Rotationally Resolved Spitzer Spectra of Comet-Asteroid Transition Object 944 Hidalgo
American Astronomical Society, Department of Planetary Sciences
Last year (Campins et al. 2006), we reported near-infrared rotational variability in ground-based spectra of comet-asteroid transition object 944 Hidalgo. Since then, we carried out a rotationally resolved study of Hidalgo at mid-infrared wavelengths using the Infrared Spectrograph (IRS) on NASA's Spitzer Space Telescope. We obtained 7 to 38 micron spectra of Hidalgo at 10 different rotational phases. These observations were carried out on July 24, 2006, when Hidalgo was at heliocentric and Spitzer-centric distances of 4.83 AU and 4.84 AU. In an initial analysis, we normalized the spectra with a thermal model fit to the continuum (which varied as the cross section of this non-spherical object changed with rotational phase). No detectable rotational variability in the emissivity was found across the wavelength range. All the spectra show clear emissions from silicates. These emissions are qualitatively similar to those seen in the spectra of Trojan asteroids (Emery et al. 2006) and in the spectrum of comet Hale-Bopp (Crovisier et al. 1997). Given the lack of emissivity variability, we averaged all our spectra and compared them with the other Spitzer spectrum of Hidalgo, which was obtained as part of the guaranteed time observations (GTO) on February 10, 2005 when Hidalgo was at heliocentric and Spitzer-centric distances of 1.96 AU and 1.71 AU. Although the 2005 spectrum has better signal-to-noise than the combined 2006 spectra, the two are identical within the uncertainties, save for changes in the thermal continuum. It is not clear why there is spectral variability in the near-infrared and not the longer wavelengths. One possible explanation is that the mineralogy across Hidalgo's surface is similar but some areas have been affected differently by space weathering, i.e., one or more collisions may have exposed fresh material on some of Hidalgo's surface.
Campins, Humberto & Kelley, Michael & Fernandez, Yanga & Ziffer, Julie & Licandro, Javier & Emery, Joshua & Cruikshank, Dale & Hergenrother, Carl & Pinilla-Alonso, Noemi & Clautice, Devon. (2007). "Rotationally Resolved Spitzer Spectra of Comet-Asteroid Transition Object 944 Hidalgo." AAS/Division for Planetary Sciences Meeting #39.