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Hiroi, T.

Publications and source records attributed to Hiroi, T..

34 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS2004

Characteristics and Current Status of Near Infrared Spectrometer for Hayabusa Mission

NIRS is a near infrared spectrometer on-boarded the spacecraft HAYABUSA (MUSESC), which aims to return samples from a near-earth asteroid, (25143) Itokawa (1998 SF36). HAYABUSA was successfully launched by Japanese M-V-5 rocket on May 19, 2003. After the earth swing-by in Jun 2004, the spacecraft will arrive at the asteroid in summer 2005. During the rendezvous phase with the asteroid, we will observe the asteroid surface using NIRS and obtain reflectance spectra of the surface materials across the wavelength range of 850nm to 2100nm. Based on ground-based observations [1],[2],[3], (25143) Itokawa appears to be an S(IV) type asteroid. NIRS can detect absorption bands due to olivine and pyroxene and investigate the mineralogical composition of the surface materials. Combining with the data from asteroid multiband imaging camera (AMICA) and X-ray spectrometer (XRS), we can reveal a relationship between asteroids and meteorites.

Abe, M.↗
NASA NTRS2004

Reflectance Spectra of CM2 Chondrite Mighei Irradiated with Pulsed Laser and Implications for Low-Albedo Asteroids and Martian Moons

Micrometeoritic bombardment is an important space weathering process modifying surface optical properties of airless solar system bodies. We have used irradiation with a microsecond pulsed laser as an experimental method to simulate such a process on various targets. The experiment discussed here was performed on a powdered sample of CM2 carbonaceous chondrite Mighei. Shingareva et al. report the details of experimental procedure as well as the results of mineralogical and chemical studies of the irradiated material. Here we present reflectance spectra of irradiated Mighei samples and discuss their spectral properties compared to those of non-irradiated meteorite and low-albedo small solar system bodies.

Moroz, L. V.↗
NASA NTRS2003

The Tagish Lake Meteorite as a Possible Sample from a T or D Type Asteroid

Since the Tagish Lake meteorite fell in January 2000, the one of the kind meteorite has become the hottest issue among meteoriticists as well as astrobiologists. Meanwhile, concerning the physical origin of the meteorite in our solar system, only Hiroi et al. reported that it likely came from a D asteroid based on comparison of their visible-near-infrared (Vis-NIR) reflectance spectra. However, because asteroid classification is usually performed based on a limited range of wavelength, the asteroid classes and surface compositions may not necessarily have one-to-one correspondence. Also, Hiroi et al. precluded the T-type asteroids simply because their average albedo is much higher than the visible reflectance of Tagish Lake although their spectral matches with Tagish Lake were good, and the 3-mm band spectra compiled in were not enough to compare the degree of hydration or kinds of hydrous minerals between primitive asteroids and Tagish Lake. In this paper, we employ more spectral datasets up to the 3-mm range to address the possible connection between two particular asteroids, 773 Irmintraud and 308 Polyxo, and the Tagish Lake meteorite.

Hiroi, T.↗
NASA NTRS2003

Effects of Microsecond Pulse Laser Irradiation on Vis-NIR Reflectance Spectrum of Carbonaceous Chondrite Simulant: Implications for Martian Moons and Primitive Asteroids

Goal of this study is to make a progress in understanding the optical effects of space weathering on small bodies believed to be similar in composition to carbonaceous chondrites: C, G, B, F, T, D, and P asteroids and possibly Martian satellites Phobos and Deimos. The companion work focuses on petrological and mineralogical aspects of this process. One of the main factors of space weathering is meteorite and micrometeorite bombardment leading, in particular, to impact melting of components of the regolith. Studies of lunar regolith and laboratory experiments simulating impact melting show that the melting products differ from the unmelted material in mineralogy and distribution of chemical components among different phases that results in spectral changes. We simulate impact melting of CM chondrite by pulse laser irradiation of an artificial analog of such a meteorite. The analog is a mixture of 46 wt.% non-magnetic fraction of L5 ordinary chondrite Tsarev, 47 wt.% serpentine, 5 wt.% kerite, and 2 wt.% calcite. It simulates rather well bulk chemistry, including volatiles such as H2O and CO2, and only approximately the CM chondrite mineralogy. Thus, we do not expect the mixture to be spectrally similar to CM chondrites, but expect the laser melting products to be similar to those formed by impact melting of natural CM chondrites.

