Space weathering

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 many weathering processes:
* collisions of galactic cosmic rays and solar cosmic rays,
* irradiation, implantation, and sputtering from solar wind particles, and
* bombardment by all sizes of meteorites and micrometeorites. Space weathering is important because these processes affect the physical and optical properties of the surface of many planetary bodies. Therefore, it is critical to understand the effects of space weathering in order to properly interpret remotely sensed data.

History

Much of our knowledge of the space weathering process comes from studies of the lunar samples returned by the Apollo program, particularly 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 first products of space weathering that were recognized in lunar soils were agglutinates. Agglutinates are created when micrometeorites melt a small amount of material, which incorporates surrounding glass and mineral fragments into a glass-welded aggregate ranging in size from a few micrometers to a few millimeters. Agglutinates appear black to the human eye, largely due to the presence of nanophase iron. Agglutinates are very common in lunar soil, accounting for as much as 60 to 70% of mature soils.

Space weathering also produces 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. It wasn't until the 1990s that improved instruments and techniques allowed for the discovery of very thin (60-200 nm) 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. [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.] These weathering processes have large affects on the spectral properties of lunar soil, particularly in the UV/Vis/NIR wavelengths.

Effects on spectral properties

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 its diognostic absorption bands are reduced [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.] 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 systems, 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 a thorough review of the current state of understanding of space weathering on Asteroids, see Chapman, C.R. (2004) Space Weathering of Asteroid Surfaces. Annual Review of Earth and Planetary Sciences 32, 539-567.] 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, the most abundant type, 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. [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.] 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.

Space weathering on Mercury

The environment at Mercury also differs substantially from the Moon. For one thing, it is significantly hotter in the day (diurnal surface temperature ~100 °C for the Moon, ~425 °C on Mercury) and colder at night, which may alter the products of space weathering. In addition, because of its location in the solar system, Mercury is also subjected to a slightly larger flux of micrometeorites that impact at much higher velocities than the Moon. These factors combine to make Mercury much more efficient than the Moon at creating both melt and vapor. Per unit area, impacts on Mercury are expected to produce 13.5x the melt and 19.5x the vapor than is produced on the Moon. [Cintala, M.J. (1992) Impact-Induced Thermal Effects in the Lunar and Mercurian Regoliths. J. Geophys. Res., 97, 947-973. ] Agglutinitic glass-like deposits and vapor-deposited coatings should be created significantly faster and more efficiently on Mercury than on the Moon.

The UV/Vis spectrum of Mercury, as observed telescopically from Earth, is roughly linear, with a red slope. There are no absorption bands related to Fe-bearing minerals, such as pyroxene. This means that either there is no iron on the surface of Mercury, or else the iron in the Fe-bearing minerals has been weathered to nanophase iron. A weathered surface would then explain the reddened slope. [Hapke, B. (2001) Space Weathering from Mercury to the asteroid belt. J. Geophys. Res., 106, 10039-10073.]

References

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