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. Diffraction peak Primary rainbow Glory Glory Secondary rainbow Fresnel reflection Supernumerary bows Scattering by Spherical Particles Hansen [1971] Rainbows, glory and supernumerary bows characteristic of spherical particles

7. 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

11. 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

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 => 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

14. Tropospheric ozone changed by 10 DU Stratospheric ozone changed by 10 DU 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

15. Earth: Tropospheric Ozone • 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

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

17. 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]

18. 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]

19. 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