832 Karin Shows No Rotational Spectral Variations
Abstract
Sasaki et al. (2004, 2005) claimed that 832 Karin, the brightest member of the very young (5.75 Myr) Karin cluster of the Koronis family, shows dramatically different colors as a function of rotational phase. It was interpreted that Karin is a fragment of the recently broken-up asteroid, showing the reddish space-weathered exterior surface of the precursor asteroid as well as an interior face, which has not had time to become space-weathered. On five nights during UT 7-14 January 2006, we observed Karin with the SpeX instrument, 0.8-2.5 microns, on the IRTF. We sampled its spectrum well throughout its rotation. We analyzed the data in 50 deg. intervals of rotational longitude; some longitudes were sampled during two different nights. We find that Karin exhibits minimal spectral variations with rotation, certainly nothing of the magnitude reported by Sasaki et al. Since our data resemble Sasaki et al.'s "blue" and "green" sets, we suggest that their "red" set is spurious. Indeed, it is difficult to understand how the reported color change could have occurred during such a modest interval ( 4%) of rotational longitude. (Note that we have not determined Karin's pole position nor the phase of the Sasaki et al. data within our own coverage, so the refutation of dramatic color change is not absolutely secure.) Karin and its family members are not quite as red as typical S-types, yet have shallow absorption bands. Perhaps the space-weathering process affecting these young asteroids has had time to reduce spectral contrast, but has not operated long enough to redden them -- an intermediate case of space weathering, which has gone to completion for older main-belt asteroids of these sizes.
Supported by the NASA Planetary Astronomy Program. T. Sasaki et al. 2004. ApJ 615, L161-L164; T. Sasaki et al. 2005. LPSC XXXVI, 1590.pdf.- Publication:
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AAS/Division for Planetary Sciences Meeting Abstracts #38
- Pub Date:
- September 2006
- Bibcode:
- 2006DPS....38.7108C