Origin of Vestoids Suggested from Their Space Weathering Trend

T. Hiroi, C. M. Pieters (Brown Univ.)

A space-weathering-related trend of 20 Vestoids [1] was studied using HED meteorites [2], laser-irradiated diogenite [3], eucrite impact melt [4], and lunar soils based on the following parameters:

1- tex2html_wrap_inline21 m band depth = tex2html_wrap_inline23 - tex2html_wrap_inline25 , Visible redness = tex2html_wrap_inline23 - tex2html_wrap_inline29

where tex2html_wrap_inline31 , tex2html_wrap_inline33 , and tex2html_wrap_inline35 indicate reflectance at the maximum around 0.74 tex2html_wrap_inline21 m, the 1- tex2html_wrap_inline21 m band center, and 0.55 tex2html_wrap_inline21 m, respectively. The relation between these two parameters is shown in Fig. 1. Vestoids are shown with circles with a size proportional to estimated ejection velocity from Vesta [1].

There is a trend formed by HEDs, majority of Vestoids, and lunar soils. About 8 Vestoids diverge from this trend. These Vestoids do not belong to Vesta family with one exception. Furthermore, the three Vestoids with the largest ejection velocity all fall in this unusual group. The data indicate that many of the Vestoids far from Vesta's orbit are dynamically and spectrally distinct from the others. Perhaps they are more associated with the projectile.

Laboratory spectra were measured at RELAB, a multiuser facility operated under NAGW-748.

References: [1] Binzel R. P. and Xu S. (1993) Science 260, 186. [2] Hiroi T. et al. (1995) Icarus 115, 374. [3] Wasson J. T. et al. (1997) Lunar Planet. Sci. 28, 1505. [4] Hiroi T. (1997) Antarct. Meteorites 22.