Reflectance spectroscopy (0.35-8μm) of ammonium-bearing minerals and qualitative comparison to Ceres-like asteroids

Breanne L. Berg, Edward A. Cloutis, Pierre Beck, Pierre Vernazza, Janice L. Bishop, Driss Takir, Vishnu Reddy, Daniel Applin, Paul Mann

Research output: Contribution to journalArticlepeer-review

25 Scopus citations

Abstract

Ammonium-bearing minerals have been suggested to be present on Mars, Ceres, and various asteroids and comets. We undertook a systematic study of the spectral reflectance properties of ammonium-bearing minerals and compounds that have possible planetary relevance (i.e., ammonium carbonates, chlorides, nitrates, oxalates, phosphates, silicates, and sulfates). Various synthetic and natural NH4+-bearing minerals were analyzed using reflectance spectroscopy in the long-wave ultraviolet, visible, near-infrared, and mid-infrared regions (0.35-8μm) in order to identify spectral features characteristic of the NH4+ molecule, and to evaluate if and how these features vary among different species. Mineral phases were confirmed through structural and compositional analyses using X-ray diffraction, X-ray fluorescence, and elemental combustion analysis. Characteristic absorption features associated with NH4 can be seen in the reflectance spectra at wavelengths as short as ~1μm. In the near-infrared region, the most prominent absorption bands are located near 1.6, 2.0, and 2.2μm. Absorption features characteristic of NH4+ occurred at slightly longer wavelengths in the mineral-bound NH4+ spectra than for free NH4+ for most of the samples. Differences in wavelength position are attributable to various factors, including differences in the type and polarizability of the anion(s) attached to the NH4+, degree and type of hydrogen bonding, molecule symmetry, and cation substitutions. Multiple absorption features, usually three absorption bands, in the mid-infrared region between ~2.8 and 3.8μm were seen in all but the most NH4-poor sample spectra, and are attributed to fundamentals, combinations, and overtones of stretching and bending vibrations of the NH4+ molecule. These features appear even in reflectance spectra of water-rich samples which exhibit a strong 3μm region water absorption feature. While many of the samples examined in this study have NH4 absorption bands at unique wavelength positions, in order to discriminate between different NH4+-bearing phases, absorption features corresponding to molecules other than NH4+ should be included in spectral analysis. A qualitative comparison of the laboratory results to telescopic spectra of Asteroids 1 Ceres, 10 Hygiea, and 324 Bamberga for the 3μm region demonstrates that a number of NH4-bearing phases are consistent with the observational data in terms of exhibiting an absorption band in the 3.07μm region.

Original languageEnglish (US)
Pages (from-to)218-237
Number of pages20
JournalIcarus
Volume265
DOIs
StatePublished - Feb 1 2016
Externally publishedYes

Keywords

  • Asteroids
  • Comets
  • Mars

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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