T. Hiroi

Brown University, Providence, Rhode Island, United States

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Publications (276)481.24 Total impact

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    ABSTRACT: Surface scattered sunlight carries important information about the composition and microtexture of surface materials, thus enabling tracing back the geological and climatic processes that occurred on the planetary body. Here we perform laboratory spectro-goniometric measurements of granular samples (45-75 μm fraction) with different composition and physical properties over the VIS-NIR spectral range (0.4-2.5 μm). To quantify the evolution of the scattering properties over the VIS-NIR, we use an inversion procedure based on a Bayesian approach to estimate photometric parameters from the Hapke radiative transfer model. The granular samples are also carefully characterized by optical and SEM techniques in order to link these scattering variations with the grains' physical properties. Results show that the scattering properties are wavelength-dependent and can vary significantly over the VIS-NIR spectral range. In particular, the phase function of a granular material is affected by both the absorptivity and the external and internal structure of the grains, from the millimeter scale down to the wavelength scale. Our results also confirm that the macroscopic roughness parameter, as defined by Hapke, is to first order correlated with the absorptivity of the particles, through multiple scattering effects, and thus mostly corresponds to a measurement of the particles shadowing. Photometric datasets, typically obtained at a given wavelength that can vary from one study to another, should therefore be compared and interpreted with caution when extrapolating across wavelengths. Our results also suggest that multi-wavelength photometry could potentially provide a much richer signature than with single-wavelength photometry, opening new perspectives into the characterization of surface materials.
    No preview · Article · Dec 2015 · Icarus
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    ABSTRACT: We report the global distribution of areas exhibiting no absorption features (featureless or FL) on the lunar surface, based on the reflectance spectral data set obtained by the Spectral Profiler onboard Kaguya/SELENE. We found that FL sites are located in impact basins and large impact craters in the Feldspathic Highlands Terrane, while there are no FL sites in the Procellarum regions nor the South Pole-Aitken basin. FL sites in each impact basin/crater are mainly found at the peak rings or rims, where the purest anorthosite (PAN) sites are also found. At the local scale, most of the FL and PAN points are associated with impact craters and peaks. Most of the FL spectra show a steeper (redder) continuum than the PAN spectra, suggesting the occurrence of space weathering effects. We propose that most of the material exhibiting a FL spectrum originate from space weathered PAN. Taking into account all the occurrence trends of FL sites on the Moon, we propose that both the FL and PAN materials were excavated from the primordial lunar crust during ancient basin formations below the megaregolith in the highlands. Since the FL and PAN sites are widely distributed over the lunar surface, our new data may support the existence of a massive PAN layer below the lunar surface.
    No preview · Article · Dec 2015 · The Journal of Geophysical Research Planets
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    Full-text · Conference Paper · Nov 2015
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    Full-text · Dataset · Oct 2015
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    ABSTRACT: The surface scattered sunlight carries major information about the composition and microtexture of surface materials, thus enabling tracing back the geological and climatic processes that occurred on the planetary body. Here we perform laboratory spectro-goniometric measurements of different kinds of granular samples over the VIS-NIR spectral range, coupling the spectral and geometric dimensions to analyze their scattering behavior. To quantify the evolution of the scattering properties with the wavelength, we use an innovative inversion procedure based on a Bayesian approach to estimate photometric parameters from the Hapke model. The granular samples are also characterized by optical and SEM techniques in order to link these scattering variations with the grains' physical properties.
    Full-text · Conference Paper · Sep 2015
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    ABSTRACT: Introduction: In an extensive review of the literature on “space weathering”, Hapke defines the term as an “aggregate of the physical and chemical changes that occur to material exposed on the surface of an airless body” [1]. The processes that can affect these changes include the solar wind, ultraviolet radiation, meteorite and micrometeorite impacts, and solar and galactic cosmic rays. It has been demonstrated that small particles (<50 nm) of npFe0 darken and redden spectral properties while large particles (>50 nm) of npFe0 only darken spectra [2,3]. Recently it has also been discovered using scanning transmission electron microscopy (STEM) that hydrogen implanted into silicates gen-erates H20 and -OH [4]. The mechanism(s) of npFe0 formation within low-iron materials and their kinetics are not well under-stood and remain contentious [5,6]. In fact, some investigators have found that hydrogen implantation causes depletion in the water content of irradiated samples [7]. Methodology: Here we present the results of the first atom probe tomography (APT) and visible/near-infrared (VNIR) spectral study of a sample of San Carlos olivine (Fo90.1 [8]) exposed to simulated solar wind-based space weathering due to hydrogen at solar wind energy (~1keV/amu). This work was accomplished using plasma source ion implantation (PSII) similar to work performed previously on orthopyroxene [9], except that here we used a fluence of 1017 H ions/cm2 instead of 1018 H ions/cm2. Pressed pellets consisting of ground olivine seived to less than 75 micrometers were implanted alongside samples of polished olivine for VNIR analysis. The polished sample was removed from the PSII chamber and returned to the U.S. where it was coated with 100 nm nickel and prepared for APT analysis using a standard focused ion beam (FIB) liftout method [10]. The pressed pellet was returned to RELAB at Brown University for VNIR analysis. Results: The VNIR analysis of the pressed pellet demonstrated no change in darkening or reddening of the sample, contrary to previous results from polished specimens implanted with 1018 H ions/cm2. The APT analysis of the polished olivine revealed a very complex structure within the outer 10 nm of the specimen’s surface. This structure includes nanophase iron particles in the region roughly 5 nm below the surface. Regions of higher concentrations of magnesium appear to reside closer to the surface than the nanophase iron. References: [1] Hapke, B. (2001) Journal of Geophysical Research-Planets 106:10039-10073. [2] Lucey, P. G. and M. A. Riner (2011) Icarus 212:451-462. [3] Noble, S. K., et al. (2007) Icarus 192:629-642. [4] Bradley, J. P. et al., Proceedings of the National Academy of Science U.S.A., 111:1732-1735. [5] Marchi S., et al. (2010) Astrophysical Journal Letters 721:L172-L176. [6] Pieters, C. M. (2000) Meteoritics & Planetary Science 35:1101-1107. [7] Burke, D. J., et al. (2011) Icarus 211:1082-1088. [8] Fournelle, J. (2009) Abstract #V31E-2009. AGU Fall Meeting. [9] Kuhlman, K. R., et al. (2015) Planetary and Space Science, DOI:10.1016/j.pss.2015.04.003. [10] Miller, M. K., et al. (2007) Microscopy and Microanalysis 13:428-436.
    Full-text · Conference Paper · Jul 2015
