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Abstract:

A laboratory study was performed using the Visible 91̽�� Space Environment Goniometer in which the broadband (350–1250 nm) bidirectional reflectance distribution functions (BRDFs) of two representative Apollo regolith samples were measured, for two surface roughness profiles, across a range of viewing angles—reflectance: 0–70°, in steps of 5°; incidence: 15°, 30°, 45°, and 60°; and azimuthal: 0°, 45°, 90°, 135°, and 180°. The BRDF datasets were fitted using the Hapke BRDF model to (1) provide a method of comparison to other photometric studies of the lunar regolith and (2) to produce Hapke parameter values which can be used to extrapolate the BRDF to all angles. Importantly, the surface profiles of the samples were characterized using an Alicona 3D® instrument, allowing two of the free parameters within the Hapke model, φ and θ ¯ $$ \overline{\theta} $$ , which represent porosity and surface roughness, respectively, to be constrained. The study determined that, for θ ¯ $$ \overline{\theta} $$ , the 500–1000 μm size‐scale is the most relevant for the BRDF. Thus, it deduced the following “best fit” Hapke parameters for each of the samples: Apollo 11 rough— w $$ w $$ = 0.315 ± 0.021, b $$ b $$ = 0.261 ± 0.007, and h S $$ {h}_S $$ = 0.039 ± 0.005 (with θ ¯ $$ \overline{\theta} $$ = 21.28° and φ = 0.41 ± 0.02); Apollo 11 smooth— w $$ w $$ = 0.281 ± 0.028, b $$ b $$ = 0.238 ± 0.008, and h S $$ {h}_S $$ = 0.032 ± 0.006 (with θ ¯ $$ \overline{\theta} $$ = 13.80° and φ = 0.60 ± 0.02); Apollo 16 rough— w $$ w $$ = 0.485 ± 0.155, b $$ b $$ = 0.155 ± 0.083, and h S $$ {h}_S $$ = 0.135 ± 0.007 (with θ ¯ $$ \overline{\theta} $$ = 21.69° and φ = 0.55 ± 0.02); Apollo 16 smooth— w $$ w $$ = 0.388 ± 0.057, b $$ b $$ = 0.063 ± 0.033, and h S $$ {h}_S $$ = 0.221 ± 0.011 (with θ ¯ $$ \overline{\theta} $$ = 14.27° and φ = 0.40 ± 0.02). Finally, updated hemispheric albedo functions were determined for the samples, which can be used to set laboratory measured visible scattering functions within thermal models.

Abstract:

The detection of hydrogen chloride (HCl) in the atmosphere of Mars was among the primary objectives of the ExoMars Trace Gas Orbiter (TGO) mission. Its discovery using the Atmospheric Chemistry Suite mid‐infrared channel (ACS MIR) showed a distinct seasonality and possible link to dust activity. This paper is part 2 of a study investigating the link between HCl and aerosols by comparing gas measurements made with TGO to dust and water ice opacities measured with the Mars Climate Sounder (MCS). In part 1, we showed, and compared, the seasonal evolution of vertical profiles of HCl, water vapor, temperature, dust opacity, and water ice opacity over the dusty periods around perihelion (solar longitudes 180°–360°) across Mars years 34–36. In part 2, we investigated the quantitative correlations in the vertical distribution between each quantity, as well as ozone. We show that there is a strong positive correlation between HCl and water vapor, which is expected due to fast photochemical production rates for HCl when reacting with water vapor photolysis products. We also show a strong positive correlation between water vapor and temperature, but are unable to show any correlation between temperature and HCl. There are weak correlations between the opacities of dust and water ice, and dust and water vapor, but only very low correlations between dust and HCl. We close with a discussion of possible sources and sinks and that interactions between HCl and water ice are the most likely for both, given the inter‐comparison.