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).

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

6. 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.”

7. Grain Size
Clark 1981

8. Scattering

10. 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

11. 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

12. The Astrochemistry Lab
15 K, 10-8 Torr

13. Good Agreement
Different Slope

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

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

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

18. 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

19. CH4/H2O
Bernstein, et al. 2005

20. H2O/NH3
Scattering!!

22. 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….