Editing 4 Vesta (section)

===Rheasilvia and Veneneia===
{{main|Rheasilvia|Veneneia}}
{{stack|
[[File:Vesta northern and southern hemispheres pia15677.jpg|thumb|Northern (left) and southern (right) hemispheres. The "Snowman" craters are at the top of the left image; Rheasilvia and Veneneia (green and blue) dominate the right. Parallel troughs are seen in both. Colors of the two hemispheres are not to scale,{{efn|1=that is, blue in the north does not mean the same thing as blue in the south.}} and the equatorial region is not shown.]]
[[File:Viewing the South Pole of Vesta.jpg|thumb|South pole of Vesta, showing the extent of Rheasilvia crater.]]
}}

The most prominent of these surface features are two enormous impact basins, the {{convert|500|km|mi|sigfig=1|adj=mid|-wide}} Rheasilvia, centered near the south pole; and the {{convert|400|km|0|abbr=on}} wide Veneneia. The Rheasilvia impact basin is younger and overlies the Veneneia.<ref name="Schenk2012"/> The ''Dawn'' science team named the younger, more prominent crater [[Rheasilvia]], after the mother of Romulus and Remus and a mythical [[vestal virgin]].<ref name="Rheasilvianamed"/> Its width is 95% of the mean diameter of Vesta. The crater is about {{convert|19|km|0|abbr=on}} deep. A central peak rises {{convert|23|km|0|abbr=on}} above the lowest measured part of the crater floor and the highest measured part of the crater rim is {{convert|31|km|0|abbr=on}} above the crater floor low point. It is estimated that the impact responsible excavated about 1% of the volume of Vesta, and it is likely that the [[Vesta family]] and [[V-type asteroid]]s are the products of this collision. If this is the case, then the fact that {{convert|10|km|mi|0|abbr=on}} fragments have survived bombardment until the present indicates that the crater is at most only about 1&nbsp;billion years old.<ref name="Binzel1997"/> It would also be the site of origin of the [[HED meteorite]]s. All the known V-type asteroids taken together account for only about 6% of the ejected volume, with the rest presumably either in small fragments, ejected by approaching the 3:1&nbsp;[[Kirkwood gap]], or perturbed away by the [[Yarkovsky effect]] or [[radiation pressure]]. [[Spectroscopy|Spectroscopic]] analyses of the Hubble images have shown that this crater has penetrated deep through several distinct layers of the crust, and possibly into the [[mantle (geology)|mantle]], as indicated by spectral signatures of [[olivine]].<ref name="Thomas1997b"/>

