Vertical Structure of Martian Dust Measured by Solar Infrared Occultations from the Phobos Spacecraft

Abstract

Solar occultation observations of the Martian atmosphere from the Phobos spacecraft resulted in nine vertical profiles of volume extinction coefficient at 3.7 and 1.9 μm in the altitude range of 12-35 km. Interpretation of the profiles in terms of the Mie scattering theory, with the adopted refraction index 1.5-0.01 i and particle size distribution of gamma-type, gives vertical profiles of the particle number density and the effective radius (weighted-mean extinction cross-section). The number density ( n) is found to be typically 0.2-1 cm -3 between 15 and 25 km; the effective radius ( r eff) varies from 0.8 μm near 25 km to 1.6 μm at 15 km. Average values of these parameters are n = 0.32 ± 0.15 cm -3 and r eff = 1.26 ± 0.2 μm. The dependence of these parameters on the refraction index and an effective variance of the size distribution is also studied. The data can be interpreted by a one-dimensional dynamic atmospheric model with a source of aerosol of 3 · 10 -12 g/cm 2/sec near 22 km. The derived mixing coefficient, K = (1.1 ± 0.5) · 10 6 cm 2/sec found to be independent of height between 15 km and 30 km ( d log K/dz ≤ 0.025 km -1). The width of the particle size distribution, v eff, is estimated with the modeling from the vertical profiles of particle number density assuming it is independent on height: v eff = 0.2 ± 0.1. Mean value of the total optical thickness obtained by extrapolation of the extinction profiles to the surface, assuming dust scale height equal to that of gas, is 0.24 ± 0.06 at 1.9 μm.