Space Weathering
By David
Polishook – Tel-
Differences between the spectral
types of NEAs and MBAs that have the same origins (Chapman 2004) led to the
conclusion that NEAs are not different from MBAs, but rather represent an evolutionary
stage of known taxonomies. Binzel et al. (1996), found a continuum distribution
of the 1μm band strength in asteroid spectra, between the relatively
shallow band depths of the S-type MBAs asteroids to the relatively deep
(therefore interpreted as fresh) Q-type NEAs. From this, it seems that the
small Q-type NEAs (the largest with a diameter of ~5 km) could be understood as
fragments, with freshly exposed surfaces, originating from larger, S-type, old
MB asteroids. According to this theory, the "old" objects have been
exposed to "space weathering", a variety of effects such as solar
wind, cosmic rays and micrometeorite bombardment that modify the optical
characteristic of their surfaces (Clark et al. 2002). The effects of space
weathering have been measured on lunar soil and on grained meteorites that was
exposed to extreme radiation bombardment in the laboratory. Images with
transmission electron microscopy (TEM) of individual lunar soil grains suggest
that the different causes for the "space weathering" effect are
vaporizing minerals on the surface. When these minerals sink back to the
surface, they are altered in a way that rims of Fe metal particles are abundant
on the "weathered" surface. These metal particles change the
reflectance spectra of the asteroid to appear redder and darker as they obscure
the spectral properties of the minerals below them.
Binary asteroids formed from a
disrupted single object might present fresh surfaces that were unexposed to
space weathering on the specific body section where the disruption took place.
This is true only if the disruption time is shorter than the timescale of space
weathering. In this way, the time of disruption of the progenitor asteroid into
a binary system can be determined. In addition, observing fresh areas allows a
glimpse to the asteroids' interior and a possibility to compare it with the exposed
material on the surface. Therefore, binary asteroids are good candidates for
finding color variation as the asteroid rotates and for linking between
different aspects of asteroids evolution such as disruption and space
weathering effects.