Hiroi, T.↗
NASA NTRS2003

Surface Material Analysis of the S-type Asteroids: Removing the Space Weathering Effect from Reflectance Spectrum

Recent years, many researchers have been observing a lot of asteroid reflectance spectra in the UV, visible to NIR at wavelength region. Reflectance spectroscopy of asteroid at this range should bring us a lot of information about its surface materials. Pyroxene and olivine have characteristic absorption bands in this wavelength range. Low-Ca pyroxene has two absorption bands around 0.9 microns and 1.9 microns. The more Ca and Fe content, the longer both absorption band centers. On the other hand, reflectance spectrum of olivine has three complicated absorption bands around 1 m, and no absorption feature around 2 microns. In general, reflectance spectra of many asteroids that are considered to be silicate rich (i.e., S- and A type asteroids) show redder slope and more subdued absorption bands than those of terrestrial minerals and meteorites. These features are now believed to be caused by the space weathering effect, which is probably caused by micrometeorite bombardment and/or solar wind. This process causes nanophase reduced iron (npFe(sup 0)) particles near the surface of mineral grains, which leads the optical change. Therefore, the space weathering effect should be removed from asteroid reflectance spectra to compare with those of meteorite and terrestrial minerals. In this report, we will apply the expanded modified Gaussian model (MGM) to the reflectance spectra of S-type asteroids 7 Iris and 532 Herculina and compare them with those of meteorites.

Ueda, Y.↗
NASA NTRS2001

Discovery of the First D-Asteroid Spectral Counterpart: Tagish Lake Meteorite

We have discovered the first meteorite, Tagish Lake, which shows a reflectance spectrum very similar to that of the D asteroids, especially 368 Haidea. D and P asteroids have been believed to be "supercarbonaceous" unlike any meteorite. Additional information is contained in the original extended abstract.

Hiroi, T.↗
NASA NTRS1994

Spectral analysis of ALH 84001, a meteorite from Mars

ALH 84001 has recently been reclassified as a meteorite from Mars (SNC) and contains more than 90% orthopyroxene with minor chromite and accessory phases of augite, maskelynite, and carbonate. This meteorite represents a new class of igneous material from Mars. We have measured reflectance spectra of ALH 84001 as a chip as a powder, dry sieved to less than 125 microns to compare with previous spectral analyses of SNCs and remote observations of Mars. Spectra of the chip and powder in the visible-to-near-infrared region are shown. These spectra are composites of data measured with the RELAB bidirectional spectrometer from 0.3 to 2.55 microns and a Nicolet FTIR for longer wavelengths. As expected, the spectra of the chip have negative slopes and are significantly darker than the spectrum of the particulate sample, which has a positive slope. The strong absorptions near 1 micron and 2 microns are characteristic of low-Ca pyroxene and have band rninima of 0.925 microns and 1.930 microns. The strong absorption near 3 microns is characteristic of water. There is a distinct flattening in the spectrum between 1.0 and 1.5 microns indicating the presence of an additional absorption. This is interpreted to be the result of Fe(2+) in the M1 site of low-Ca pyroxene. Mid-infrared spectra showing the Christiansen feature and the reststrahlen bands are shown for spectra of the powder and of three different locations on the chip. These spectra exhibit several features in this range, some of which are associated with a specific region on the chip. Each of the spectra includes a doublet reststrahlen peak near 1100/cm, and peaks near 880 and 500/cm, which are typical for low-Ca pyroxenes. Weaker features at 940-1000/cm, 600-750/cm, and 530-560/cm are present in spectra from some locations on the chip, but not others, implying compositional and textural variation.

Bishop, J.↗
NASA NTRS1994

Mineralogy of artificially heated carbonaceous chondrites

We have examined suites of heated Murchison (CM2) and Allende (CV3) samples heated in the range 400-1200 C, in a H2 atmosphere with a pressure of 10(exp 5) bar for periods of 1 to 4 weeks. We used a combination of X-ray diffraction, electron microprobe and TEM analyses to determine the mineralogy of these samples.

Zolensky, M. E.↗
NASA NTRS1994

Possible spinel absorption bands in S-asteroid visible reflectance spectra

Minor absorption bands in the 0.55 to 0.7 micron wavelength range of reflectance spectra of 10 S asteroids have been found and compared with those of spinel-group minerals using the modified Gaussian model. Most of these S asteroids are consistently shown to have two absorption bands around 0.6 and 0.67 micron. Of the spinel-group minerals examined in this study, the 0.6 and 0.67 micron bands are most consistent with those seen in chromite. Recently, the existence of spinels has also been detected from the absorption-band features around 1 and 2 micron of two S-asteroid reflectance spectra, and chromite has been found in a primitive achondrite as its major phase. These new findings suggest a possible common existence of spinel-group minerals in the solar system.

