Ultraviolet spectral reflectance properties of common planetary minerals
Introduction
The ultraviolet (UV) spectral region is one which has been largely underexploited for remote sensing of solid planetary surfaces. This arises largely from the fact that the Earth's atmosphere is relatively opaque below ∼350 nm. Thus, space-based observations are required in order to fully utilize this wavelength region for planetary surface mapping. Indeed, recent observational studies have suggested that this wavelength region has some potential for geological mapping of Solar System bodies and may provide otherwise unobtainable geological, compositional, and/or physical properties information (e.g., Hendrix et al., 2003, Hendrix and Vilas, 2006, Robinson et al., 2007, Bell and Ansty, 2007).
The Moon was the first planetary target studied in the UV spectral region (here defined as the interval from 200 to 400 nm). Early studies include Zond-3 spacecraft measurements (Lebedinsky et al., 1967a, Lebedinsky et al., 1967b, Lebedinsky et al., 1968), rocket-based whole-disk images (Carver et al., 1974), and a number of other more restricted studies (e.g., Stair and Johnson, 1953, Lucke et al., 1973, Lucke et al., 1976, and references therein). Asteroids were also surveyed at UV wavelengths and many show evidence for shallow absorption bands, spectral differences among various taxonomic classes, and “anomalous” UV properties (e.g., Butterworth and Meadows, 1985). UV spectra of meteorites have also hinted at compositional information that may be obtained from this wavelength region (e.g., Dollfus et al., 1980, Wagner et al., 1987).
More recently, space-based telescopes have enabled high spatial resolution UV observations of planetary surfaces, including Hubble Space Telescope imaging of the Moon (Garvin et al., 2006, Robinson et al., 2006, Robinson et al., 2007) and Europa (Noll et al., 1995). Recently deployed and upcoming planetary orbiters will be capable of conducting planetary surface observations in the UV, including LROC on the Lunar Reconnaissance Orbiter (300 and 360 nm imaging bands; Chin et al., 2007), SPICAM on Mars Express (118–320 nm; Bertaux et al., 2006), MARCI on the Mars Reconnaissance Orbiter (imaging bands at 260 and 320 nm; Malin, 2006), and the MESSENGER MASCS instrument (spectroscopy from 115 to 600 nm with ∼1 nm resolution; Gold et al., 2001).
UV spectra of planetary surfaces are, in general, characterized by decreasing reflectance toward shorter wavelengths, with few, if any, well-defined absorption bands. For example, in Mariner 9 ultraviolet observations of Mars airborne dust, Pang et al. (1982) detected spectral features near 210, 250, and 300 nm that they assigned to TiO charge transfers in the anatase polymorph of TiO2. This assignment was bolstered by the detection of Ti by the Viking landers and the lack of other spectral analogues for these features. Space weathering of asteroids and the Moon may also manifest itself in the UV region (e.g., Shkuratov et al., 1985, Hendrix and Vilas, 2006), and the UV may be a more sensitive indicator of space weathering than longer wavelength regions (Hendrix and Vilas, 2006, Vilas and Hendrix, 2006). Carlson et al. (1999) and Hendrix et al., 1998, Hendrix et al., 2003 detected hydrogen peroxide on Europa and Ganymede and hemispheric asymmetries on Europa using Galileo spectra, partially on the basis of spectral behavior in the 200–300 nm interval. Recent HST observations of the Moon (Garvin et al., 2006, Robinson et al., 2007) suggest that unique compositional information may be derivable from UV observations. Much of this information relates to the presumed UV spectral uniqueness of ilmenite compared to other common lunar minerals, and thus the UV region may be useful for determination of ilmenite abundance.
Analysis of observational UV data has been largely hampered by the lack of systematic spectral-compositional laboratory spectroscopic data for minerals known or presumed to exist on various Solar System bodies. The vast majority of previous laboratory studies that have been undertaken either have included only a limited range of samples and/or are not accompanied by compositional information from which correlations between spectral variations and composition or structure can be derived. The availability of high-quality laboratory spectra of planetary materials has also been hampered by a number of technical issues, including low UV intensities for most illumination sources, the unavailability of suitable spectrometers capable of reproducing planetary observing conditions (e.g., broadband target illumination coupled with wavelength selective detectors), generally low UV sensitivity for the most common types of detectors, and a lack of suitable reflectance standards.
In addition to planetary mapping, knowledge of the UV spectral properties of geologically significant minerals may also enable better modeling of heat flow in the interiors of the Earth and other planets. For example, the significance and magnitude of heat transport in the Earth's mantle through radiative heat transport is strongly dependent on the spectral properties (transparency, opacity) of mantle materials as a function of wavelength. These properties are very sensitive to factors such as grain size, temperature, structure, composition, and Fe oxidation state and electron spin configuration (e.g., Shankland et al., 1979, Smith and Langer, 1982, Badro et al., 2004, Hofmeister, 2005).
