¹Jean-Philippe Combe et al. (>10)*
¹Bear Fight Institute, 22 Fiddler’s Road, P.O. Box 667, Winthrop, WA 98862, USA
*We currently do not have a copyright agreement with this publisher and cannot display the abstract here

The surface composition of the northern regions of Vesta, observed by the Dawn spacecraft, offers the possibility to test several hypotheses related to impact-related processes. We used mostly imaging spectrometry in the visible and near infrared to assess the distribution of mafic lithologies, hydrated components and albedo properties, and use the link with howardite, eucrite and diogenite meteorites (HEDs) to investigate the origin of those materials. We established that Rheasilvia ejecta reached part of the northern regions, and have a diogenitic-rich composition characteristic of the lower crust. Investigations of the antipodes of the two major impact basins (Rheasilvia and Veneneia) did not reveal any correlation between geographic location, geological features and the surface composition. The northern wall of Mamilia crater, which is one of the freshest craters above 22°N, contains relatively pure eucritic-rich, diogenitic-rich and dark, hydrated materials, which are representative of the rest of the northern regions (and most of Vesta), with the exception of an olivine-like component found in Bellicia crater by Ammannito et al. (2013, Nature Volume 504, Issue 7478, pp. 122-125). We determined that similar types of materials are found in various proportions over a large region, including Bellicia, Arruntia and Pomponia craters, and their origin does not seem to be related to Rheasilvia ejecta. These materials are hydrated, which could indicate an exogenous origin, and not as dark as expected for carbonaceous chondrites, which likely compose the majority of dark hydrated materials on Vesta. Spectral mixture analysis reveals that mixtures of pyroxenes (hypersthene, pigeonite and diopside) could offer an alternative interpretation to olivine in this area.

Reference
Combe J-P et al. (2015) Composition of the northern regions of Vesta analyzed by the Dawn Mission. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.04.026]

Copyright Elsevier

¹Sherry K. Fieber-Beyer, ¹Michael J. Gaffey
¹Department of Space Studies, University Stop 9008, University of North Dakota, 58202

The research is an integrated effort beginning with telescopic observations and extending through detailed mineralogical characterizations to provide constraints on the composition and meteorite affinities of a subset of fourteen asteroids in/near the 3:1 Kirkwood Gap. Eight asteroids were identified as having either one or two absorption features, while six were deemed featureless. The compositional analysis of asteroids (355) Gabriella and (1447) Utra reveal Fs and Fa values which are consistent with values for the L-type ordinary chondrites (Fs19-22 and Fa22-26). The location of these two bodies with respect to each other and to the previously identified L-chondrite parent body asteroid (1722) Goffin, suggests a small L-chondrite genetic family. These results support the model that the L-chondrites come from an asteroid family rather than from a single object. Asteroids (1368) Numidia, (1587) Kahrstadt, (1854) Skortsov, (2497) Kulikovskij, and (5676) Voltaire were analyzed and determined to have “basaltic” silicate mineralogies similar to those of the HED (howardite – eucrite – diogenite) meteorite group. In particular, we found that the compositions of (1368), (1587) and (1854) are consistent with olivine-orthopyroxenitic diogenites, while (2497) and (5676)’s compositions are consistent with harzburgitic diogenites. The Band I and Band II absorption feature depths are much shallower than seen in diogenite spectra, typically ∼70% depth ( Burbine et al. 2000). The nature of the weak features seen in the asteroid spectra when compared to measured band depths of in situ diogenite samples indicate an additional mechanism(s) acting to weaken the features, most likely space weathering. The aforementioned five asteroids are plausible sources for the olivine-orthopyroxenitic diogenites and harzburgitic diogenites, and very well may be fragments of Vesta. Asteroid (46) Hestia is an interesting object whose surface minerals may be consistent with a CR2 chondrite; however, the unique spectrum deserves further study in the future. Featureless asteroids (248) Lameia, (1960) Guisan, (3345) Tarkovskij and (6212) 1993 MS1 surface materials are likely organic assemblages consistent with the Type 1 or 2 carbonaceous chondrite meteorite class; however specific terrestrial meteorite analog could not be identified. The spectra of asteroids (3228) Pire and (3999) Aristarchus are consistent with each other and have been assigned to the Eulalia by Walsh et al. (2013). Spectrally they are similar to (495) in terms of blue-slope and albedo ( Fieber-Beyer et al. 2012), thus increasing our confidence the three bodies are truly related dynamically and genetically. By extrapolation and due to their location adjacent to the 3:1 Kirkwood Gap, (3228) and (3999) are plausible sources of the CV3OXB carbonaceous chondrites.

Reference
Fieber-Beyer SK, Gaffey MJ (2015) Near-infrared Spectroscopy of 3:1 Kirkwood Gap Asteroids III. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.04.034]

Copyright Elsevier