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.
Surface materials on airless solar system bodies exposed to interplanetary are gradually changed their visible to near-infrared reflectance spectra by the process called space weathering, which makes the spectra darker and redder. Hapke et al. proposed a model of weathering: vapor deposition of nanophase reduced iron (npFe(sup 0)) on the surfaces of the grains within the very surface of lunar regolith. This model has been proved by detailed observation of the surfaces of the lunar soil grains by transmission electron microscope (TEM). They demonstrated that npFe(sup 0) was formed by a combination of vapor deposition and irradiation effects. In other words, both micrometeorite impacts and irradiation by solar wind and galactic cosmic ray play roles on the weathering on the Moon. Because there is a continuum of reflectance spectra from those of Q-type asteroids (almost the same as those of ordinary chondrites) to those of S-type asteroids, it is strongly suggested that reflectance spectra of asteroids composed of ordinary chondrite-like materials were modified over time to those of S-type asteroids due to weathering. It is predicted that a small amount of npFe(sup 0) on the surface of grains in the asteroidal regolith composed of ordinary chondrite-like materials is the main agent of asteroidal weathering.
PARTICLES: A SUMMARY OF THE INITIAL ANALYSIS. T. Noguchi, M. Kimura, T. Hashimoto, M. Konno, T. Nakamura, A. Nakato, T. Ogami, H. Ishida, R. Sagae, S. Tsujimoto, A. Tsuchiyama, M. E. Zolensky, M. Tanaka, A. Fujimura, M. Abe, T. Yada, T. Mukai, M. Ueno, T. Okada, K. Shirai, Y. Ishibashi, and R. Okazaki Ibaraki University (2-1-1 Bunkyo, Mito, Ibaraki 310-8512, Japan, tngc@mx.ibaraki.ac.jp), Hitachi High-Technologies Corporation (882 Ichige, Hitachinaka, Ibaraki 312-8504 Japan), Tohoku University (Aoba, Sendai, Miyagi 980-8578, Japan), Department of Earth and Space Science, Osaka University (1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan), NASA/JSC (Houston, Texas 77058, USA), National Institute for Materials Science (1-1-1 Kouto, Sayo, Hyogo 679-5148, Japan), ISAS/JAXA (3-1-1 Yoshinodai, Sagamihara, Kanagawa 252-5210, Japan), Kyushu University (Hakozaki, Fukuoka 812-8581, Japan).
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.
Abstract The mineralogy and mineral chemistry of Itokawa dust particles captured during the first and second touchdowns on the MUSES ‐C Regio were characterized by synchrotron‐radiation X‐ray diffraction and field‐emission electron microprobe analysis. Olivine and low‐ and high‐Ca pyroxene, plagioclase, and merrillite compositions of the first‐touchdown particles are similar to those of the second‐touchdown particles. The two touchdown sites are separated by approximately 100 meters and therefore the similarity suggests that MUSES ‐C Regio is covered with dust particles of uniform mineral chemistry of LL chondrites. Quantitative compositional properties of 48 dust particles, including both first‐ and second‐touchdown samples, indicate that dust particles of MUSES ‐C Regio have experienced prolonged thermal metamorphism, but they are not fully equilibrated in terms of chemical composition. This suggests that MUSES ‐C particles were heated in a single asteroid at different temperatures. During slow cooling from a peak temperature of approximately 800 °C, chemical compositions of plagioclase and K‐feldspar seem to have been modified: Ab and Or contents changed during cooling, but An did not. This compositional modification is reproduced by a numerical simulation that modeled the cooling process of a 50 km sized Itokawa parent asteroid. After cooling, some particles have been heavily impacted and heated, which resulted in heterogeneous distributions of Na and K within plagioclase crystals. Impact‐induced chemical modification of plagioclase was verified by a comparison to a shock vein in the Kilabo LL 6 ordinary chondrite where Na‐K distributions of plagioclase have been disturbed.
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 Fe(2+).
Remote sensing by the spacecraft Hayabusa suggested that outermost surface particles of Muses-C regio of the asteroid Itokawa consist of centimeter and sub-centimeter size small pebbles. However, particles we found in the sample catcher A stored in the Hayabusa capsule, where Muses-C particles were captured during first touchdown, are much smaller. i.e., most are smaller than 100 microns in size. This suggests that only small fractions of Muses-C particles were stirred up due to the impact of the sampling horn onto the surface, or due to jets from chemical thrusters during the lift off of the spacecraft from the surface. X-ray fluorescence and near-infrared measurements by the Hayabusa spacecraft suggested that Itokawa surface materials have mineral and major element composition roughly similar to LL chondrites. The particles of the Muses-C region are expected to have experienced some effects of space weathering. Both of these prospects can be tested by the direct mineralogical analyses of the returned Itokawa particles in our study and another one. This comparison is most important aspect of the Hayabusa mission, because it finally links chemical analyses of meteorites fallen on the Earth to spectroscopic measurements of the asteroids.
One of the fundamental aspects of any astromaterial is its shock history, since this factor elucidates critical historical events, and also because shock metamorphism can alter primary mineralogical and petrographic features, and reset chronologies [1]. Failure to take shock history into proper account during characterization can result in seriously incorrect conclusions being drawn. Thus the Hayabusa Preliminary Examination Team (HASPET) made shock stage determination of the Itokawa samples a primary goal [2]. However, we faced several difficulties in this particular research. The shock state of ordinary chondrite materials is generally determined by simple optical petrographic observation of standard thin sections. The Itokawa samples available to the analysis team were mounted into plastic blocks, were polished on only one side, and were of non-standard and greatly varying thickness, all of which significantly complicated petrographic analysis but did not prevent it. We made an additional estimation of the sample shock state by a new technique for this analysis - electron back-scattered diffraction (EBSD) in addition to standard petrographic techniques. We are also investigating the crystallinity of Itokawa olivine by Synchrotron X-ray diffraction (SXRD).
