Olivine or impact melt: Nature of the “Orange” material on Vesta from Dawn

Abstract

NASA's Dawn mission observed a great variety of colored terrains on asteroid (4) Vesta during its survey with the Framing Camera (FC). Here we present a detailed study of the orange material on Vesta, which was first observed in color ratio images obtained by the FC and presents a red spectral slope. The orange material deposits can be classified into three types, a) diffuse ejecta deposited by recent medium-size impact craters (such as Oppia), b) lobate patches with well-defined edges (nicknamed "pumpkin patches"), and c) ejecta rays from fresh-looking impact craters. The location of the orange diffuse ejecta from Oppia corresponds to the olivine spot nicknamed "Leslie feature" first identified by Gaffey (1997) from ground-based spectral observations. The distribution of the orange material in the FC mosaic is concentrated on the equatorial region and almost exclusively outside the Rheasilvia basin. Our in-depth analysis of the composition of this material uses complementary observations from FC, the visible and infrared spectrometer (VIR), and the Gamma Ray and Neutron Detector (GRaND). Several possible options for the composition of the orange material are investigated including, cumulate eucrite layer exposed during impact, metal delivered by impactor, olivine-orthopyroxene mixture and impact melt. Based on our analysis, the orange material on Vesta is unlikely to be metal or olivine (originally proposed by Gaffey, 1997). Analysis of the elemental composition of Oppia ejecta blanket with GRaND suggests that its orange material has ~25% cumulate eucrite component in a howarditic mixture, whereas two other craters with orange material in their ejecta, Octavia and Arruntia, show no sign of cumulate eucrites. Morphology and topography of the orange material in Oppia and Octavia ejecta and orange patches suggests an impact melt origin. A majority of the orange patches appear ! %! to be related to the formation of the Rheasilvia basin. Combining the interpretations from the topography, geomorphology, color and spectral parameters, and elemental abundances, the most probable analog for the orange material on Vesta is impact melt. ! &! ! '! 1.0 µm. Dawn FC color data have confirmed several ground-based and HST observations of Vesta including its hemispherical dichotomy, rotational color variations (Reddy et al. 2012b, Reddy et al., in review; Li et al., in review) and the presence of dark carbonaceous chondrite xenoliths (Reddy et al. 2012d). The high-resolution color images obtained by the FC allowed the discrimination of a variety of color units and provided data to study their distribution on the vestan surface. We also use the visible and infrared spectrometer (VIR) an imaging spectrometer combining two data channels: the visible-infrared (VIS) from 0.25 to 1.07 !m, and the infrared (IR) from 0.95 to 5.

The geology of the Marcia quadrangle of asteroid Vesta: Assessing the effects of large, young craters

“…The deposit displays a low albedo in monochrome FC images, but displays a distinctive bright orange color in FC false color ratio images. The composition of this colored material remains unclear, but comparison of FC color and VIR spectra with laboratory spectra of HED meteorites is most consistent with a mixture of impact melt and howardite (Le Corre et al, 2013). This unit generally displays decreasing opacity with increasing distance from source.…”

“…16). The distribution of this diffuse mantling unit is not symmetrical around the crater, but rather extends $190 km to the west-northwest (Le Corre et al, 2013) and into the Av-7 Lucaria quadrangle (Schäfer et al, 2014). To date, FC color and VIR data analysis has not resulted in a specific mineralogical composition of this diffuse mantling material.…”

“…Type area: lat 6°S, long 152°E. Interpretation: Diffuse deposit of impact ejecta and/or impact melt that thinly mantles underlying topography, perhaps including excavated Veneneia ejecta (cf., Le Corre et al, 2013). Distinctive color in color ratio images suggests unique composition, different from that at Oppia crater (Le Corre et al, 2013).…”

“…It has a strong 2.8 mm-absorption indicative of pyroxene, but no other diagnostic features of particular mineral compositions have been detected. Le Corre et al (2013) found from comparison of FC color and VIR data to laboratory spectra of HED meteorites that this material most closely resembles howardite mixed with impact melt, and they suggested the Octavia impact may be excavating Veneneia ejecta as a possible source of the melt. We performed crater counts of parts of the Octavia ejecta blanket, and our results provide a model age of formation of $280-360 Ma using the lunar-derived chronology and $730-990 Ma using the asteroid flux-derived chronology (Fig.…”

“…This unit forms a radial halo where the underlying topography is still apparent, extending beyond one crater diameter. The halo is asymmetric with a wider dispersal to the northwest of the crater, spanning $190 km on its longest axis (Le Corre et al, 2013). The deposit displays a low albedo in monochrome FC images, but displays a distinctive bright orange color in FC false color ratio images.…”