Abstract

We have utilized the Spitzer Space Telescope InfraRed Spectrograph (IRS) to directly observe thermal emission from the nucleus of comet 9P/Tempel 1 on UT 2004 March 25 to 27. We obtained 8-35 μm low-resolution (R ~ 100) spectra and contemporaneous 16 and 22-μm photometric imaging over a 39-hour period. The comet was 3.7 AU from the Sun at the time and showed no evidence of extended emission beyond a point source. Visual inspection of the absolute photometry implies a rotation period of 40 ± 2 hours, consistent with earlier results. Snapshot photometry by Spitzer at 8 and 24 μm, taken on UT 2004 March 10 and 15 respectively, are consistent with this lightcurve phasing and with the IRS-measured flux. The spectra closely follow the spectral energy distribution (SED) of a thermal model with a thermal inertia between 0 and 50 J/K/m2/s1/2. The spectra are inconsistent with a rapid rotator. The mean effective radius at the middle of the lightcurve is 3.3 ± 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 about 3, implying a = 7.0 ± 0.2 km and b = 2.3 ± 0.2 km. Combining our SST infrared lightcurve with visible observations of the nucleus, we obtain a visible geometric albedo of 0.04 ± 0.01. With this sized nucleus and the published water production rates, we estimate that 9 ± 2% of the surface area is actively emitting volatile material at perihelion.