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“Space weathering” is a blanket term used for a number of processes that act on any body exposed to the harsh space environment. Airless bodies (including the Moon , Mercury , the Asteroids , Comets , and some of the moons of other planets) incur Galactic Cosmic Ray s and Solar Cosmic Ray s; Irradiation , implantation, and Sputtering from Solar Wind particles; and bombardment by all sizes of Meteorites and Micrometeorites . LUNAR SPACE WEATHERING Much of our knowledge of the space weathering process comes from studies of the lunar samples returned by the Apollo Program , in particular, the lunar soils, or Regolith . The constant flux of high energy particles and micrometeorites, along with larger meteorites, act to comminute, melt, sputter and vaporize components of the lunar soil, as well as to garden (or overturn) it. The products produced by these weathering processes include agglutinates as well as surface-correlated products on individual soil grains, such as glass splashes; implanted hydrogen, helium and other rare gases; solar flare tracks; and accreted components, including nanophase iron. Very thin (60-200nm) patinas, or rims, develop on individual lunar soil grains as a result of the redepositing of vapor from nearby micrometeorite impacts and the redeposition of material sputtered from nearby grains. These weathering processes have large effects on the spectral properties of lunar soil, particularly in the UV/Vis/NIR Wavelengths . The spectral effects of space weathering are threefold: as a surface matures it becomes darker (the Albedo is reduced), redder (reflectance increases with increasing wavelength), and the depth of it's diognostic Absorption Band s are reduced. These effects are largely due to the presence of Nanophase Iron in both the agglutinates and in the accreted rims on individual grains. The darkening effects of space weathering are readily seen by studying lunar craters. Young, fresh craters have bright Ray System s, because they have exposed fresh, unweathered material, but over time those rays disappear as the weathering process darkens the material. SPACE WEATHERING ON ASTEROIDS Space weathering is also thought to occur on asteroids. For years there had been a so-called "conundrum" in the planetary science community because, at that time, the spectra of asteroids did not generally match the spectra of our collection of meteorites. Particularly, the spectra of S-type Asteroid s, the most abundant type, did not match the spectra of the most abundant type of meteorites, Ordinary Chondrite s (OCs). The asteroid spectra tended to be redder with a steep curvature in the visible wavelengths. However, Binzel et al. (1996) 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. More recent 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. REFERENCES Binzel R.P., Bus S.J., Burbine T.H. and Sunshine J.M. 1996. Spectral Properties of Near-Earth Asteroids: Evidence for Sources of Ordinary Chondrite Meteorites. Science. 273, 946-948. Chapman, C.R. 2004. Space Weathering of Asteroid Surfaces. Annual Review of Earth and Planetary Sciences 32, 539-567. Hapke, B. 2001. Space Weathering from Mercury to the asteroid belt. J. Geophys. Res., 106, 10039-10073. Keller L. P and McKay D. S. (1997) The nature and origin of rims on lunar soil grains. Geochimica et Cosmochimica Acta 61:2331-2341. Pieters C. M., Fischer E. M., Rode O. and Basu A. (1993) Optical Effects of Space Weathering: The Role of the Finest Fraction. Journal of Geophysical Research 98, 20,817-20,824. |
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