1. Page 1 1 of 20, EGU General Assembly, Apr 21, 2009 Polarization as a Tool for Remote Sensing of Planetary Atmospheres Vijay Natraj (Caltech) EGU General Assembly, Vienna, Austria April 22, 2009

2. Page 2 2 of 20, EGU General Assembly, Apr 21, 2009 Outline Introduction to Polarization Rainbows Applications –Venus Clouds –Earth: Tropospheric Ozone –Circular Polarization Conclusions

3. Page 3 3 of 20, EGU General Assembly, Apr 21, 2009 Introduction to Polarization Light is a transverse wave Amplitude and phase of electric field determine polarization

4. Page 4 4 of 20, EGU General Assembly, Apr 21, 2009 Stokes Parameters Polarization state represented by 4 parameters Called Stokes parameters (Stokes, 1852) Represent intensity, linear and circular polarization

5. Page 5 5 of 20, EGU General Assembly, Apr 21, 2009 Ray Paths for Spherical Particles Hansen and Travis [1974]

6. Page 6 6 of 20, EGU General Assembly, Apr 21, 2009 Scattering by Spherical Particles Diffraction peak Primary rainbow Supernumerary bowsSecondary rainbow Fresnel reflection Glory Rainbows, glory and supernumerary bows characteristic of spherical particles Hansen [1971]

7. Page 7 7 of 20, EGU General Assembly, Apr 21, 2009 Effect of Droplet Size on Rainbow Scattering Bailey [2007]

8. Page 8 8 of 20, EGU General Assembly, Apr 21, 2009 Rainbow Scattering for Different Liquids Bailey [2007]

9. Page 9 9 of 20, EGU General Assembly, Apr 21, 2009 Effect of Particle Shape on Rainbow Scattering Bailey [2007]

10. Page 10 10 of 20, EGU General Assembly, Apr 21, 2009 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. Page 11 11 of 20, EGU General Assembly, Apr 21, 2009 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. Page 12 12 of 20, EGU General Assembly, Apr 21, 2009 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. Page 13 13 of 20, EGU General Assembly, Apr 21, 2009 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. Page 14 14 of 20, EGU General Assembly, Apr 21, 2009 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 Tropospheric ozone changed by 10 DU Stratospheric ozone changed by 10 DU

15. Page 15 15 of 20, EGU General Assembly, Apr 21, 2009 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. Page 16 16 of 20, EGU General Assembly, Apr 21, 2009 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. Page 17 17 of 20, EGU General Assembly, Apr 21, 2009 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. Page 18 18 of 20, EGU General Assembly, Apr 21, 2009 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. Page 19 19 of 20, EGU General Assembly, Apr 21, 2009 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. Page 20 20 of 20, EGU General Assembly, Apr 21, 2009 Acknowledgments Javier Martin-Torres Yuk Yung Run-Lie Shia Jack Margolis Yung research group