Editing 4 Vesta (section)

==Surface features==
{{Further|List of geological features on Vesta}}
Before the arrival of the [[Dawn (spacecraft)|''Dawn'' spacecraft]], some Vestan surface features had already been resolved using the [[Hubble Space Telescope]] and ground-based telescopes (e.g., the [[W. M. Keck Observatory|Keck Observatory]]).<ref name="Zellner2005"/> The arrival of ''Dawn'' in July 2011 revealed the complex surface of Vesta in detail.<ref name="Jaumann2012"/>

[[File:PIA18788-VestaAsteroid-GeologicMap-DawnMission-20141117.jpg|upright=3|center|thumb|Geologic map of Vesta ([[Mollweide projection]]).<ref name="Williams2014"/> The most ancient and heavily cratered regions are brown; areas modified by the [[Veneneia]] and [[Rheasilvia]] impacts are purple (the Saturnalia Fossae Formation, in the north)<ref name="Scully2014"/> and light cyan (the Divalia Fossae Formation, equatorial),<ref name="Williams2014"/> respectively; the Rheasilvia impact basin interior (in the south) is dark blue, and neighboring areas of Rheasilvia ejecta (including an area within Veneneia) are light purple-blue;<ref name="Schäfer2014"/><ref name="Kneissl2014"/> areas modified by more recent impacts or mass wasting are yellow/orange or green, respectively.]]

===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" /> 

===Other craters===
{{stack|
[[File:PIA17661-NASA-DawnMission-Asteroid-Vesta-20131216.jpg|thumb|The crater Aelia]]
[[File:Old unnamed equatorial basin on 4 Vesta.png|thumb|[[Feralia Planitia]], an old, degraded impact basin or impact basin complex near Vesta's equator (green and blue). It is {{convert|270|km|0||abbr=on}} across and predates Rheasilvia (green at bottom)]]
}}
Several old, degraded craters approach Rheasilvia and Veneneia in size, although none are quite so large. They include [[Feralia Planitia]], shown at right, which is {{convert|270|km|0|abbr=on}} across.<ref name="Av-10"/> More-recent, sharper craters range up to {{convert|158|km|0|abbr=on}} Varronilla and {{convert|196|km|0|abbr=on}} Postumia.<ref name="Planetary Names"/>

Dust fills up some craters, creating so-called [[dust ponds]]. They are a phenomenon where pockets of dust are seen in celestial bodies without a significant atmosphere. These are smooth deposits of dust accumulated in depressions on the surface of the body (like craters), contrasting from the Rocky terrain around them.<ref>{{cite news |first=J. Kelly |last=Beatty |date=25 June 2004 |title=Eros's puzzling surface |magazine=[[Sky and Telescope]] |quote=To geologists' surprise, the asteroid Eros has more than 250&nbsp;'ponds' thought to contain compacted deposits of finely ground dust. |url=https://skyandtelescope.org/astronomy-news/eross-puzzling-surface/ |access-date=18 October 2023 |via=skyandtelescope.org }}</ref> On the surface of Vesta, we have identified both type&nbsp;1 (formed from impact melt) and type&nbsp;2 (electrostatically made) [[dust ponds]] within 0˚–30°N/S, that is, Equatorial region. 10&nbsp;craters have been identified with such formations.<ref>{{cite journal |first1=R.  |last1=Parekh |first2=K.A. |last2=Otto |first3=K.D. |last3=Matz |first4=R. |last4=Jaumann |first5=K. |last5=Krohn |first6=T. |last6=Roatsch |first7=E. |last7=Kersten |first8=S. |last8=Elgner |first9=C.T. |last9=Russell |first10=C.A. |last10=Raymond |display-authors=6 |orig-date=1 November 2021 |date=28 February 2022 |title=Formation of ejecta and dust pond deposits on asteroid Vesta |journal=Journal of Geophysical Research: Planets |volume=126 |issue=11 |page=e2021JE006873 |doi=10.1029/2021JE006873 |doi-access=free }}</ref>

===="Snowman craters"====
The "snowman craters" are a group of three adjacent craters in Vesta's northern hemisphere. Their official names, from largest to smallest (west to east), are Marcia, Calpurnia, and Minucia. Marcia is the youngest and cross-cuts Calpurnia. Minucia is the oldest.<ref name="Williams2014"/>
<!-- [[File:Vesta Snowman craters.jpg|thumb|right|upright=1.5|{{center|"Snowman craters" by ''[[Dawn (spacecraft)|Dawn]]'' from {{convert|5,200|km|-1|abbr=on}} in July 2011}}]]
{{stack|[[File:Vesta snowman.jpg|thumb|upright=1.5|"Snowman craters" to the left by ''Dawn'' from orbit (2011).]]}}
-->
{{multiple image
|total_width=600
| align     = center
| direction = horizontal
| image1    = Vesta Snowman craters.jpg
| alt1      =
| caption1  = "Snowman" craters by ''[[Dawn (spacecraft)|Dawn]]'' from 5,200&nbsp;km (3,200&nbsp;mi) in 2011
| image2    = Vesta Snowman craters close-up.jpg
| alt=2     =
| caption2  = Detailed image of the "Snowman" craters
}}

