Low-temperature reflectivity spectra of red hematite and the color of Mars

Richard V. Morris, D. C. Golden, James F. Bell

Research output: Contribution to journalArticlepeer-review

54 Scopus citations

Abstract

Reflectivity spectra (visible and near IR) were measured near 141, 210, and 300 K for four red and well-crystalline powders of hematite (red hematite) used as commercial pigments, two samples of volcanic tephra from Mauna Kea volcano that contain red hematite as their dominant pigment, three samples of palagonitic tephra from the same location that contain nanophase ferric oxide as their dominant pigment, and two mixtures of the two types of pigmenting phases. Relative proportions of red hematite and nanophase ferric oxide were determined by Mössbauer spectroscopy. For samples containing red hematite as the dominant pigment, the positions of the ferric electronic transitions near 430, 500, 630, and 860 nm are essentially independent of temperature, but their widths decrease with decreasing temperature. This decrease results in a well-defined minimum for the band at 630 nm at low temperatures and in significant increases in reflectivity in spectral regions near 1050 and 600 nm. For example, the reflectivity ratios A600/R530 and A600/R860 both increase by a factor as large as ∼1.4 between 300 and 140 K. The spectral features from nanophase ferric oxide in samples of palagonitic tephra are nearly independent of temperature. Spectral data of Martian bright regions that are characterized by a shallow band minimum near 860 nm, a reflectivity maximum near 740 nm, a distinct bend near 600 nm, and a shallow absorption edge from ∼400 to 740 nm are attributed to the presence of nanophase ferric oxide plus subordinate amounts of red hematite. The 600-, 740-, and 860-nm features are associated with red hematite. Because the reflectivity of red hematite at 600 nm is strongly dependent on temperature and because this wavelength is in the red part of the visible spectrum, the color of the Martian surface may vary as a function of its temperature. A conservative upper limit for the red hematite content of the optical surface of Mars is 5%.

Original languageEnglish (US)
Article number96JE03993
Pages (from-to)9125-9133
Number of pages9
JournalJournal of Geophysical Research E: Planets
Volume102
Issue numberE4
DOIs
StatePublished - 1997
Externally publishedYes

Keywords

  • Charge-exchange processes
  • Coincidence technique
  • Electrospray ionization
  • Electrostatic ion guide
  • Frank-Condon factors
  • Orthogonal acceleration
  • Reaction cross-sections
  • Supersonic beam
  • Thermally induced dissociation
  • Time-of-flight

ASJC Scopus subject areas

  • Geophysics
  • Forestry
  • Oceanography
  • Aquatic Science
  • Ecology
  • Water Science and Technology
  • Soil Science
  • Geochemistry and Petrology
  • Earth-Surface Processes
  • Atmospheric Science
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science
  • Palaeontology

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