Galileo Photometry of Asteroid 243 Ida

Galileo imaging observations over phase angles 19.5° to 109.8° are combined with near-opposition Earth-based data to derive the photometric properties of Ida. To first order these properties are uniform over the surface and well modeled at λ = 0.55 μm by Hapke parameters ω₀= 0.22,h= 0.020,B₀= 1.5,g= -0.33, and θ = 18° with corresponding geometric albedop= 0.21±^0.03_0.01and Bond albedoA_B= 0.081±^0.017_0.008. Ida's photometric properties are more similar to those of "average S-asteroids" (P. Helfenstein and J. Veverka 1989,Asteroids II, Univ. of Arizona Press, Tucson) than are those of 951 Gaspra. Two primary color units are identified on Ida: Terrain A exhibits a spectrum with relatively shallower 1-μm absorption and a relatively steeper red spectral slope than average Ida, while Terrain B has a deeper 1-μm absorption and a less steep red slope. The average photometric properties of Ida and Terrain A are similar while those of Terrain B differ mostly in having a slightly higher value of ω₀(0.22 versus 0.21), suggesting that Terrain B consists of slightly brighter, more transparent regolith particles. Galileo observations of Ida's satellite Dactyl over phase angles 19.5° to 47.6° suggest photometric characteristics similar to those of Ida, the major difference being Dactyl's slightly lower albedo (0.20 compared to 0.21).