AAS/Division for Planetary Sciences Meeting Abstracts
The data number of the raw image pixels are converted [2] to I/F. The observation geometry of each pixel is calculated using the Ryugu shape model [4]. We fit a linear function to the phase function plot within the narrow phase angle range (0.2-1.7°). Using this function, each pixel's I/F was extrapolated to normal albedo. Finally, we create a mosaic map of normal albedo for each band.
From this map, a v-band (0.55 μm) average normal albedo is derived as 4.06 ± 0.10%. This value is consistent with a geometric albedo (albedo in disk-integrated definition) of 4.0 ± 0.5%, derived from recent photometry study [5]. The recent color studies [e.g. 1, 4, 5, 6] of Ryugu reported that the spectral slope from b-band (0.48 μm) to x-band (0.86μm) exhibits the greatest regional variation on Ryugu. Our normal albedo map also confirms this spectral character of Ryugu. Additionally, we found that the v-band normal albedo is well correlated with the spectral slope.
Since the observed brightness at the opposition condition is less affected by shadows or topographic undulation than other geometries, the derived map successfully shows the albedo distribution under minimal noise conditions. This approach may be useful for a future mission of a bumpy surface asteroids.
1: Sugita et al. (2019) Science 364, eaaw0422. ; 2: Tatsumi E. et al. (2019) Icarus 325, 153-195. ; 3: Hapke B.W. (1981) JGR 86, 3039-3054. ; 4: Watanabe S. et al. (2019) Science 364, 268-272. ; 5: Tatsumi E. et al. (2020), A&A 639, A83. ; 6: Morota et al. (2020) Science 368, 654-659.