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    ABSTRACT: The Hayabusa2 spacecraft has three framing cameras (ONC-T, ONC-W1, and ONC-W2) for optical navigation to asteroid 1999 JU3. The ONC-T is a telescopic camera with seven band-pass filters in the visible and near-infrared range. These filters are placed on a wheel, which rotates to put a selected filter for different observations, enabling multiband imaging. Previous ground-based observations suggesting that hydrous materials may be present on the surface of 1999 JU3 and distributed in relatively limited areas. The presence of hydrous minerals indicates that this asteroid experienced only low to moderate temperatures during its formation, suggesting that primordial materials are preserved. In order to find the best sampling sites, we will perform reflectance spectroscopic observations using the ONC-T near the asteroid after arrival. Finding regions rich in hydrous minerals is the key for this remote sensing observation. In preparation for this, we conducted ground-based experiments for the actual ONC-T flight model to confirm the detectability of the absorption band of Fe-rich serpentine. As a result, we detected the absorption band near 0.7 μm by reflectance spectroscopy of CM2 chondrites, such as Murchison and Nogoya, which are known to contain the Fe-rich serpentine, and did not detect any 0.7 μm absorption in Jbilet Winselwan CM2 chondrite with decomposed Fe-rich serpentine.
    No preview · Article · Jul 2015 · Advances in Space Research
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    ABSTRACT: We performed pulse-laser irradiation experiments of a primitive meteorite to simulate space weathering by micrometeorite bombardments on C-type asteroids. Pellets of powdered Murchison CM2 chondrite were set in vacuum and exposed to pulse laser with a diameter of 0.5 mm and delivered energies of 5, 10, and 15 mJ. We measured reflectance spectra of unirradiated and irradiated surfaces of the pellets. During analysis the pellet was heated to approximately 100 °C and purged in N2 gas in order to reduce absorption of ambient water. The spectra become darker and bluer with increasing laser energies. Their UV reflectance increases and 0.7- and 3-μm band depths decrease from 0 to 15 mJ. The spectral bluing observed in our experiments reproduces the bluing occurred during space weathering of C-type asteroids. High-resolution observation by a transmission electron microscope showed that the laser heating causes preferential melting and evaporation in FeS-rich fine-grained portions, which results in dispersion and deposition of numerous FeS-rich amorphous silicate particles 20–1000 nm in size on the surface of the pellet. In addition, at the laser-irradiated but unmelted areas, heat-induced amorphization and decomposition of serpentine occur. These mineralogical changes make the reflectance spectra of the Murchison CM chondrite darker and bluer.
    No preview · Article · Jul 2015 · Icarus
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    ABSTRACT: We present details of the global distribution of high-Ca pyroxene (HCP)-rich sites in the lunar highlands based on the global dataset of hyper-spectral reflectance obtained by the SELENE Spectral Profiler. Most HCP-rich sites in the lunar highlands are found at fresh impact craters. In each crater, most of the detection points are distributed on the ejecta, rim, and floor of the impact craters rather than the central peaks, while the central peaks are dominated by purest anorthosite (PAN). This indicates that HCP-rich materials originate from relatively shallower regions of the lunar crust than PAN. In addition, while all ray craters with sizes larger than ~40km possess HCP-rich materials, small fresh craters with sizes less than ~6−−10km do not, indicating that the uppermost mixing layers in the lunar crust are not dominated by HCP. Based on these results, we propose that in the upper lunar crust, a HCP-rich zone overlying the PAN layer exists below the uppermost mixing layer. This HCP-rich zone may originate from interstitial melt during the formation of the flotation anorthositic cumulate, while an impact ejecta origin, impact melt origin, and/or magmatic intrusion into the upper lunar crust may also account for the occurrence of HCP-rich sites in the highlands.
    No preview · Article · Mar 2015 · The Journal of Geophysical Research Planets
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    Full-text · Dataset · Jan 2015
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    ABSTRACT: Sulfate minerals are important indicators for aqueous geochemical environments. The geology and mineralogy of Mars have been studied through the use of various remote-sensing techniques, including thermal (mid-infrared) emission and visible/near-infrared reflectance spectroscopies. Spectral analyses of spacecraft data (from orbital and landed missions) using these techniques have indicated the presence of sulfate minerals on Mars, including Fe-rich sulfates on the iron-rich planet. Each individual Fe-sulfate mineral can be used to constrain bulk chemistry and lends more information about the specific formational environment [e.g., Fe2+ sulfates are typically more water soluble than Fe3+ sulfates and their presence would imply a water-limited (and lower Eh) environment; Fe3+ sulfates form over a range of hydration levels and indicate further oxidation (biological or abiological) and increased acidification]. To enable better interpretation of past and future terrestrial or planetary data sets, with respect to the Fe-sulfates, we present a comprehensive collection of mid-infrared thermal emission (2000 to 220 cm−1; 5-45 μm) and visible/near-infrared (0.35-5 μm) spectra of 21 different ferrous- and ferric-iron sulfate minerals. Mid-infrared vibrational modes (for SO4, OH, H2O) are assigned to each thermal emissivity spectrum, and the electronic excitation and transfer bands and vibrational OH, H2O, and SO4 overtone and combination bands are assigned to the visible/near-infrared reflectance spectra. Presentation and characterization of these Fe-sulfate thermal emission and visible/near-infrared reflectance spectra will enable the specific chemical environments to be determined when individual Fe-sulfate minerals are identified.
    No preview · Article · Dec 2014 · American Mineralogist
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    ABSTRACT: Olivine-rich asteroids appear to be common in the main asteroid belt as well as present in the near-Earth asteroid population. There are a number of meteorite classes that are dominated by olivine ± metal. To determine whether relationships exist between these asteroids and meteorites, we spectrally character-ized a number of olivine + meteoritic metal powder intimate and areal mixtures, pallasite slabs, and olivine powders on a metal slab. Our goal is to understand the spectral characteristics of olivine + metal assem-blages and develop spectral metrics that can be used to analyze reflectance spectra of olivine-dominated asteroids. We found that the major olivine absorption band in the 1 lm region is resolvable in intimate mixtures for metal abundances as high as $90 wt.%. The wavelength position of the 1 lm region olivine
    Full-text · Article · Nov 2014 · Icarus
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    ABSTRACT: The Spectral Profiler (SP) is a visible–near infrared spectrometer onboard the Japanese Selenological and Engineering Explorer (SELENE), which was launched in 2007 and observed the Moon until June 2009. The SP consists of two gratings and three linear-array detectors: VIS (0.5–1.0 $muhbox{m}$), NIR 1 (0.9–1.7 $muhbox{m}$), and NIR 2 (1.7–2.6 $muhbox{m}$). In this paper, we propose a new method for radiometric calibration of NIR 2, specifically for the dark output (background) estimate, which is different from the previous method used for VIS and NIR 1. We show that the reflectance spectra of NIR 2 derived from the new radiometric calibration show less noise than those of the previous method. Based on an analysis of the reflectance spectra at exposure sites of the end-member minerals on the lunar surface, we demonstrated that the spectral features of the 2- $muhbox{m}$ band in the NIR 2 spectra are consistent with those expected from the minerals inferred from the features of the 1- $muhbox{m}$ band in the VIS and NIR 1 spectra. Finally, we examined the repeatability of the radiometric calibration of NIR 2 using the SP data near the Apollo 16 landing site observed at four different times. The typical difference in the reflectance at wavelengths $< sim!! 2.1 muhbox{m}$ was a few percent, which is within the uncertainty due to the error in the background estimate, suggesting that there was no significant change in the sensitivity of NIR 2 over the mission period.
    No preview · Article · Nov 2014 · IEEE Transactions on Geoscience and Remote Sensing

