科研之友
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摘要
Photometry of asteroids gives fundamental information about their spectral and physical properties. The aim of this work is two-fold: (1) to calculate phase functions of Vesta and Lutetia in the visible spectral range; and (2) to compare photometric properties of all the asteroids visited by space missions, as inferred from disk-resolved photometry. The phase functions of Vesta and Lutetia have been retrieved by performing a statistical analysis on data provided by the VIR-Dawn and the VIRTIS-Rosetta imaging spectrometers, respectively. The approach is based on the empirical procedure defined in Longobardo et al. (Longobardo, A. et al. [2014]. Icarus 240, 20-35). The Vesta phase functions have been calculated at two wavelengths, one outside (0.75 mu m) and one inside (0.95 mu m) the pyroxene absorption band at 0.9 mu m. The steepness of the phase function at 0.75 mu m decreases from dark to bright regions, due to the increasing role of multiple scattering. Otherwise, the phase function at 0.95 mu m results in uniformity across Vesta surface, since darkening agents are spectrally featureless and their influence at wavelengths inside the pyroxene absorption band is negligible. Moreover, it is, on average, steeper than the phase functions at 0.75 mu m, due to the more important role of single scattering at 0.95 mu m. The Lutetia phase function is instead constant across the surface due to the homogeneous spectral properties of this asteroid. The obtained photometric curves (reflectance versus phase angle) of Vesta and Lutetia have been then compared with those retrieved in previous works on asteroids visited by space missions. Differently from comparisons of disk-integrated phase functions of asteroids performed in previous works at low phase angles (lower than 25 degrees), this work restricts to asteroid observations that are disk-resolved and occur at solar phase angles between 20 degrees and 60 degrees. The S-type asteroids (Gaspra, Ida, Eros and Annefrank) show similar photometric curves. The phase functions found in bright material units on Vesta are similar to those found for Steins (E-type in the Tholen taxonomy, Xe-type in the Bus one), suggesting a photometric analogy between achondritic surfaces. The latter are brighter and with a flatter phase function with respect to chondritic surfaces: we argued that this behavior is driven by optical properties of asteroid surfaces (e.g. albedo, role of multiple scattering) rather than by physical ones (e.g. grain size, roughness). Dark material units on Vesta show an intermediate behavior between achondrites and the C-type Mathilde, confirming once again that these regions are characterized by mixtures of HED and carbonaceous chondrites. While a clear anti-correlation is observed between reflectance and steepness of phase function for V, S and C asteroids, Lutetia shows an anomalous photometric behavior, presenting both a low reflectance and a flat phase curve, and hence cannot be grouped with other spectral classes here considered. This behavior is similar to some X-type asteroids ground-observed at low phase angles and is consistent with a chondritic composition of its surface.
- 出版日期2016-3-15
