Analysis of the First Disk-resolved Images of Ceres from Ultraviolet Observations with the Hubble Space Telescope

We present Hubble Space Telescope Faint Object Camera observations of the asteroid 1 Ceres at near-, mid-, and far-UV wavelengths (λ = 3636, 2795, and 1621 Å, respectively) obtained on 1995 June 25. The disk of Ceres is well resolved for the first time, at a scale of ∼50 km. We report the detection of a large, ∼250 km diameter surface feature for which we propose the name "Piazzi"; however, it is presently uncertain whether this feature is due to a crater, albedo variegation, or other effect. From limb fits to the images, we obtainsemimajor and semiminor axes of R₁ = 484.8 ± 5.1 km and R₂ = 466.4 ± 5.9 km, respectively, for the illumination-corrected projected ellipsoid. Although albedo features are seen, they do not allow for a definitive determination of the rotational motion and pole position of Ceres, particularly because of the sparse sampling (two epochs) of the 9 hr rotation period. From full-disk integrated albedo measurements, we find that Ceres has a red spectral slope from the mid- to near-UV, and a significant blue slope shortward of the mid-UV. Despite the presence of Piazzi, we detect no significant global differences in the integrated albedo as a function of rotational phase for the two epochs of data we obtained. From Minnaert surface fits to the near- and mid-UV images, we find an unusually large Minnaert parameter of k ≈ 0.9, suggesting a more Lambertian than lunar-like surface.