Kei Shirai

Itokawa Dust Particles: A Direct Link Between S-Type Asteroids and Ordinary Chondrites

Laboratory analysis of samples returned from an asteroid establishes a direct link between asteroids and meteorites and provides clues to the complex history of the asteroid and its surface. The Hayabusa spacecraft successfully recovered dust particles from the surface of near-Earth asteroid 25143 Itokawa. Synchrotron-radiation x-ray diffraction and transmission and scanning electron microscope analyses indicate that the mineralogy and mineral chemistry of the Itokawa dust particles are identical to those of thermally metamorphosed LL chondrites, consistent with spectroscopic observations made from Earth and by the Hayabusa spacecraft. Our results directly demonstrate that ordinary chondrites, the most abundant meteorites found on Earth, come from S-type asteroids. Mineral chemistry indicates that the majority of regolith surface particles suffered long-term thermal annealing and subsequent impact shock, suggesting that Itokawa is an asteroid made of reassembled pieces of the interior portions of a once larger asteroid.

Incipient Space Weathering Observed on the Surface of Itokawa Dust Particles

Laboratory analysis of samples returned from an asteroid establishes a direct link between asteroids and meteorites and provides clues to the complex history of the asteroid and its surface. The reflectance spectra of the most abundant meteorites, ordinary chondrites, are different from those of the abundant S-type (mnemonic for siliceous) asteroids. This discrepancy has been thought to be due to space weathering, which is an alteration of the surfaces of airless bodies exposed to the space environment. Here we report evidence of space weathering on particles returned from the S-type asteroid 25143 Itokawa by the Hayabusa spacecraft. Surface modification was found in 5 out of 10 particles, which varies depending on mineral species. Sulfur-bearing Fe-rich nanoparticles exist in a thin (5 to 15 nanometers) surface layer on olivine, low-Ca pyroxene, and plagioclase, which is suggestive of vapor deposition. Sulfur-free Fe-rich nanoparticles exist deeper inside (<60 nanometers) ferromagnesian silicates. Their texture suggests formation by metamictization and in situ reduction of Fe2+.

X-ray Fluorescence Spectrometry of Asteroid Itokawa by Hayabusa

X-ray fluorescence spectrometry of asteroid 25143 Itokawa was performed by the x-ray spectrometer onboard Hayabusa during the first touchdown on 19 November 2005. We selected those data observed during relatively enhanced solar activity and determined average elemental mass ratios of Mg/Si = 0.78 ± 0.09 and Al/Si = 0.07 ± 0.03. Our preliminary results suggest that Itokawa has a composition consistent with that of ordinary chondrites, but primitive achondrites cannot be ruled out. Among ordinary chondrites, LL- or L-chondrites appear to be more likely than H-chondrites. No substantial regional difference was found on the asteroid surface, indicating its homogeneity in composition.

Irradiation History of Itokawa Regolith Material Deduced from Noble Gases in the Hayabusa Samples

Laboratory analysis of samples returned from an asteroid establishes a direct link between asteroids and meteorites and provides clues to the complex history of the asteroid and its surface. Noble gas isotopes were measured in three rocky grains from asteroid Itokawa to elucidate a history of irradiation from cosmic rays and solar wind on its surface. Large amounts of solar helium (He), neon (Ne), and argon (Ar) trapped in various depths in the grains were observed, which can be explained by multiple implantations of solar wind particles into the grains, combined with preferential He loss caused by frictional wear of space-weathered rims on the grains. Short residence time of less than 8 million years was implied for the grains by an estimate on cosmic-ray–produced 21Ne. Our results suggest that Itokawa is continuously losing its surface materials into space at a rate of tens of centimeters per million years. The lifetime of Itokawa should be much shorter than the age of our solar system.

Lunar X-ray spectrometer experiment on the SELENE mission

Abstract Major elemental mapping of lunar surface has been planned using the XRS instrument on the SELENE orbiter. Arrayed charge-coupled devices (CCD) are used due to its high energy resolution. By thermal design of the XRS with radiation cooling, CCD is kept cool enough to reduce excitation of thermal electrons and improve signal-to-noise ratio. During lunar x-ray fluorescence spectrometry, concurrent monitoring of solar x-rays as well as in situ calibration of x-ray fluorescence using a standard sample is performed for quantitative elemental analysis. Observed data are processed with on-board electronics that effectively extracts x-ray events. In one-year mission, the XRS will cover all the lunar surfaces except for polar region and map elemental composition with spatial resolution of 20km. We present instrumentation of the XRS and its current status of development, as well as showing the XRS observation plan of lunar elemental mapping.

