Article

Dependence of Mercurian Atmospheric Column Abundance Estimations on Surface-Reflectance Modeling

Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, Texas, 77058, .govf1; University of Arizona, Lunar and Planetary Laboratory, Tucson, Arizona, 85721
Icarus (Impact Factor: 3.16). 07/1997; DOI: 10.1006/icar.1997.5725

ABSTRACT Column abundance estimates of sodium, and analogously, potassium, in Mercury's exosphere are strongly correlated to the surface reflection model used to calibrate the spectral data and the surface reflection model incorporated into the atmospheric radiative transfer solution. Depending on the surface reflection model parameters used, there can be differences in calibration factors of up to ±30% and differences in estimated column abundance of up to ±35%. Although the surface reflectance may not be used in the calibration of spacecraft measurements, the interaction between the reflected surface light and the atmospheric brightness remains important.

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    ABSTRACT: A comparison of the photometric properties of Mercury and the Moon is performed, based on their integral phase curves and disk-resolved image data of Mercury obtained with the Swedish Vacuum Solar Telescope. Proper absolute calibration of integral V-band magnitude observations reveals that the near-side of the Moon is 10–15% brighter than average Mercury, and 0–5% brighter for the “bolometric” wavelength range 400–1000 nm. As shown, this is supported by recent estimates of their geometric albedos. Hapke photometric parameters of their surfaces are derived from identical approaches, allowing a contrasting study between their surface properties to be performed. Compared to the average near-side Moon, Mercury has a slightly lower single-scattering albedo, an opposition surge with smaller width and of marginally smaller amplitude, and a somewhat smoother surface with similar porosity. The width of the lobes of the single-particle scattering function are smaller for Mercury, and the backward scattering anisotropy is stronger. In terms of the double Henyey–Greenstein b–c parameter plot, the scattering properties of an average particle on Mercury is closer to the properties of lunar maria than highlands, indicating a higher density of internal scatterers than that of lunar particles. The photometric roughness of Mercury is well constrained by the recent study of Mallama et al. (2002, Icarus 155, 253–264) to a value of about 8°, suggesting that the surfaces sampled by the highest phase angle observations (Borealis, Susei, and Sobkou Planitia) are lunar mare-like in their textural properties. However, Mariner 10 disk brightness profiles obtained at intermediate phase angles indicate a surface roughness of about twice this value. The photometric parameters of the Moon are more difficult to constrain due to limited phase angle coverage, but the best Hapke fits are provided by rather small surface roughnesses. Better-calibrated, multiple-wavelength observations of the integral and disk-resolved brightnesses of both bodies, and obtained at higher phase angle values in the case of the Moon, are urgently needed to arrive at a more consistent picture of the contrasting light scattering properties of their surfaces.
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    ABSTRACT: Aims.The variation of albedo and color of Mercury's surface is studied with disk-resolved image data obtained at six evenly spaced wavelengths in the optical to near infrared wavelength range (447-944 nm) with the 1-m Swedish Solar Telescope on La Palma in April, 2003. Methods: .Disk images have been modeled and photometrically normalized with the light scattering theory of Hapke to derive albedo-color properties of a poorly known region (unimaged by Mariner 10) of Mercury's surface between longitudes 210°W and 270°W. Maps of relative abundances of ferrous iron, titanium and optical maturation are derived on the basis of a feldspathic model for the crustal composition and previous results for the Moon, assuming the validity of the general maturation model for mafic silicate regoliths of atmosphereless bodies. Results: .The albedo-color scatterplot distributions of Mercury's surface are uniform with respect to wavelength in the near-ultraviolet to near-infrared due to the absence of strong absorption bands in the reflectance spectrum. The extents of the distributions are less than for the global Moon and similar to that of the lunar farside, which is related to the relatively subdued color contrasts of Mercury's primarily feldspathic surface. At the attained 500-km spatial resolution scale, these maps do not indicate the existence of surface regions chemically similar to the lunar maria, which have a high FeO and TiO2 content. Variations in abundances of ferrous iron and titanium are shown to be less than for the global Moon and similar to the lunar farside at the same spatial scale. Optically bright regions on Mercury are less mature and less opaque than their surroundings consistent with geologically recent immature crater ejecta, while localized dark regions generally have intermediate maturities and iron abundances and higher-than-average titanium abundances. The smaller relative intensity range of spatial variations of spectral parameters in the near infrared compared to the near ultraviolet may imply that relative abundance variations in ferrous iron are smaller than variations in opaque minerals. Conclusions: .The results reinforce the similar natures of the Mariner 10-imaged and the poorly known hemispheres of Mercury, as well as their superficial similarity to the lunar farside, and demonstrate that geological interpretation of ground-based observations of albedo features on Mercury is possible.
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