“Space weathering” is the term applied to darkening albedo, reddening spectral slope, and obscuring absorption bands of planetary surface materials with time. The mismatch between reflectance spectra of most common asteroids (S-type asteroids) and most common meteorites (ordinary chondrites) might be caused by the space weathering. Recent laboratory experiments simulating micrometeorite impact heating confirmed Hapke’s old hypothesis that the spectral darkening / reddening are caused by formation of nanophase iron particles. In the meantime, a recent study of celestial mechanics discovered a new-born group of asteroids, “Karin cluster group”, which is thought to be remnants of a collisional breakup only 5.8 million years ago.
We have conducted a near-infrared (J, H, and K bands) spectroscopy of the brightest asteroid 832 Karin among the Karin cluster group. The spectroscopic observation was performed by the Subaru telescope with the Cooled Infrared Spectrograph and Camera for OHS on 2003 September 14. For different rotational phases of Karin, we derived different spectra such as reddened spectrum like that of S-type asteroid and un-reddened spectrum like that of ordinary chondrite. Karin could be an impact fragment preserving an old surface and is probably one of the cone-shaped fragments at low-velocity impact that formed the Karin cluster group. Our result supports the idea that S-type asteroids are parent bodies of ordinary chondrites.
Observational phase of 832 Karin
Light-curve of 832 Karin is based on data obtained by the 1.8-m VATT at the Vatican observatory at Mt. Graham, Arizona, by the 1-m Schmidt telescope at the Kiso ovservatory, Japan, and by the 40-cm telescope in Fukuoka University of Education, Japan. Our observation corresponds to the phase of rotations of red, green, and blue regions. This figure was adopted and modified from Yoshida et al. (2004).
Reflectance specctra of 832 Karin (2 sets)
Red points: first set of 832 Karin
Blue points: last set of 832 Karin
Violet squares: S(IV)-type asteroid 584 Semiramis (from SBN Data Set 52 Color)
Green diamonds: L6 ordinary chondrite Paranaiba (from RELAB Public Spectroscopy Database)
These data are normalized to the unity at 1.0 um. The data points corresponding to the mean values for every 10 pixels are plotted with their error-bars of standard deviation. The data are removed in the wavelength range where the telluric absorptions were strong and the error-bars are large.
In the same season, Yoshida et al. (2004) performed a multispectral (B, V, R, and I bands) Karin observation, and their visible observations of Karin are consistent with our observation. Thus, the spectral changes according to the rotational phase due to space weathering were detected in both the visible and the infrared observations.