Spectral diversity and photometric behavior of main-belt and near-Earth vestoids and (4) Vesta: A study in preparation for the Dawn encounter

Kurzfassung

In anticipation of the Dawn Mission to 4 Vesta, we conducted a ground-based campaign of Bessel BVRI filter photometry of five V-type near-Earth asteroids over a wide range of solar phase angles. We also obtained medium-resolution optical spectroscopy (0.38 μm < λ < 0.92 μm; R ∼ 500) of sixteen near-Earth and main-belt V-type asteroids in order to investigate their spectral diversity and to draw connections between spacecraft data of Vesta and V-type asteroids. Our disk-integrated photometry extended the excursion in solar phase angle beyond the maximum of 24° available from Earth for Vesta to 87°, which is more typical of the geometry during the Dawn approach and mapping phases. The majority of our broad-band observations were obtained at the JPL 0.6-m Table Mountain Observatory but multiple nights were also contributed by the Calar Alto 1.2-m and 2.2-m telescopes, as well as by the Purple Mountain 1-m Schmidt. Our results include a determination of rotation periods for 4 asteroids, identification of a binary candidate and four new V-type asteroids, including a confirmation of two main-belt V-type asteroids beyond the Jupiter 1:3 resonance (Cruikshank, D.P., Tholen, D.J., Bell, J.F., Hartmann, W.K., Brown, R.H. [1991]. Icarus 89, 1–13; Lazzaro, D. et al. [2000]. Science 288, 2033–2035; Roig, F., Gil-Hutton, R. [2006]. Icarus 183(2), 411–419; Moskovitz, N.A., Jedicke, R., Gaidos, E., Willman, M., Nesvorný, D., Fevig, R., Ivezić, Ž. [2008]. Icarus 198, 77–90). This latter finding supports the hypothesis that some vestoids may be crustal fragments of a disrupted basaltic parent body compositionally similar to 4 Vesta. We also obtained rotationally resolved medium resolution spectra of Vesta during the Dawn orbit insertion phase, which will be valuable for calibration and comparison of spacecraft data. Modeling of a composite V-type asteroid phase curve yielded a generic photometric model for V asteroids. We also find that a significant amount of the spectral diversity in the V class comes from changes in solar phase angle. A fit of a composite solar phase curve containing our vestoid observations, previously published groundbased observations of Vesta, and early disk-integrated Dawn observations show important differences with other asteroids. The macroscopic surface roughness of V-type asteroids is significantly larger than that of C-type or S-types (Helfenstein, P., Veverka, J. [1989]. Physical characterization of asteroid surfaces from photometric analysis. In: Binzel, R., Gehrels, T., Matthews, M.S. (Eds.), Asteroids II. University of Arizona Press, Tucson, pp. 557–593). This result is consistent with radar studies showing that igneous rocky asteroids – the E and V types – exhibit the largest surface roughness (Benner, L. et al. [2008]. Icarus 198, 294–304). The effects of what appears to be space weathering can be largely explained by phase reddening in our collection of V-type NEOs, but our finding that smaller vestoids, which have shorter lifetimes, are more similar to Vesta suggests that some type of alteration of the surface through time occurs. Our observations confirm that the south polar region of Vesta has a more diogenitic composition than its equatorial regions. The south pole, which is dominated by a large impact feature, thus may offer a view into the interior of Vesta. We derive a visible phase integral of 0.44 ± 0.02 and a corresponding Bond albedo of 0.15 ± 0.03 from our composite V-type asteroid solar phase curve.