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Curvilinear gullies, lobate deposits and fractures, and their implications for the geologic evolution and hydration of Vesta


Much of our understanding of the early solar system is based on observations of Vesta and its associated howardite-eucrite-diogenite (HED) meteorites. Vesta is a largely intact protoplanet that formed at ~4.56 Ga (Russell et al., 2012). From 2011-2012, the Dawn spacecraft characterized in situ the physical, compositional and geological properties of this important body. Here we contribute to this characterization through geologic observations and interpretations of gully systems, troughs, grooves, (pit) crater chains and minor ridges. In detailed geologic maps, we identify curvilinear gullies and lobate deposits in eight impact craters. Using associated geomorphic features, analogs, quantitative modeling and experimental work, we propose transient meltwater, released from impact-heated, localized, sub-surface ice-bearing deposits, formed curvilinear gullies in a debris-flow-like process within newly formed impact craters. The meltwater rapidly evaporated because of the surface conditions, and contributed to the formation of pitted terrain. In the northern hemisphere and equatorial regions, we map in detail troughs, grooves, (pit) crater chains and minor ridges. Troughs may be interpreted as (half-)grabens, formed by strains induced by basin-forming impacts in the southern hemisphere. We additionally propose that the basin-forming impacts resulted in the formation of grooves and (pit) crater chains as ejecta ray systems and as the surface expression of dilational cracks. While we show Vesta has been modified at length by impact cratering and impact-induced fracturing, it has remained largely intact since the beginning of the solar system. Therefore, we test hypotheses about the early evolution of the Earth against the Dawn-derived knowledge of Vesta. The proposed sub-surface ice-bearing deposits are consistent with an expanded understanding of water in the solar system (Saal et al., 2008; Hsieh and Jewitt, 2006; Küppers et al., 2014), which suggests that our solar system may contain a continuum of bodies with many intermediate hydration states. Based on our study, and the D/H ratios of vestan meteorites (Sarafian et al., 2014), we postulate that within the first ~10 million years of the solar system, hydrated impactors could have been the source of localized sub-surface ice-bearing deposits on Vesta, and could also have been the source of water on the Earth.

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