Irradiation history of Itokawa regolith material deduced from noble gases in the Hayabusa samples.
ABSTRACT 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 (21)Ne. 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.
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ABSTRACT: Meteorites are the oldest rocks in our solar system, and therefore record the earliest stages of its evolution. On the basis of their mineralogy, petrography, bulk chemistry, and oxygen-isotope compositions, meteorites are classified into more than 70 groups. Each group is believed to represent samples of one or a few larger bodies that formed early in the solar system and may still be represented by asteroids (1) in the main asteroid belt, which lies between the orbits of Mars and Jupiter. Various types of asteroids have been identified based on their spectral properties. Linking the meteorite groups to these asteroid types is critical for assessing the meteorite data and using it as a basis for inferring, for example, the composition and growth of Earth and other planets. Because we have lacked direct samples of specific asteroids, we have had to rely on remote observations of asteroids and relate them to the meteorite groups. This has been problematic because the surfaces of asteroids have been heavily modified by space weathering, which changes their spectral properties. To establish such a connection and understand the role of space weathering, the JAXA (Japan Aerospace Exploration Agency) spacecraft Hayabusa was sent to near-Earth asteroid 25143 Itokawa, which is 0.5 by 0.3 by 0.2 km in size to collect and bring back to Earth samples of its surface (see the figure). Itokawa is an S-type asteroid, the most abundant spectral type of asteroid in the inner asteroid belt.Science 08/2011; 333(6046):1098-9. · 31.20 Impact Factor
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ABSTRACT: 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+).Science 08/2011; 333(6046):1121-5. · 31.20 Impact Factor