Polarization as a Tool for Remote Sensing of Planetary Atmospheres

Page 1 1 of 20, EGU General Assembly, Apr 21, 2009

Polarization as a Tool for Remote Polarization as a Tool for Remote Sensing of Planetary AtmospheresSensing of Planetary Atmospheres

Vijay Natraj (Caltech)

EGU General Assembly, Vienna, AustriaEGU General Assembly, Vienna, Austria

April 22, 2009April 22, 2009


Outline

• Introduction to Polarization

• Rainbows

• Applications

– Venus Clouds

– Earth: Tropospheric Ozone

– Circular Polarization

• Conclusions


Introduction to Polarization

• Light is a transverse wave

• Amplitude and phase of electric field determine polarization


Stokes Parameters

• Polarization state represented by 4 parameters

• Called Stokes parameters (Stokes, 1852)

• Represent intensity, linear and circular polarization

onpolarizaticircular

onpolarizati

onpolarizativerticalhorizontal

irradianceincidenttotal

V

U

Q

I

o45

/


Ray Paths for Spherical Particles

Hansen and Travis [1974]


Scattering by Spherical Particles

Diffraction peak

Primary rainbow

Supernumerary bows

Secondary rainbow

Fresnel reflection

Glory

Glory

Rainbows, glory and supernumerary bows characteristic of spherical particles

Hansen [1971]


Effect of Droplet Size on Rainbow Scattering

Bailey [2007]


Rainbow Scattering for Different Liquids

Bailey [2007]


Effect of Particle Shape on Rainbow Scattering

Bailey [2007]


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


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 1.44±0.015 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

– Large positive polarization near terminator and poles at 935 nm• strong negative polarization expected for sulfuric acid

– Polarization characteristics require optically thin layer of small particles above clouds


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


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 => 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



• 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

Tropospheric ozone changed by 10 DU

Stratospheric ozone changed by 10 DU



• Less ozone => more scattering => polarization smoothed due to multiple scattering

• Concentration of scatterers high in troposphere

• Concentration of scatterers low in stratosphere => 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


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


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]


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]


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]


Acknowledgments

• Javier Martin-Torres

• Yuk Yung

• Run-Lie Shia

• Jack Margolis

• Yung research group

Documents

Polarization as a Tool for Remote Sensing of Planetary Atmospheres