[Previous] | [Session 72] | [Next]
K.J. Snook, C. P. McKay (NASA-Ames Research Center), O. B. Toon (U. Colorado), F. Forget (Lab. de Meteorologie Dynamique, CNRS, Paris), B. J. Cantwell (Stanford U.)
Methods for iteratively determining the infrared optical constants for dust suspended in the Mars atmosphere are presented. In this research, theoretical infrared emission spectra of the emergent intensity from Martian dust clouds were generated using a 2-stream source-function radiative transfer code. The code computed the radiation field in a plane-parallel, vertically homogeneous, multiple-scattering atmosphere. Calculated intensity spectra were compared with Mariner 9 Infrared Interferometric Spectrometer (IRIS) spacecraft data to iteratively retrieve the optical properties and opacity of the dust, as well as the surface temperature of Mars at the time and location of each measurement. The data were not sufficient to constrain the particle size with any accuracy, so many different sizes were investigated to determine the best fit to the data. The particles were assumed spherical and the temperature profile was obtained from the CO2 band shape. Given a reasonable initial guess for the indices of refraction, the searches converged in a well-behaved fashion, producing a fit with error of less than 1.2 K (rms) to the observed brightness spectra for a wide range of particle sizes. The particle size distributions corresponding to the best fits were lognormal with size parameters in the range of r\rm eff = 1.4 - 2.0 \mum, and \nu\rm eff = 0.2 - 1.0, in close agreement with the size distributions found to be the best fit in the visible wavelengths in previous studies. The optical properties and the associated single scattering properties indicated a mineralogy of a mixture of poorly crystalline silicates, but not matching any particular Earth analog material. They were shown to be a significant improvement over those used in existing models in one-dimensional heating flux calculations.
The author(s) of this abstract have provided an email address for comments about the abstract: ksnook@mail.arc.nasa.gov