  • No preview · Conference Paper · Sep 2014
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    ABSTRACT: Although petrologic, chemical and isotopic studies of ordinary chondrites and meteorites in general have largely helped establish a chronology of the earliest events of planetesimal formation and their evolution, there are several questions that cannot be resolved via laboratory measurements and/or experiments only. Here we propose rationale for several new constraints on the formation and evolution of ordinary chondrite parent bodies (and by extension most planetesimals) from newly available spectral measurements and mineralogical analysis of main belt S-type asteroids (83 objects) and unequilibrated ordinary chondrite meteorites (53 samples). Based on the latter, we suggest spectral data may be used to distinguish whether an ordinary chondrite was formed near the surface or in the interior of its parent body. If these constraints are correct, the suggested implications include that: i) large groups of compositionally similar asteroids are a natural outcome of planetesimal formation and, consequently, meteorites within a given class can originate from multiple parent bodies; ii) the surfaces of large (up to ~200km) S-type main-belt asteroids expose mostly the interiors of the primordial bodies, a likely consequence of impacts by small asteroids (D<10km) in the early solar system (Ciesla et al. 2013); iii) the duration of accretion of the H chondrite parent bodies was likely short (instantaneous or in less then ~10^5 yr but certainly not as long as 1 Myr); iv) LL-like bodies formed closer to the Sun than H-like bodies, a possible consequence of radial mixing and size sorting of chondrules in the protoplanetary disk prior to accretion.
    Full-text · Article · May 2014 · The Astrophysical Journal

