Other
Scientific paper
Aug 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011psrd.repte.158t&link_type=abstract
Planetary Science Research Discoveries
Other
Itokawa, Asteroid, S-Type, S-Type Asteroid, Ll Chondrite, Chondrite Meteorite, Ordinary Chondrite, Sample Return, Hayabusa
Scientific paper
The Hayabusa spacecraft, flown by the Japanese Aerospace Exploration Agency (JAXA), returned samples from asteroid 25143 Itokawa on June 13, 2010. Though the sampling device did not operate properly, the mission was able to return a couple thousand particles, from a few- to a few hundred-micrometers across. A battery of laboratory analyses (electron microscopy, elemental analysis, and oxygen isotopic measurements) shows that the particles derive from materials like those in thermally metamorphosed LL group ordinary chondrites. Astronomical observations had classified Itokawa as a stony S(IV) type of asteroid. The nature of S-type asteroids has been debated for decades; some astronomers argued that S-type asteroids are ordinary chondrites while others suggested that they were more likely to be differentiated objects (i.e., melted or partially melted to make igneous rocks). The problem was that we did not know enough about space weathering on asteroids to know how the spectra of chondritic or differentiated asteroids changed with exposure to micrometeorites and solar wind. The examinations of Hayabusa's treasure have settled the argument: S-type asteroid, Itokawa, indeed has an ordinary chondrite composition whose spectrum has been reddened by space weathering. This conclusion is supported by detailed studies of the surfaces of 10 Itokawa particles, half of which have glassy rims (5-50 nanometers thick) containing nano-sized particles of iron sulfide and metallic iron, signatures of space weathering. Analysis of noble gases in three particles from the asteroid indicate that they were exposed on the surface of Itokawa for surprisingly short times, less than 8 million years. The Hayabusa science team suggests that the short exposure time indicates loss of particles into space through small impacts at the surprisingly fast rate of tens of centimeters per million years. This might not seem fast, but the asteroid, only 535 x 294 x 209 meters in size, would become just a dusty memory in no more than a cosmically-short billion years.
Jeffrey Taylor G.
Martel Linda M. V.
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