Space Weathering - Space Weathering On Asteroids

Space Weathering On Asteroids

Space weathering is also thought to occur on asteroids, though the environment is quite different from the Moon. Impacts in the asteroid belt are slower, and therefore create less melt and vapor. Also, fewer solar wind particles reach the asteroid belt. And finally, the higher rate of impactors and lower gravity of the smaller bodies means that there is more overturn and the surface exposure ages should be younger than the lunar surface. Therefore, space weathering should occur more slowly and to a lesser degree on the surfaces of asteroids.

However, we do see evidence for asteroidal space weathering. For years there had been a so-called "conundrum" in the planetary science community because, in general, the spectra of asteroids do not match the spectra of our collection of meteorites. Particularly, the spectra of S-type asteroids, did not match the spectra of the most abundant type of meteorites, ordinary chondrites (OCs). The asteroid spectra tended to be redder with a steep curvature in the visible wavelengths. However, Binzel et al. have identified near Earth asteroids with spectral properties covering the range from S-type to spectra similar to those of OC meteorites, suggesting an ongoing process is occurring that can alter the spectra of OC material to look like S-type asteroids. There is also evidence of regolith alteration from Galileo's flybys of Gaspra and Ida showing spectral differences at fresh craters. With time, the spectra of Ida and Gaspra appear to redden and lose spectral contrast. Evidence from NEAR Shoemaker's x-ray measurements of Eros indicate an ordinary chondrite composition despite a red-sloped, S-type spectrum, again suggesting that some process has altered the optical properties of the surface. Results from the Hayabusa spacecraft at the asteroid Itokawa, also ordinary chondrite in composition, shows spectral evidence of space weathering. In addition, definitive evidence of space weathering alteration has been identified in the grains of soil returned by the Hayabusa spacecraft. Because Itokawa is so small (550 m diameter), it was thought that the low gravity would not allow for the development of a mature regolith, however, preliminary examination of the returned samples reveals the presence of nanophase iron and other space weathering effects on several grains. In addition, there is evidence that weathering patinas can and do develop on rock surfaces on the asteroid. Such coatings are likely similar to the patinas found on lunar rocks.

There is evidence to suggest most of the color change due to weathering occurs rapidly, in the first hundred thousands years, limiting the usefulness of spectral measurement for determining the age of asteroids.