Spectral Diversity on (7) Iris and (10) Hygiea Revealed by Broadband Colors

ABSTRACT We present evidence of surface spectral diversity on asteroids (7) Iris and (10) Hygiea based on Johnson–Cousins (B–V, V–R) and ATLAS (c–o) color indices. For Iris, B–V shows significant rotational modulation, revealing localized heterogeneities associated with features such as the Xanthos and Porphyra craters. Its B–R spectral slope, per , matches that of Q/S‐type asteroids, while phase‐resolved variations suggest combined effects of composition, topography, and illumination geometry. In contrast, Hygiea is spectrally uniform: neither B–V nor V–R varies with rotation, and its B–R slope, per , is typical of the B/C‐complex. The c–o distributions for both objects do not show a clear dependence on Earth‐based viewing geometry. We estimate the c–o index of near‐Earth asteroid as 0.5, consistent with a redder K/L‐type surface. This result indicates that c–o can discriminate between S‐ and C‐complex asteroids, similar to V–R. These findings demonstrate the diagnostic value of broadband colors for characterizing asteroid surfaces and provide new insights into the compositional and evolutionary diversity of main‐belt asteroids.