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: Mariner 10 and Earth-based observations have revealed Mercury, the innermost of the terrestrial planetary bodies, to be an exciting laboratory for the study of Solar System geological processes. Mercury is characterized by a lunar-like surface, a global magnetic field, and an interior dominated by an iron core having a radius at least three-quarters of the radius of the planet. The 45% of the surface imaged by Mariner 10 reveals some distinctive differences from the Moon, however, with major contractional fault scarps and huge expanses of moderate-albedo Cayley-like smooth plains of uncertain origin. Our current image coverage of Mercury is comparable to that of telescopic photographs of the Earth’s Moon prior to the launch of Sputnik in 1957. We have no photographic images of one-half of the surface, the resolution of the images we do have is generally poor (∼1km), and as with many lunar telescopic photographs, much of the available surface of Mercury is distorted by foreshortening due to viewing geometry, or poorly suited for geological analysis and impact-crater counting for age determinations because of high-Sun illumination conditions. Currently available topographic information is also very limited. Nonetheless, Mercury is a geological laboratory that represents (1) a planet where the presence of a huge iron core may be due to impact stripping of the crust and upper mantle, or alternatively, where formation of a huge core may have resulted in a residual mantle and crust of potentially unusual composition and structure; (2) a planet with an internal chemical and mechanical structure that provides new insights into planetary thermal history and the relative roles of conduction and convection in planetary heat loss; (3) a one-tectonic-plate planet where constraints on major interior processes can be deduced from the geology of the global tectonic system; (4) a planet where volcanic resurfacing may not have played a significant role in planetary history and internally generated volcanic resurfacing may have ceased at ∼3.8Ga; (5) a planet where impact craters can be used to disentangle the fundamental roles of gravity and mean impactor velocity in determining impact crater morphology and morphometry; (6) an environment where global impact crater counts can test fundamental concepts of the distribution of impactor populations in space and time; (7) an extreme environment in which highly radar-reflective polar deposits, much more extensive than those on the Moon, can be better understood; (8) an extreme environment in which the basic processes of space weathering can be further deduced; and (9) a potential end-member in terrestrial planetary body geological evolution in which the relationships of internal and surface evolution can be clearly assessed from both a tectonic and volcanic point of view. In the half-century since the launch of Sputnik, more than 30 spacecraft have been sent to the Moon, yet only now is a second spacecraft en route to Mercury. The MESSENGER mission will address key questions about the geologic evolution of Mercury; the depth and breadth of the MESSENGER data will permit the confident reconstruction of the geological history and thermal evolution of Mercury using new imaging, topography, chemistry, mineralogy, gravity, magnetic, and environmental data.
    Space Science Reviews 08/2007; 131(1):41-84. · 5.52 Impact Factor
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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.
    Icarus 01/2004; · 3.16 Impact Factor
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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.
    Astronomy and Astrophysics 01/2006; 460(2):625-633. · 5.08 Impact Factor

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