Photometric properties of the nucleus of Comet 103P/Hartley 2

We have studied the photometric properties of the nucleus of a hyperactive comet, 103P/Hartley 2, at visible wavelengths using the DIXI flyby images with both disk-integrated and disk-resolved analyses. The disk-integrated phase function of the nucleus has a linear slope of 0.046 ± 0.002 mag/deg and an absolute magnitude of 18.4 ± 0.1 at V-band. The nucleus displays an overall linear, featureless spectrum between 400 nm and 850 nm. The linear spectral slope is 7.6 ± 3.6% per 100 nm, corresponding to broadband solar-illuminated color indices B–V of 0.75 ± 0.05 and V–R of 0.43 ± 0.04. Disk-resolved photometric analysis with a Hapke model returns a best-fit single-scattering albedo of 0.036 ± 0.006, an asymmetry factor of the Henyey–Greenstein single-particle phase function of −0.46 ± 0.06, and a photometric roughness of 15 ± 10°. The model yields a geometric albedo of 0.045 ± 0.009 and a Bond albedo of 0.012 ± 0.002. The overall photometric variations of the nucleus are small, with an equivalent albedo variation of 15% FWHM, and a color variation of 12% FWHM. Some areas near the terminator visible in the inbound images show an albedo of more than twice the global average value, and a much bluer color than the average nucleus. The overall photometric properties and variations of the nucleus of Hartley 2 are similar to those of the nuclei of Comets Wild 2 and Tempel 1 as studied from previous spacecraft flyby missions at similar resolutions.