| 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 are generated using a 2-stream source-function radiative transfer code. The code computes the radiation field in a plane-parallel, vertically homogeneous, multiple- scattering atmosphere. Calculated intensity spectra are 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 are not sufficient to constrain the particle size with any accuracy, so many different sizes are investigated to determine the best fit to the data. The particles are 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 converge 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 reff = 1.4–2.0 μm, and ν eff = 0.2–1.0, in close agreement with the size distributions found to be the best fit in the visible wavelengths in recent studies by Pollack et al., [1995], Ockert-Bell et al. [1997], and Tomasko et al. [1999]. The optical properties and the associated single scattering properties indicate a mineralogy of a mixture of poorly crystalline silicates, but not matching any particular Earth analog material. They are shown to be a significant improvement over those used in existing models in one-dimensional heating flux calculations. |