Hiroi, T.↗
NASA NTRS1994

Moessbauer mineralogy of calcined Murchison meteorite

The three Antarctic meteorites B7904, Y82162, and Y86720 are unusual because they have characteristics in common with both CI and CM groups and because they apparently underwent thermal alteration after hydrous alteration on their parent body. They are also spectrally similar (visible and near-IR) to C, G, B, and F asteroids, which may imply that the surface materials on those asteroids may have undergone thermal alteration. Based on the reflectance spectra of samples of Murchison (CM2 carbonaceous chondrite) that were thermally altered in the laboratory (cryopumped and initial 10(exp -5) atm H2), Hiroi et al. concluded that putative thermal alteration occurred at temperatures of 600 to 1000 C. Similar experiments have been done on Murchison and reported mineralogical changes based on data from transmission electron diffraction microscopy, electron diffraction, and analytical electron microscopy. We report here the Mossbauer mineralogy of the same samples of thermally-altered Murchison. Mossbauer mineralogy gives the molar distribution of Fe among its oxidation states and iron-bearing mineralogies.

Morris, Richard V.↗
NASA NTRS1993

Comparison of reflectance spectra of C asteroids and unique C chondrites Y86720, Y82162, and B7904

Reflectance spectra (0.3-2.6 microns) of 11 carbonaceous (C) chondrite powders (less than 100 or less than 125 microns) including 'unique' ones Y86720, Y82162, and B7904, have been measured and compared with those of 14 C asteroids. Among those C chondrites, only three 'unique' ones had close counterparts among C asteroids (Y86720 and 1 Ceres, Y82162 and 704 Interamnia, and B7904 and 31 Euphrosyne). Mixing calculations of those C chondrites by Hapke's isotropic model were also performed to improve fits of reflectance spectra of all the 14 C asteroids. If the grain-size distributions of those asteroid surfaces are similar to those C chondrite powders, the result suggests that all tested C asteroids contain a high amount of heated C chondrite material, such as Y86720, Y82162, and B7904.

Hiroi, T.↗
NASA NTRS1993

Comparison of visible and near-infrared reflectance spectra of CM2 carbonaceous chondrites and primitive asteroids

Spectra of primitive asteroids (defined as C, P, and D classes and associated subclasses) were compared to the limited number of spectra of CM2 carbonaceous chondrites. An absorption feature located at 0.7 microns attributed to an Fe(+2) - Fe(+3) charge transfer absorption in iron oxides in phyllosilicates is apparent in some of the CM2 carbonaceous chondrite spectra and many of the asteroid spectra. Sawyer found a correlation between the area of the 0.7 micron feature and the mean semimajor axis of the asteroids. Spectra of a larger sample of carbonaceous chondrites, including 7 CM2 chondrites, covering a spectral interval of 0.30-2.5 microns were recently obtained using the Relab instrument at Brown University. These spectra were compared with spectrophotometric asteroid observations in a separate abstract. Those spectra of CM2 chondrites were isolated into the UV, visible and near-infrared spectral regions in order to compare them with high-quality narrowband reflectance spectra.

Vilas, F.↗
NASA NTRS1992

Effects of grain size and shape in modeling reflectance spectra of mineral mixtures

An 'isograin model' of reflectance from particulate material is presented. The model treats reflectance as a series of grain interactions, incorporating optical constants of each material and two parameters that model the activity of scattering and absorption in the vertical and the horizontal directions, respectively. Mixing ratios of components in mixture are treated as variables and are calculated using least-squares fit. Grain size and shape have a significant effect on the accuracy of predicting mixing ratios but can be modeled by introducing the concept of effective grain size. The model calculates spectra in good agreement with measured spectra and models reflectance within 1 percent and mixing ratios within 5 wt pct.

Hiroi, T.↗
NASA NTRS1991

Effects of grain size and shape in modeling reflectance spectra of mineral mixtures

The effects of grain size and shape on the reflectance spectra of mineral mixtures are investigated to improve a reflectance model called the isograin model, whose prototype was proposed by M. Kinoshita in 1985. The sample powder was assumed to consist of an infinite number of layers, each of which has the same thickness with the grain size d.

Hiroi, T.↗