The dusty atmosphere of Mars: A study of the properties of martian aerosol dust, using Imager for Mars Pathfinder and Hubble Space Telescope observations

Abstract

The properties of aerosol dust on Mars may be deduced from photometric observations of its atmosphere. By comparing sky images taken by the Imager for Mars Pathfinder to numerical models, the size distribution and reflective properties of the dust particles can be determined. The format, quality, and reduction of the IMP images is described herein, as is the computational radiative transfer model used, with its various parameters. After discovering and compensating for an imprecision in the pointing of the camera, models were successfully fit to the IMP solar aureole datasets. Key results include determining the particle size (reff = 1.6 ± 0.15 μm); quadratic parameters G and Θmin describing the phase function for internally scattered light as functions of wavelength; and the imaginary refractive index n i (and single scattering albedo o) of the aerosols as a function of wavelength (presented in Table 4.1). Preliminary indications of temporal variation in ni turn out to be due to an unplanned change in the time of day of the measurements, coupled with a limitation in the algorithm for correcting the aforementioned pointing imprecision. Excluding unreliable datasets leads to a set of particle properties which shows no significant variation over the first two months of the Pathfinder mission. A multispectral sky patch from MPF Sol 22 gives greater wavelength resolution, as well as showing the sensitivity of the results to variations in key model parameters. In addition, images from the Hubble Space Telescope are used to refine the surface reflectance used in the model. The final result is a model for the aerosol dust which is consistent with the IMP solar aureole observations and the HST observations. Evidence for a minor component of water ice is also discussed. Dust reflectances derived for comparison with ground spectra show a feature in the near infrared which is not present in most MPF spectra of bright surface regolith. This is an indication that there are components visible in the bright soil which are not present in the airborne dust.