Subsequent analysis of data from the Dawn mission provided much greater detail on Rheasilvia's structure and composition, confirming it as one of the largest impact structures known relative to its parent body size.<ref name="Schenk2012"/> The impact clearly modified the pre-existing very large, Veneneia structure, indicating Rheasilvia's younger age.<ref name="Schenk2012"/> Rheasilvia's size makes Vesta's southern topography unique, creating a flattened southern hemisphere and contributing significantly to the asteroid's overall oblate shape.<ref name="Jaumann2012"/> Rheasilvia's ~22 km central peak stands as one of the tallest mountains identified in the Solar System.<ref name="Schenk2012"/> Its base width of roughly 180 km and complex morphology distinguishes it from the simpler central peaks seen in smaller craters.<ref name="Ivanov2013">{{cite journal |last1=Ivanov |first1=B. A. |last2=Melosh |first2=H. J. |year=2013 |title=Rheasilvia impact basin on Vesta: Constraints on formation models from the central peak topography |journal=Journal of Geophysical Research: Planets |volume=118 |issue=7 |pages=1545–1555 |doi=10.1002/jgre.20108 |bibcode=2013JGRE..118.1545I }}</ref> Numerical modeling indicates that such a large central structure within a ~505 km diameter basin requires formation on a differentiated body with significant gravity. Scaling laws for craters on smaller asteroids fail to predict such a feature; instead, impact dynamics involving transient crater collapse and rebound of the underlying material (potentially upper mantle) are needed to explain its formation.<ref name="Ivanov2013"/> Hydrocode simulations suggest the impactor responsible was likely 60–70 km across, impacting at roughly 5.4 km/s.<ref name="Bowling2013">{{cite journal |last1=Bowling |first1=T. J. |last2=Richard |first2=G. |last3=Melosh |first3=H. J. |year=2013 |title=Numerical simulations of the Rheasilvia impact basin on Vesta |journal=Journal of Geophysical Research: Planets |volume=118 |issue=8 |pages=1622–1639 |doi=10.1002/jgre.20113 |bibcode=2013JGRE..118.1622B }}</ref> Models of impact angle (around 30-45 degrees from vertical) better match the detailed morphology of the basin and its prominent peak.<ref name="Ivanov2013"/> Crater density measurements on Rheasilvia's relatively unmodified floor materials and surrounding ejecta deposits, calibrated using standard lunar chronology functions adapted for Vesta's location, place the impact event at approximately 1 billion years ago.<ref name="Marchi2012">{{cite journal |last1=Marchi |first1=S. |last2=McSween |first2=H. Y. |last3=O'Brien |first3=D. P. |year=2012 |title=The Violent Collisional History of Vesta |journal=Science |volume=336 |issue=6082 |pages=690–694 |doi=10.1126/science.1218405 |bibcode=2012Sci...336..690M }}</ref><ref name="Williams2014"/> This age makes Rheasilvia a relatively young feature on a protoplanetary body formed early in Solar System history. The estimated excavation of ~1% of Vesta's volume<ref name="Schenk2012"/> provides a direct link to the Vesta family of asteroids (Vestoids) and the HED meteorites. Since Vesta's spectral signature matches that of the Vestoids and HEDs, this strongly indicates they are fragments ejected from Vesta most likely during the Rheasilvia impact.<ref name="McSween2013"/><ref name="Marchi2012"/> 

The Dawn mission's VIR instrument helped to confirm the basin's deep excavation and compositional diversity. VIR mapping revealed spectral variations across the basin consistent with the mixing of different crustal layers expected in the HED meteorites. Signatures matching eucrites (shallow crustal basalts) and diogenites (deeper crustal orthopyroxenites) were identified, which usually correlate with specific morphological features like crater walls or slump blocks.<ref name="DeSanctis2012">{{cite journal |last1=De Sanctis |first1=M. C. |last2=Combe |first2=J.-P. |last3=Ammannito |first3=E. |year=2012 |title=Spectroscopic Characterization of Mineralogy and Its Diversity on Vesta |journal=Science |volume=336 |issue=6082 |pages=697–700 |doi=10.1126/science.1219270 |bibcode=2012Sci...336..697D }}</ref><ref name="McSween2013" /> The confirmed signature of olivine-rich material, which were first hinted at by Hubble observations is strongest on the flanks of the central peak and in specific patches along the basin rim and walls, suggesting it is not uniformly distributed but rather exposed in distinct outcrops.<ref name="Clenet2014">{{cite journal |last1=Clénet |first1=H. |last2=Jutzi |first2=M. |last3=Barrat |first3=J.-A. |year=2014 |title=Constraints on Vesta's crustal structure and evolution from VIR/Dawn data: Olivine detection and analysis |journal=Icarus |volume=244 |pages=146–157 |doi=10.1016/j.icarus.2014.04.010 |bibcode=2014Icar..244..146C }}</ref><ref name="DeSanctis2012" /> As the dominant mineral expected in Vesta's mantle beneath the HED-like crust,<ref name="Russell2012" /> the presence of olivine indicates the Rheasilvia impact penetrated Vesta's entire crust (~20-40 km thick in the region) and excavated material from the upper mantle.<ref name="Clenet2014" /> Furthermore, the global stresses resulting from this massive impact are considered the likely trigger for the formation of the large trough systems, like Divalia Fossa, that encircle Vesta's equatorial regions.<ref name="Buczkowski2012" /><ref name="Jaumann2012" />