With the increasing evidence that UV spectral measurements of planetary surfaces may be able to provide geological information that is unique or complementary to that derivable from longer wavelength observations, we initiated a comprehensive laboratory-based investigation of the UV spectral reflectance properties of a wide range of geologically-important materials, coupled to comprehensive compositional and structural data for these samples. Our approach is designed to enable more detailed and quantitative interpretation of planetary UV observations.
Section snippets
Previous UV laboratory observations
As mentioned above, previous laboratory-based UV spectral reflectance studies of geological materials generally have been limited in terms of number of samples, accompanying sample information, spectral resolution, or wavelength coverage. For completeness, and as a baseline against which our new study can be compared, the results of many of these previous studies, and the experimental procedures they employed, are provided in Appendix A.
Experimental conditions of previous studies
Most of the previous studies discussed in Appendix A provided useful information on the UV properties of minerals. However, a number suffer from various shortcomings that limit their utility for quantitative analysis of UV observational data. These limitations are both technical and procedural. They include: the reliability/linearity of the detector systems; the use of a monochromatic illumination source and broadband detector; the absolute reflectance of the standards used for the
Experimental procedure of this study
A diverse suite of samples was included in this study (Table 1a, Table 1b, Table 1c, Table 1d, Table 1e, Table 1f, Table 1g, Table 1h, Table 1i). The selected samples were designed to largely encompass the major mineral species known to occur on the Moon and Mars, but also to include minerals of asteroidal and meteoritic relevance. The sample suite encompasses meteoritic metal, eleven oxides/hydroxides (three hematites, goethite, magnetite, two TiO2 polymorphs, and four ilmenites), seven
Causes of UV absorption
The 200–400 nm spectra of most geological minerals are dominated by the intense metal–O charge transfer absorption that leads to generally decreasing reflectance toward shorter wavelengths, often with a local reflectance minimum in the 200–300 nm interval (e.g., Wagner et al., 1987). Few additional resolvable absorption bands are expected in this region for common rock-forming minerals (e.g., Burns, 1970, Burns, 1981, Cloutis, 2002). Most planetary targets also generally exhibit a decrease in
Results
The results of our UV spectral measurements are presented by major mineral groups. A number of these groups include multiple samples in the case of groups with solid solution series (e.g., olivine, pyroxene, plagioclase feldspar), and/or a range of grain sizes.
Discussion of laboratory spectra
As demonstrated above, the UV reflectance spectra of common rock-forming minerals are dominated by metal–O charge transfer absorptions, with the most common metal being Fe. The intensity and wavelength position of UV absorption bands will be determined by the types and abundances of the metals, oxidation state, and their coordination environment. The strongest Fe3+O and Fe2+O charge transfer bands occur near 217 and 250 nm, respectively, and the vast majority of the mineral spectra in this
UV planetary spectroscopy
In many remote sensing situations, reflectance data are acquired at discrete wavelengths rather than in contiguous band passes. This necessitates the use of different analytical tools, such as reflectance ratios, rather than absorption band positions, to attempt to constrain surface mineralogies. While a number of recent and upcoming planetary missions will be acquiring UV reflectance data across contiguous band passes, discrete band pass data is being, and will continue to be, widely used. We
Summary and conclusions
The UV spectral reflectance properties of various geological materials are dominated by the particular transition series elements that are present. For the inner Solar System and common rock-forming minerals, the most important of these are Fe3+, Fe2+, and Ti4+. The presence of these elements leads to the appearance of metal–O charge transfer bands. These bands are generally broad and intense, with Fe3+O bands being particularly intense. The intensity of metal–O charge transfer bands is a
Acknowledgments
Thanks to Sarah Noble for providing some of the lunar sample spectra used in this analysis. The laboratory spectral measurements were made possible through the design and fabrication skills of Gundars Reinfelds, Joel Smigelski, and Harald Weigeldt of the University of Winnipeg. The kind assistance of Dr. Jeffrey Post (Smithsonian Institution National Museum of Natural History), Dr. Ted Roush (NASA Ames), Dr. Dick Morris (NASA JSC), Ms. Susan Atkinson (University of Alberta), and Mr. George
References (145)
- et al.
Optical and chemical analysis of iron in Luna 20 plagioclase
Geochim. Cosmochim. Acta
(1973) Charge-coupled device imaging spectroscopy of Mars. 2. Results and implications for martian ferric mineralogy
Icarus
(1992)- et al.
High spectral resolution UV to near-IR observations of Mars during 1999, 2001, and 2003 using HST/STIS
Icarus
(2007) - et al.
Groundbased imaging spectroscopy of Mars in the near-infrared: Preliminary results
Icarus
(1993) - et al.
Spectroscopy of Mars from 2.04 to 2.44 μm during the 1993 opposition: Absolute calibration and atmospheric vs. mineralogic origin of narrow absorption features
Icarus
(1994) - et al.
