832 Karin Shows No Rotational Spectral Variations

1. 832 Karin Shows No Rotational Spectral Variations Clark R. Chapman, B. Enke, W.J. Merline, D. Nesvorný, P. Tamblyn, and E.F. Young SouthwestResearchInstitute Boulder, Colorado, USA Presentation No. 71.08, Fri. 1:30-3:00 pm 38th Annual AAS/DPS Meeting Pasadena, California USA 8-13 October 2006

2. Spatial Variations of Color (hence Composition) on Asteroids? • 1928: Bobrovnikoff first found spectral variations on Vesta, determined 5h rotation period from their periodicity • Since then: No other asteroid has been found to have such significant color variations • Many that have been reported (e.g. for 433 Eros) have been found later not to be real • By far the most prominent color variations reported have been for 832 Karin (Sasaki et al. 2004)

3. 832 Karin, a Very Young Asteroid • Karin is the brightest member of a “cluster” within the large, well-known Koronis family • Nesvorný et al. (2002) integrated the orbits of Karin cluster members backwards in time and showed that orbital parameters converged at 5.8 My ago, precisely defining the age of the break-up…very young! • Thus Karin cluster members became obvious targets, in comparison with regular Koronis members, for studying time-variable phenomena like space-weathering

4. Sasaki et al. (ApJ, 2004): Big Color Differences! • IR spectra: biggest ever color variation on any asteroid • Rotational phases plotted on Yoshida et al. lightcurve • “Green” spectrum (identical to “blue”) just 15° from “red” • Interpretation: “blue” = fresh, “red” = space-weathered

5. Our IRTF SpeX Observations of Karin, 7-14 January 2006 • IRTF SpeX • 0.8 - 2.4 μm • Good rotation- al phase coverage, analyzed in 7 50° intervals • Solar analog stars for airmass cor. and solar calibration • Generally excellent skies during 5 nights • Remote & on-site observing • Funding from NASA PAST

6. Result: No Significant Spectral Variations with Rotation • Average spectra for 7 bins of rotational phase • Error bars shown for (a) noisiest and (b) best spectrum (sta-tistical errors only) • There is no obvious-ly significant differences between these spectra • Certainly, no dramatic differences are apparent

7. Comparison of Sasaki et al. and Chapman et al. Results • Two results are overlaid • Fair agreement between our average spectrum (black) and their “blue” spectrum • Their “red” result is dramatically inconsistent with our result.

8. Observing Geometry • Only way to explain the spectral constancy of our observations, if there were a real large reddish area, would be if we were looking pole-on while Sasaki et al. were not. • But we observed just 0.5 Karin-year later, in opposite direction from where Sasaki and Yoshida et al. observations showed 0.7 mag. lightcurve amplitude. • It is not plausible that we were observing nearly pole-on. • There is no plausible way Karin could show a “red” hemisphere and we would have missed it in our rotational phase sampling. From JPL “orbit viewer” website

9. Karin’s Reflectance Spectrum:Preliminary Interpretation • As previous observers have noted, Karin’s spectrum is: • Slightly less red than other S-types • Has slightly weaker pyroxene absorption bands • Possibly Karin, as a relatively fresh asteroid, has had time for space-weathering to reduce spectral contrast but not enough time to show substantial reddening • We are currently reducing our other SpeX data on Koronis family members Average spectral reflectance for 832 Karin Wavelength (μm)