Deep Impact photometry of Comet 9P/Tempel 1

The photometric properties of the nucleus of Comet 9P/Tempel 1 are studied from the disk-resolved color images obtained by Deep Impact (DI). Comet Tempel 1 has typical photometric properties for comets and dark asteroids. The disk-integrated spectrum of the nucleus of Tempel 1 between 309 and 950 nm is linear without any features at the spectral resolution of the filtered images. At V-band, the red slope of the nucleus is 12.5 ± 1 % per 100 nm at 63° phase angle, translating to B – V = 0.84 ± 0.01 , V – R = 0.50 ± 0.01 , and R – I = 0.49 ± 0.02 . No phase reddening is confirmed. The phase function of the nucleus of Tempel 1 is constructed from DI images and earlier ground-based observations found from the literature. The phase coefficient is determined to be β = 0.046 ± 0.007   mag / deg between 4° and 117° phase angle. Hapkeʹs theoretical scattering model was used to model the photometric properties of this comet. Assuming a single Henyey–Greenstein function for the single-particle phase function, the asymmetry factor of Tempel 1 was fitted to be g = − 0.49 ± 0.02 , and the corresponding single-scattering albedo (SSA) was modeled to be 0.039 ± 0.005 at 550 nm wavelength. The SSA spectrum shows a similar linear slope to that of the disk-integrated spectrum. The roughness parameter is found to be 16 ° ± 8 ° , and independent of wavelength. The Minnaert k parameter is modeled to be 0.680 ± 0.014 . The photometric variations on Tempel 1 are relatively small compared to other comets and asteroids, with a ∼ 20 % full width at half maximum of albedo variation histogram, and ∼ 3 % for color. Roughness variations are evident in one small area, with a roughness parameter about twice the average and appearing to correlate with the complex morphological texture seen in high-resolution images.