Mars: Near-infrared comparative spectroscopy during the 1986 opposition

James F. Bell, Thomas B. McCord

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Abstract

Near-infrared (0.7-2.5 μm) spectral observations of Mars during the 1986 opposition were carried out at the Mauna Kea Observatory utilizing the University of Hawaii 2.2-m telescope. Spectra were obtained of several Martian locations using a continuously variable filter (CVF) spectrometer with a resolution of ∼1.25% (Δλ/λ). During two separate runs in June and August, approximately 60 distinct spots between 354°W and 163°W and 32°N and 53°S were observed at an angular resolution of ≅0.5 to 1.5 arcsec, corresponding to a spatial resolution on Mars of ≅200 to 460 km, varying with nightly seeing conditions. These different spots fall roughly into a set of eight distinct geologic regions: volcanic regions, ridged plains, ridged volcanic plains, scoured plains, impact basins, channels and canyons, densely cratered regions, and layered terrain and ice. The spectra exhibit typical noise-related or weather-induced errors of less than 4% of the full-scale signal. To analyze these spectral data, spot-to-spot ratios, or relative reflectance spectra, were produced between spectra taken in different geologic regions. Spectral features observed in these ratios can act as indicators of mineralogic differences between areas under consideration. Perhaps the most striking result obtained from the many ratios taken close in time and under similar viewing geometries was the consistent lack of noticeable differences between spectra taken of areas which, in geologic maps and Viking Orbiter images, appear to have very different morphologies. This observation leads to several possible conclusions: (a) The spectral feature differences are below the detection limit of these measurements. (b) The spatial resolution of spots observed on Mars was not high enough to merit comparison with medium- to high-resolution Viking Orbiter images and geologic maps (i.e., the local morphology varies significantly within the aperture region and a mixing or averaging effect operates to reduce the spectral feature differences). (c) All of these regions are the same spectrally in the near-infrared. It is likely that all three of these effects operate to some degree to prevent spectral features from being observed among the areas measured. Possibility (c) may best explain these results, as it is consistent with several previous studies indicating a grossly uniform mantle of global dust over much of the observed surface. The message to Mars orbiting spectroscopy experiments such as Mars Observer VIMS is that the spectral differences are small (only a few percent of the continuum) or nonexistent among many areas on Mars on the scale of several hundred kilometers.

Original languageEnglish (US)
Pages (from-to)21-34
Number of pages14
JournalIcarus
Volume77
Issue number1
DOIs
StatePublished - Jan 1989

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ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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