New Ground-based Spectral Observations of Mercury and Comparison with the Moon

Introduction: Spectroscopic observations (400-670 nm) of Mercury were made at La Palma with the Nordic Optical Telescope (NOT) in June and July of 2002. Extensive observations of solar analog standard stars and validation spectra of 7 Iris and a variety of locations on the Moon were also collected. The 2002 Mercury data were also combined with previous observations (520-970 nm) from the Swedish Solar Vacuum Telescope (SVST) [1]. A spectrum (400-970 nm) calibrated to standard bidirectional geometry (α=i=30°, e=0°) was constructed based on the spectral slopes from 2002. The combined spectrum permits analysis with the Lucey lunar abundance relations for FeO and TiO 2 [2]. Validation of Mercury Spectra: A well-studied asteroid and locations on the Moon were observed on the same nights with the same instrument as the Mercury observations. These observations provide a check on the calibration procedures used for the more difficult collection and reduction of data for Mercury. Spectra were successfully obtained for five lunar sites. All the sites have previously been observed with Earth-based telescopes and published spectra are available for comparison. The five sites include Mare Serenitatis-2 (MS2), an often-observed spectral standard site; a sample-return site (Apollo 12); and three locations composed of pure anorthosite (Alphonsus central peak, Mersenius C, and Gassendi E). "Pure anorthosite" is a lithologic term used to describe lunar rock containing over 90% plagioclase feldspar [3]. Pure anorthosite has a low-Fe composition and is a plausible analog for Mercury [4, 5]. All NOT spectra for the validation targets show good agreement with previously collected data. This boosts our confidence in the quality of the NOT data and calibration procedures for Mercury. Examining the NOT data in terms of the Hapke model for reflectance of a silicate regolith: Hapke [5] conceived a general model for the reflectance properties of a lunar-like silicate regolith. The model considers the reflectance to be largely controlled by these regolith components: transition ions (chiefly Fe +2) in silicate minerals and glasses, submicroscopic metallic iron created by reduction and vapor-phase deposition of the iron in the silicates by space-weathering processes , and opaque phases (such as ilmenite or coarse-grained metallic iron). This model was used by Lucey and coworkers [2 and references therein] to develop algorithms for quantitative compositional mapping of lunar FeO, TiO 2 , and optical maturity using Clementine images. It is instructive to examine the NOT spectra in terms of the Hapke model. Opaque …