  • No preview · Article · Apr 2014 · Geochimica et Cosmochimica Acta
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    ABSTRACT: We report the global distribution of high-Ca pyroxene on lunar highland regions revealed by the Spectral Profiler onboard the lunar explorer SELENE (Kaguya).
    No preview · Article · Feb 2014
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    ABSTRACT: We analyzed the continuous 3-µm spectra of dark asteroids using PCA, suggesting that the shape of absorption band around 2.7 µm may be controlled by serpentine.
    Preview · Article · Feb 2014
  • S. Koga · S. Sugita · S. Kamata · M. Ishiguro · T. Hiroi · S. Sasaki
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    ABSTRACT: We performed PCA for spectra of Itokawa using high-resolution multiband images and found a possible spectral evolution track of S-type asteroids.
    No preview · Article · Feb 2014
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    ABSTRACT: We report the performance of Near-Infrared Spectrometer (NIRS3) on Hayabusa-2, confirmed by ground tests using the flight mode.
    Full-text · Article · Feb 2014

Publication Stats

3k Citations
481.24 Total Impact Points


  • 1993-2015
    • Brown University
      • Department of Geological Sciences
      Providence, Rhode Island, United States
  • 2011
    • The University of Aizu
      • School of Computer Science and Engineering
      Hukusima, Fukushima, Japan
  • 2009
    • SETI Institute
      Mountain View, California, United States
  • 1992-1997
    • NASA
      Вашингтон, West Virginia, United States
  • 1995
    • Massachusetts Institute of Technology
      • Department of Earth Atmospheric and Planetary Sciences
      Cambridge, Massachusetts, United States
  • 1990-1994
    • The University of Tokyo
      • Department of Earth and Planetary Science
      Tōkyō, Japan