Shock wave and fracture propagation in water ice by high velocity impact

In order to clarify the elementary processes of impact disruption, we conducted impact experiments with water ice at an impact velocity of 3.6 km/s and observed shock wave and fracture propagation in it by means of ultra-high speed photography. We observed that a region in which HEL (Hugoniot elastic limit) followed the elastic precursor wave, expanded with a velocity of 3–2.5 km/s until the pressure fell below 240 MPa. Below that pressure, a damage region appeared 0.8–3 µs after the passage of precursor wave. In this region, dynamic shear strength of water ice was estimated to be 21 MPa. Below 80 MPa, the several radial cracks proceeded toward the rear surface and broke the sample before the tensile fracture caused by reflection waves from an antipodal point became visible. Therefore, the main mechanism to cause the largest fragment is the radial crack growth rather than a spallation at the rear.

Shock pressure attenuation in water ice at a pressure below 1 GPa

Shock pressure attenuation in water ice was studied at an impact pressure below 1 GPa and a temperature of 255 K. The observed shock wave showed a multiple shock wave structure: A precursor wave was followed by a main wave, which had a longer rise time and higher amplitude. The Hugoniot elastic limit (HEL) of water ice was measured to be in the range from 0.1 to 0.3 GPa when associated with precursor waves traveling at 3.86 km/s. The peak amplitude of the main wave Pm was observed to decrease with its propagation x from 3 to 60 mm (from 0.4 to 8 times as large as a projectile radius) in two series of experiments in which initial shock pressures Pi at the impact point were 0.60 and 0.87 GPa. The Pm was described as the power law relation Pm/Pi = (x/2.6 mm)−89. The precursor wave disappears as the Pm attenuated to a pressure <0.1 GPa. The measured wave profiles were used to calculate the loading path of water ice in shock compression between the HEL and 0.6 GPa. The loading path obtained by Lagrangian analysis was closely consistent with previous Hugoniot data regarding water ice.

Numerical estimation of lunar X-ray emission for X-ray spectrometer onboard SELENE

We conducted a numerical estimation of lunar X-ray spectra, which is applicable for lunar X-ray fluorescence observations using an X-ray spectrometer (XRS) onboard the SELENE orbiter, with an improved simulation model. We investigated the integration times of measurements for six elements (Mg, Al, Si, Ca, Ti, and Fe) to achieve signal-to-background ratio of over 10 under various solar conditions. The results of these calculations indicate that expected along-the-track spatial resolutions of a single orbital path for Mg, Al and Si will be <90 km and 20 km under normal and active Sun conditions, respectively. Ca, Ti and Fe will be also detectable with a spatial resolution of 20 km during the periods active solar flares over M1 class happen to occur.

Instrumentation and performance evaluation of the XRS on SELENE orbiter

The x-ray fluorescence spectrometer (XRS) on board Japanese lunar polar orbiter SELENE (Kaguya) will provide global distribution of major elemental composition on the lunar surface in energy range of characteristic K-α x-ray line emission for Mg, Al, Si, Ca, Ti, and Fe. These measurements will contribute to research of lunar origin and its evolution. The XRS shows a good energy resolution within 200 eV at 5.9 keV relying on charge coupled device (CCD) as photon energy dispersive detector. Total collective area of 100 cm2 for main detector facing the lunar surface is composed of 16 CCD chips. Instrumentation of the XRS and its performance evaluated in laboratory are presented.

X-ray fluorescence/diffraction analyzer for the SELENE-B lander/rover mission

Abstract A miniaturized X-ray fluorescence and diffraction analyzer is being developed for the SELENE-B, a future Japanese lunar lander and rover mission, to analyze major elemental composition and mineralogy. An onboard micro X-ray tube with a fine focus collimator is used to generate primary X-rays that excite fluorescence X-rays characteristic of major elements in collected samples; the primary X-rays are also scattered into X-ray diffraction pattern reflecting lattice structures of the component minerals. By using a two-dimensional charge-coupled device, pulse height analysis for XRF and diffraction pattern extraction for XRD will be simultaneously carried out. The instrument covers an energy detection range from 1 to 10 KeV and measures diffraction angles from 20 to 60 degrees for elemental and mineral analysis, respectively. We show the results of laboratory experiments conducted with alumina powder.