832 Karin Shows No Rotational Spectral Variations

832 Karin Shows No Rotational Spectral Variations

Clark R. Chapman, B. Enke, W.J. Merline, D. Nesvorný,

P. Tamblyn, and E.F. Young

Clark R. Chapman, B. Enke, W.J. Merline, D. Nesvorný,

P. Tamblyn, and E.F. Young

Southwest Research InstituteBoulder, Colorado, USA

Presentation No. 71.08, Fri. 1:30-3:00 pm 38th Annual AAS/DPS Meeting

Pasadena, California USA 8-13 October 2006

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)

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

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

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

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

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.

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

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)