Optical Properties of the Dust Suspended in the Mars Atmosphere - Mariner 9 Infrared Interferometric Spectrometer Data Revisited

K. J. Snook (Stanford University), C. P. McKay (NASA Ames Research Center), O. B. Toon (University of Colorado)

Thermal emission spectra taken by the Mariner 9 Infrared Spectrometer during a global dust storm are revisited. High quality infrared (200-2000 cm tex2html_wrap_inline12 ) spectra were measured from orbit throughout the duration of the 1971 storm for many different orbits and view angles. Previous attempts to infer dust particle size and composition involved fitting the measured spectra with experimental optical properties of known earth-analog materials. The technique used in this current work provides a more general approach by inverting the data iteratively until a set of optical properties is found to match the spectra for all orbits simultaneously. Beginning with an arbitrary set of imaginary indices of refraction, a Kramers-Kronig routine is used to compute corresponding real indices of refraction. Given this initial guess for the optical properties and a lognormal particle size distribution, a Mie single scattering code calculates the extinction and scattering coefficients for the dust particles. A theoretical emission spectrum is generated by a radiative transfer routine which uses a 2-stream approximation for the intensity field and a linear temperature gradient throughout the atmospheric layer. Brightness temperatures are matched at every wavelength, then Kramers-Kronig is again used to recalculate the real indices of refraction from the improved set of imaginary indices. This process is repeated until the real indices of refraction converge. Results for dust particle optical properties will be presented.