Albedo–color distribution on Mercury
A photometric study of the poorly known hemisphere
Institutionen för Astronomi och Rymdfysik, Uppsala Universitet, Box 515, 751 20 Uppsala, Sweden e-mail: email@example.com
Accepted: 29 August 2006
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.
Key words: planets and satellites: individual: Mercury / Moon
© ESO, 2006