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

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).

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

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.”

Grain Size Clark 1981

Scattering

Absorption coefficients 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

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

The Astrochemistry Lab 15 K, 10-8 Torr

Good Agreement Different Slope

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

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

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

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

CH4/H2O Bernstein, et al. 2005

H2O/NH3 Scattering!!

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….