paths for Spherical Particles
Hansen and Travis 1974
6
Scattering by Spherical Particles
Hansen 1971
Rainbows, glory and supernumerary bows
characteristic of spherical particles
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Effect of Droplet Size on Rainbow Scattering
Bailey 2007
8
Rainbow Scattering for Different Liquids
Bailey 2007
9
Effect of Particle Shape on Rainbow Scattering
Bailey 2007
10
Venus Clouds

• Very little known about composition of clouds
  till late 60s
• Measurements of spectral reflectivity
  insufficient to identify composition
• Gaseous absorber or lower cloud layer could
  provide observed absorption
• Horak 1950 Rayleigh scattering could not
  account for observations
• Arking and Potter 1968 Angular distribution of
  reflected light gives refractive index that is
  too wide in range
• Hansen 1971 Polarization observations more
  sensitive to cloud particle characteristics

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Venus Clouds

• Hansen and Hovenier 1974
• Used ground-based measurements at 365 nm, 550 nm
  and 990 nm
• Refractive index of cloud particles found to be
  1.440.015 at 0.55 µm
• Spherical particles with effective radius 1.05 µm
  and effective variance 0.07
• Cloud top 50 mbar
• Composition of cloud particles probably
  concentrated sulfuric acid solution
• Travis et al. 1979
• Pioneer Venus Orbital Cloud Photopolarimeter
  measurements at 270 nm, 365 nm, 550 nm, 935 nm
• Results mostly consistent with sulfuric acid
  cloud particles

12
Venus Clouds

• Kawabata et al. 1980
• Haze top at 5 mbar
• Top of main cloud layer at 40 mbar
• Haze optical thickness 0.25 at 935 nm and 0.83 at
  365 nm
• Haze optical thickness larger in polar regions
  than near equator

13
Earth Tropospheric Ozone

• Ozone cycle primarily driven by interaction of
  ultraviolet (UV) radiation with oxygen and ozone
  in the stratosphere
• Important ozone formation processes take place in
  the troposphere
• Fishman et al. 1990 enhanced tropospheric
  ozone over Indonesia during biomass burning
  season
• Koelemeijer and Stammes 1999
• Clouds affect ozone retrieval
• Enhance reflectivity compared to clear sky gt
  scattering altitude changed
• Screen tropospheric ozone below
• Multiple scattering inside clouds enhances
  optical path length
• Clouds change air mass factor by changing path
  length of light in atmosphere

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Earth Tropospheric Ozone

• Jiang et al. 2004
• Tropospheric column ozone 10 stratospheric
  column
• Signal in intensity of radiation from troposphere
  overwhelmed by that from stratosphere
• Change in polarization due to ozone change 10X
  larger for troposphere

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Earth Tropospheric Ozone

• Less ozone gt more scattering gt polarization
  smoothed due to multiple scattering
• Concentration of scatterers high in troposphere
• Concentration of scatterers low in stratosphere
  gt single scattering dominates
• Change in aerosol/cloud and tropospheric ozone
  have opposite effects on linear polarization
• Change in linear polarization due to
  aerosol/cloud has weak wavelength dependence
• Strong wavelength dependence of linear
  polarization change due to tropospheric ozone

16
Circular Polarization

• Kolokolova and Sparks 2007
• Light in optical continuum of cometary spectra
  circularly polarized
• Arises due to asymmetry in scattering medium
• Multiple scattering in anisotropic medium
• Scattering by aligned non-spherical particles
• Scattering by chiral particles
• Evidence of presence of complex organics
• Chirality is a property of organic molecules
• Non-living systems contain equal numbers of L and
  D molecules
• Not so for terrestrial biomolecules

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Other Applications

• Earth
• Polarization of ground features with similar
  reflectance Fitch, 1981
• Cloud optical thickness, thermodynamic phase and
  shape Masuda and Takashima, 1992 Chepfer et
  al., 1998 Masuda et al., 2002
• Aerosol vertical distribution Aben et al., 1999
  Stam et al. 1999
• Cloud top pressure Knibbe et al., 2000 Acarreta
  et al., 2004
• Aerosol properties Chowdhary et al., 2001
  Cairns et al., 2001 Chowdhary et al., 2002
  Veihelmann et al., 2004
• Mars
• Aerosol optical thickness Petrova, 1999
• Dust and ice clouds Snik et al., 2008

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Other Applications

• Jupiter
• Haze and cloud properties Smith and Tomasko,
  1984 Braak et al., 2002
• Cloud vertical structure Smith, 1986
• Saturn
• Distribution and properties of clouds and
  aerosols Tomasko and Doose, 1984
• Titan
• Stratospheric haze layer West and Smith, 1991
• Extrasolar Planets
• Detection of liquid water Bailey, 2007

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Conclusions

• Polarization used widely to study planetary
  atmospheres
• Venus, Earth, Mars, Jupiter, Saturn, Titan,
  Exoplanets
• Aerosols, clouds, ozone, liquid water, organic
  molecules
• Microphysical properties, phase and optical
  thickness of scattering particles can be inferred
  from polarimetric observations
• Rainbows characteristic of polarization by
  spherical particles
• Absence of rainbows may indicate presence of
  non-spherical particles
• Circular polarization can be useful biosignature
• Polarization measurements now possible to
  accuracy of 10-6 Hough et al., 2006

20
Acknowledgments

• Javier Martin-Torres
• Yuk Yung
• Run-Lie Shia
• Jack Margolis
• Yung research group