Rotationally Resolved 8-35 Micron Spitzer Space Telescope Observations of the Nucleus of Comet 9P/Tempel 1

We have utilized the Spitzer Space Telescope (SST) Infrared Spectrograph (IRS) to directly observe thermal emission from the nucleus of comet 9P/Tempel 1 on UT 2004 March 25-27. We obtained 8-35 km low-resolution (R 6 100) spectra and contemporaneous 16 and 22 km photometric imaging over a 39 hr period. The comet was 3.7 AU from the Sun at the time, approximately 464 days before perihelion on 2005 July 5, and showed no evidence of extended emission beyond a point source. Visual inspection of the absolute photometry implies a rotation period of 40 c 2 hr, consistent with earlier results. Snapshot photometry by Spitzer at 8 and 24 km, taken on UT 2004 March 10 and 15, respectively, are consistent with this light-curve phasing and with the IRS-measured flux. The spectra agree well with the predictions of the standard thermal model for a slowly rotating body with thermal inertia between 0 and 50 J K super(-1) m super(-2) s super(-1/2), and are inconsistent with any rapid rotator model. The mean effective radius at the middle of the light curve is 3.3 c 0.2 km. The maximum-to-minimum flux ratio of 1.8 in the light curve is consistent with an axial ratio a/b of 3.2 c 0.4, implying a = 7.2 c 0.9 km and b = 2.3 c 0.3 km. Combining our SST infrared light curve with visible observations of the nucleus, we obtain a visible geometric albedo of 0.04 c 0.01. With this sized nucleus and the published water production rates, we estimate that 9% c 2% of the surface area is actively emitting volatile material at perihelion.