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New Optical Constants for Amorphous and Crystalline H 2 O-ice and H 2 O-mixtures

New Optical Constants for Amorphous and Crystalline H 2 O-ice and H 2 O-mixtures. R. M. E. Mastrapa, M. Bernstein, S. Sandford NASA Ames Research Center TNO Workshop, Catania 5 July 2006. Amorphous H 2 O-ice in the Solar System. Not pure amorphous H 2 O-ice, but mixtures with crystalline.

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New Optical Constants for Amorphous and Crystalline H 2 O-ice and H 2 O-mixtures

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  1. New Optical Constants for Amorphous and Crystalline H2O-ice and H2O-mixtures • R. M. E. Mastrapa, M. Bernstein, S. Sandford • NASA Ames Research Center • TNO Workshop, Catania • 5 July 2006

  2. Amorphous H2O-ice in the Solar System • Not pure amorphous H2O-ice, but mixtures with crystalline. • Europa and Ganymede (Hansen and McCord 2004) • Enceladus (Brown et al. 2006) • H2O 3 mm fundamental • At 100 K, amorphous H2O-ice converts to crystalline H2O-ice in 104 years (Jenniskens, et al. 1998).

  3. Irradiation converts crystalline H2O into amorphous H2O at a dose of 10-20 eV/molecule.(e.g. Leto and Baratta 2003, Moore and Hudson 1992, Strazzula et al. 1992) Mastrapa and Brown 2006

  4. Goals for Lab Work • What we want to know: surface composition, history, etc. • Produce optical constants used in spectral modeling. • Can not perfectly simulate all of the surface properties in the lab (grain size, contamination, mixture ratio) • Produce high spectral resolution optical constants from a pure sample with all of the variables minimized. • Biggest problems: grain size and “scattering.”

  5. Grain Size Clark 1981

  6. Scattering

  7. Previous NIR Spectra Closed cell: No Amorphous H2O-ice 270 K = Amorphous? Absorption coefficients a = -ln(I/I0)/h h - thickness Grundy and Schmitt 1998

  8. Previous NIR Spectra Wavelength (mm) 1.67 2.0 1.53 1.82 Amorphous strength 50% crystal Schmitt, et al. 1998

  9. The Astrochemistry Lab • 15 K, 10-8 Torr

  10. Good Agreement Different Slope

  11. Warm crystalline H2O-ice is not a good proxy for the amorphous phase.

  12. Amorphous features are much stronger than previously suggested. Band centers are important identifiers.

  13. Next Steps • Calculate optical constants. • Post absorption coefficients and optical constants online. • Analyze and calculate optical constants for mixtures.

  14. H2O/CO2 • Mixtures can create new features • Mixtures can change the location and shape of features depending on mixing ratio, temperature , and H2O phase. Bernstein et al. 2005

  15. CH4/H2O Bernstein, et al. 2005

  16. H2O/NH3 Scattering!!

  17. Conclusions • New absorption coefficients for amorphous and crystalline H2O-ice. • Amorphous fraction is related to radiation history. (* ignoring all other processes) • Identification of mixtures can be used to determine surface conditions. • The Astrochemistry Lab is taking requests….

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