The electronic spectra of titanium dioxide
Spectrochim. Acta
(1958) Optical absorption of MgO:Fe
J. Phys. Chem. Solids
(1977)Rates and mechanisms of chemical weathering of ferromagnesian silicate minerals on Mars
Geochim. Cosmochim. Acta
(1993)- et al.
Ultraviolet reflectance properties of asteroids
Icarus
(1985) - et al.
Mafic silicate mapping on Mars: Effects of palagonitic material, multiple mafic silicates, and spectral resolution
Icarus
(2004)
Reflectance spectra of glass-bearing mafic silicate mixtures and spectral deconvolution procedures
Icarus
Reflectance spectroscopy extended to u.v. for terrestrial, lunar and meteoritic samples
Geochim. Cosmochim. Acta
The MESSENGER mission to Mercury: Scientific payload
Planet. Space Sci.
Chemical weathering on Mars
Icarus
Europa: Disk-resolved ultraviolet measurements using the Galileo ultraviolet spectrometer
Icarus
Dependence of diffusive radiative transfer on grain-size, temperature, and Fe-content: Implications for mantle processes
J. Geodyn.
Pigmenting agents in martian soils: Inferences from spectral, Mossbauer, and magnetic properties of nanophase and other iron oxides in Hawaiian palagonitic soil PN-9
Geochim. Cosmochim. Acta
Spatial variations in the spectral properties of bright regions on Mars
Icarus
Visible and near-infrared diffuse reflectance spectra of pyroxenes as applied to remote sensing of solid objects in the Solar System
J. Geophys. Res.
Plagioclase feldspars: Visible and near-infrared diffuse reflectance spectra as applied to remote sensing
Proc. Lunar Sci. Conf.
Vitrification darkening in the lunar highlands and identification of Descartes material at the Apollo 16 site
Proc. Lunar Sci. Conf.
Absolute fluorescent quantum efficiency of sodium salicylate
J. Opt. Soc. Am.
Absorption spectra of lunar sections from different lunar areas
Space Res.
Electronic transitions in perovskite: Possible nonconvecting layers in the lower mantle
Science
Global mineral distributions on Mars
J. Geophys. Res.
Acidic volatiles and the Mars soil
J. Geophys. Res.
Mariner ultraviolet spectrometer: Topography and polar cap
Science
Mars surface mineralogy from Hubble Space Telescope imaging during 1994–1995: Observations, calibration, and initial results
J. Geophys. Res.
SPICAM on Mars Express: Observing modes and overview of UV spectrometer data and scientific results
J. Geophys. Res.
Indications of sulfate minerals in the martian soil from Earth-based spectroscopy
J. Geophys. Res.
Clementine images of the lunar sample-return stations: Refinement of FeO and TiO2 mapping techniques
J. Geophys. Res.
Lunar pure anorthosite as a spectral analog for Mercury
Meteorit. Planet. Sci.
A Mariner 10 color study of mercurian craters
J. Geophys. Res.
Crystal field spectra and evidence of cation ordering in olivine minerals
Am. Mineral.
Intervalence transitions in mixed-valence minerals of iron and titanium
Annu. Rev. Earth Planet. Sci.
Ferric sulfates on Mars
J. Geophys. Res.
Mineralogical Applications of Crystal Field Theory
Further characterization of spectral features attributable to titanium on the Moon
Proc. Lunar Sci. Conf.
Spectra of the icy Galilean satellites from 0.2 to 5 μm: A compilation, new observations, and a recent summary
J. Geophys. Res.
Hydrogen peroxide on the surface of Europa
Science
The ultraviolet reflectivity of the Moon
Moon
Comparison of lunar ultraviolet reflectivity with that of terrestrial rock samples
Moon
Application of remote sensing spectral reflectance measurements to lunar geology classification and determination of titanium content of lunar soils
J. Geophys. Res.
Lunar Reconnaissance Orbiter overview: The instrument suite and mission
Space Sci. Rev.
Formation of the hematite-bearing unit in Meridiani Planum: Evidence for deposition in standing water
J. Geophys. Res.
Global mapping of martian hematite mineral deposits: Remnants of water-driven processes on early Mars
J. Geophys. Res.
High-resolution reflectance spectra of Mars in the 2.3-μm region: Evidence for the mineral scapolite
J. Geophys. Res.
Pyroxene reflectance spectra: Minor absorption bands and effects of elemental substitutions
J. Geophys. Res.
Cited by (110)
Spectral and mineralogical alteration process of naturally-heated CM and CY chondrites
2022, Geochimica et Cosmochimica ActaCompositional and spectrochemical analyses of Cr-spinels in the Sittampundi Anorthosite Complex, Southern India: Implications for remote observation of spinels on the Moon
2021, Spectrochimica Acta - Part A: Molecular and Biomolecular Spectroscopy