1. 832 Karin Shows No Rotational Spectral Variations
Clark R. Chapman, B. Enke, W.J. Merline, D. Nesvorný, P. Tamblyn, and E.F. Young
Southwest Research Institute
Boulder, Colorado, USA
Presentation No , Fri. 1:30-3:00 pm
38th Annual AAS/DPS Meeting
Pasadena, California USA 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 2006μm
Good rotational phase coverage, analyzed in 7 50° intervals
Solar analog stars for air-mass correc-tion and solar calibration
Generally excellent skies during 5 nights
Remote & on-site observing
Funding from NASA PAST
Bins
(assuming 18.3h period)

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 are no ob-viously significant differences between these spectra
Certainly, nothing resembling the Sasaki et al. “red” spectrum is 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
Could our constant spectra be because we were observing pole-on?
We observed just 0.5 Karin-year later, in the opposite direction from where Sasaki and Yoshida et al. observations showed 0.7 mag. lightcurve amplitude.
Pole-on views would be approximately 90° away from Karin’s ecliptic longitude in Jan
There is no plausible way we could have missed a “red” hemisphere 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
We thank Bobby Bus and the IRTF staff, Cathy Olkin, and NASA’s Planetary Astronomy Program.
Wavelength (μm)