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Imaging spectroscopy of geological samples and outcrops:
Novel insights from microns to meters
GSA TODAY | DECEMBER 2015 Rebecca N. Greenberger, Dept. of Earth, Environmental and presence of clays, sulfates, carbonates, and other minerals formed
Planetary Sciences, Brown University, Providence, Rhode Island through interaction with water on Mars, illuminating potentially
02912, USA, and Jet Propulsion Laboratory, California Institute of habitable past environments (e.g., Bibring et al., 2006; Mustard et
Technology, 4800 Oak Grove Drive, Pasadena, California 91109, al., 2008; Murchie et al., 2009). The Moon Mineralogy Mapper
USA, Rebecca.N.Greenberger@jpl.nasa.gov; John F. Mustard, (M3) provided new insights into the formation, igneous evolution,
Dept. of Earth, Environmental and Planetary Sciences, Brown and composition of the Moon and discovered small and varying
University, Providence, Rhode Island 02912, USA; Bethany L. amounts of hydroxylated or water-bearing materials in its regolith
Ehlmann, Jet Propulsion Laboratory, California Institute of (Green et al., 2011; Pieters et al., 2009, 2011). The Near Infrared
Technology, 4800 Oak Grove Drive, Pasadena, California 91109, Mapping Spectrometer (NIMS) on the Galileo spacecraft (Carlson
USA, and Division of Geological & Planetary Sciences, California et al., 1992) detected hydrated salts on Europa (McCord et al.,
Institute of Technology, Pasadena, California 91125, USA; Diana L. 1998) and mapped SO2 volcanism on Io (Douté et al., 2001). The
Blaney, Jet Propulsion Laboratory, California Institute of Visible and Infrared (VIR) Mapping Spectrometer mapped litho-
Technology, 4800 Oak Grove Drive, Pasadena, California 91109, logic units on Vesta’s surface (de Sanctis et al., 2012a, 2012b) and
USA; Edward A. Cloutis, Dept. of Geography, University of has arrived at the dwarf planet Ceres. The Visual and Infrared
Winnipeg, 515 Portage Ave., Winnipeg, Manitoba R3B 2E9, Mapping Spectrometer (VIMS) on the Cassini spacecraft mapped
Canada; Janette H. Wilson, Headwall Photonics, Inc., 601 River surface compositions on satellites of Saturn and discovered a large
Street, Fitchburg, Massachusetts 01420, USA; Robert O. Green, ethane cloud on Titan (Brown et al., 2006; Griffith et al., 2006).
Jet Propulsion Laboratory, California Institute of Technology, 4800 Closer to home, imaging spectrometers flown on aircraft, such as
Oak Grove Drive, Pasadena, California 91109, USA; and Abigail A. the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS)
Fraeman, Division of Geological & Planetary Sciences, California and HyMap, and in space, such as Hyperion, have mapped miner-
Institute of Technology, Pasadena, California 91125, USA alogies and monitored dynamic changes in ice, vegetation, and
other surface processes on Earth (e.g., Vane et al., 1993; Cocks et
ABSTRACT al., 1998; Green et al., 1998; Painter et al., 2003; Pearlman et al.,
2003; Asner et al., 2004, 2007).
Imaging spectroscopy is a powerful, non-destructive mineral-
ogic tool that provides insights into a variety of geological For geological applications, at the typical tens to hundreds of
processes. This remote measurement technique has been used for meters spatial resolutions of these imaging spectrometers (Fig. 1),
decades from orbital or aerial platforms to characterize surface regional or global lithologic units can be distinguished, and some
compositions of Earth and other solar system bodies. These components of the mineral assemblages can be identified. The
instruments have now been miniaturized for use in the laboratory highest-resolution airborne imaging spectrometers currently
and field, thereby enabling petrologic analyses of samples and achieve spatial resolutions of meters, permitting discrimination of
outcrops. Here, we review the technique and present four exam- mineralogies at scales of boulders or larger outcrops. However,
ples showing the exciting science potential and new insights into spatial resolutions of a centimeter or less are generally necessary
geological processes. to investigate the mineralogic and petrologic relationships within
rocks—essential to understanding the geologic history—and
INTRODUCTION airborne and orbital imaging spectrometers cannot achieve these
resolutions. The next revolution is field- and laboratory-based
Imaging spectroscopy is a technique whereby images are imaging spectroscopy at sub-millimeter to centimeter resolutions
acquired in hundreds of wavelengths simultaneously, permitting capable of petrologic analyses (e.g., Fig. 1).
spectral analysis of each discrete pixel (Goetz et al., 1985).
Compositionally distinct materials reflect and absorb light differ- Recently, visible-shortwave infrared (VSWIR) imaging spec-
ently as a function of wavelength, creating unique spectra that are trometers have been miniaturized and are now commercially
used to identify and map compositional units remotely. The appli- available for use in the field and laboratory (e.g., manufactured by
cation of imaging spectroscopy to planetary surfaces has trans- Headwall Photonics, Inc., Norskk Elektro Optikk AS, and
formed our understanding of surface compositions throughout SPECIM), and prototypes have been deployed and demonstrated
the solar system. The Observatoire pour la Minéralogie, l’Eau, les for use on planetary missions (Blaney et al., 2014; Ehlmann et al.,
Glaces et l’Activité (OMEGA) and the Compact Reconnaissance 2014; Van Gorp et al., 2014; Pilorget and Bibring, 2013).
Imaging Spectrometer for Mars (CRISM) have revealed the Specifically, the Ultra Compact Imaging Spectrometer (UCIS) is
in development by the Jet Propulsion Laboratory for a future
GSA Today, v. 25, no. 12, doi: 10.1130/GSATG252A.1.
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