Vesta’s north pole quadrangle Av-1 (Albana): Geologic map and the nature of the south polar basin antipodes

Abstract

As part of systematic global mapping of Vesta using data returned by the Dawn spacecraft, we have produced a geologic map of the north pole quadrangle, Av-1 Albana. Extensive seasonal shadows were present in the north polar region at the time of the Dawn observations, limiting the ability to map morphological features and employ color or spectral data for determination of composition. The major recognizable units present include ancient cratered highlands and younger crater-related units (undi-vided ejecta, and mass-wasting material on crater floors). The antipode of Vesta's large southern impact basins, Rheasilvia and Veneneia, lie within or near the Av-1 quadrangle. Therefore it is of particular interest to search for evidence of features of the kind that are found at basin antipodes on other planetary bodies. Albedo markings known as lunar swirls are correlated with basin antipodes and the presence of crustal magnetic anomalies on the Moon, but lighting conditions preclude recognition of such albedo features in images of the antipode of Vesta's Rheasilvia basin. ''Hilly and lineated terrain,'' found at the antipodes of large basins on the Moon and Mercury, is not present at the Rheasilvia or Veneneia antip-odes. We have identified small-scale linear depressions that may be related to increased fracturing in the Rheasilvia and Veneneia antipodal areas, consistent with impact-induced stresses (Buczkowski, D. et al. [2012b]. Analysis of the large scale troughs on Vesta and correlation to a model of giant impact into a differentiated asteroid. Geol. Soc. of America Annual Meeting. Abstract 152-4; Bowling, T.J. et al. [2013]. J. Geophys. Res.-Planets, 118. http://dx.doi.org/10.1002/jgre.20123). The general high elevation of much of the north polar region could, in part, be a result of uplift caused by the Rheasilvia basin-forming impact, as predicted by numerical modeling (Bowling, T.J. et al. [2013]. J. Geophys. Res.-Planets, 118. http://dx.doi.org/10.1002/jgre.20123). However, stratigraphic and crater size-frequency distribution analysis indicate that the elevated terrain predates the two southern basins and hence is likely a remnant of the ancient vestan crust. The lack of large-scale morphological features at the basin antipodes can be attributed to weakened antipodal constructive interference of seismic waves caused by an oblique impact or by Vesta's non-spherical shape, or by attenuation of seismic waves because of the physical properties of Vesta's interior. A first-order analysis of the Dawn global digital elevation model for Vesta indicates that areas of permanent shadow are unlikely to be present in the vicinity of the north pole.

Geologic map of the northern hemisphere of Vesta based on Dawn Framing Camera (FC) images

“…The observed broad N-S trend of some lineations and their appearance as incised grooves suggests that material ejected from the Rheasilvia has impacted in the northern hemisphere. Possible Rheasilvia antipodal effects are discussed further by Blewett et al (2014).…”

“…Alternatively, an uneven excavation by subsequent impacts would be required. The topographic high could also be due to antipodal uplift, as discussed in Blewett et al (2014), or be a remnant of a large-scale ridge. The elevated area might also be, in part, the result of an event predating the basins.…”

“…Additional studies, for example of tectonic features observed in the northern hemisphere, are presented in Scully et al (2014). A study of the effects of the south pole basins antipodes (Rheasilvia and Veneneia) is presented in Blewett et al (2014).…”

Composition of the northern regions of Vesta analyzed by the Dawn mission

“…The northern regions, because of their distance from the Rheasilvia and Veneneia basins, should be least disturbed by the giant impacts. As evidence of this, geological mapping of the northern regions indicates terrains older than most of Vesta (Yingst et al, 2014;Blewett et al, 2014;Ruesch et al, 2014a;Scully et al, 2014), and visible-near-infrared spectroscopy from telescopic data indicates a surface primarily eucritic (Shestopalov et al, 2010). Old surfaces, typical of the northern regions, present challenges to the analysis of their composition, because meteoritic impacts over a long period of time resulted in mixing of pure materials, eventually masking the original endmember materials and making the composition gradually more homogenous over time.…”

Introduction: The geologic mapping of Vesta

“…Blewett et al (2014) investigate antipodal terrains associated with the Rheasilvia basin, mostly in the Av-1 (Albana) north polar quadrangle.…”