A COMPOSITIONAL AND GEOLOGICAL VIEW OF FRESH EJECTA OF SMALL IMPACT CRATERS ON ASTEROID 4 VESTA

Since the arrival of NASA’s Dawn spacecraft [1] at Vesta the Visible and InfraRed Imaging Spectrometer (VIR) detects the spectral properties of Vesta’s surface between 0.4 and 5 µm [2, 3]. As part of the analysis of Vesta’s surface composition and geology small morphologically fresh impact craters with distinct ejecta blankets offer the possibility to study more or less unweathered surface material. The ejecta appear bright or dark in the visible light with a sharp contrast to the surrounding region. Mostly, the craters themselves are characterized by a similar albedo. Only a few impact craters show bright ejecta and a dark crater floor [4]. In general, bright ejecta exhibit howardite/eucrite-like spectra with relatively deep pyroxene absorptions near 1 and 2 µm compared to their surroundings. In the case of dark ejecta the pyroxene absorptions are strongly reduced possibly due to the addition of carbonaceous material. Although, the absorption near 2µm is barely visible at all, the position of the absorption near 1µm is similar to the position measured for the bright ejecta, which implies no significant changes in the pyroxene composition. Changes in the pyroxene composition indicated by a shift in the band positions could only be identified in the vicinity of one impact crater characterized by bright ejecta and dark crater material, which is directly located at a huge scarp marking the rim of the Rheasilvia impact basin [4]. Both materials show a pronounced pyroxene signature, even if they have different albedos. The bright ejecta show the typical spectral characteristics as mentioned above. The positions of the pyroxene absorptions measured in the crater and slumping material extending from the crater itself, however, occur at shorter wavelength, more characteristic for diogenitic material, and strengthen the thesis that deeper parts of Vesta’s subsurface are of more diogenitic composition [4].

We gratefully acknowledge the support of the Dawn Instrument, Operations, and Science Teams and especially Th. Roatsch, F. Preusker, J. Sunshine and L. McFadden.

References: [1] Russell C.T et al. (2012) Science, 336. [2] De Sanctis M.C. et al., (2010) Space Sci. Rev. [3] De Sanctis M.C. et al. (2012) Science, 336. [4] Stephan, K. et al. (2012), ACM, #6300.