===Troughs===
The majority of the equatorial region of Vesta is sculpted by a series of parallel troughs designated [[Divalia Fossae]]; its longest trough is {{convert|10|–|20|km}} wide and {{convert|465|km}} long. Despite the fact that Vesta is a one-seventh the size of the Moon, Divalia Fossae dwarfs the [[Grand Canyon]]. A second series, inclined to the equator, is found further north. This northern trough system is named [[Saturnalia Fossae]], with its largest trough being roughly 40&nbsp;km wide and over 370&nbsp;km long. These troughs are thought to be large-scale [[graben]] resulting from the impacts that created Rheasilvia and Veneneia craters, respectively. They are some of the [[List of largest rifts and valleys in the Solar System|longest chasms in the Solar System]], nearly as long as [[Ithaca Chasma]] on [[Tethys (moon)|Tethys]]. The troughs may be graben that formed after another asteroid collided with Vesta, a process that can happen only in a body that is differentiated,<ref name="Buczkowski2012"/> which Vesta may not fully be. Alternatively, it is proposed that the troughs may be radial sculptures created by secondary cratering from Rheasilvia.<ref name = "Hirata et al. 2023">{{cite journal |url=https://doi.org/10.1029/2022JE007473 |title=Secondary Cratering From Rheasilvia as the Possible Origin of Vesta's Equatorial Troughs |first=N.|last=Hirata |journal=Journal of Geophysical Research: Planets |date=2023 |volume=128 |issue=3 |doi=10.1029/2022JE007473 |arxiv=2303.14955 |bibcode=2023JGRE..12807473H |hdl=20.500.14094/0100482053 |access-date=2024-03-04 }}</ref>
{{multiple image
|total_width=660
| align     = center
| direction = horizontal
| image1    = Divalia Fossa IOTD-260.jpg
| alt1      =
| caption1  = A section of Divalia Fossae, with parallel troughs to the north and south
| image2    = Divalia Fossa PIA15673.jpg
| alt2      =
| caption2  = A computer-generated view of a portion of Divalia Fossae
}}
{{Clear}}

===Surface composition===
Compositional information from the visible and infrared spectrometer (VIR), gamma-ray and neutron detector (GRaND), and framing camera (FC), all indicate that the majority of the surface composition of Vesta is consistent with the composition of the howardite, eucrite, and diogenite meteorites.<ref name="DeSanctis2012a"/><ref name="Prettyman2012"/><ref name="Reddy2012"/> The Rheasilvia region is richest in diogenite, consistent with the Rheasilvia-forming impact excavating material from deeper within Vesta. The presence of olivine within the Rheasilvia region would also be consistent with excavation of mantle material. However, olivine has only been detected in localized regions of the northern hemisphere, not within Rheasilvia.<ref name="Ammannito2013"/> The origin of this olivine is currently unclear. Though olivine was expected by astronomers to have originated from Vesta's mantle prior to the arrival of the ''Dawn'' orbiter, the lack of olivine within the Rheasilvia and Veneneia impact basins complicates this view. Both impact basins excavated Vestian material down to 60–100&nbsp;km, far deeper than the expected thickness of ~30–40&nbsp;km for Vesta's crust. Vesta's crust may be far thicker than expected or the violent impact events that created Rheasilvia and Veneneia may have mixed material enough to obscure olivine from observations. Alternatively, ''Dawn'' observations of olivine could instead be due to delivery by olivine-rich impactors, unrelated to Vesta's internal structure.<ref name="Palomba2015"/>

===Features associated with volatiles===
Pitted terrain has been observed in four craters on Vesta: Marcia, Cornelia, Numisia and Licinia.<ref name="Denevi2012"/> The formation of the pitted terrain is proposed to be degassing of impact-heated volatile-bearing material. Along with the pitted terrain, curvilinear gullies are found in Marcia and Cornelia craters. The curvilinear gullies end in lobate deposits, which are sometimes covered by pitted terrain, and are proposed to form by the transient flow of liquid water after buried deposits of ice were melted by the heat of the impacts.<ref name="Scully2014"/> Hydrated materials have also been detected, many of which are associated with areas of dark material.<ref name="DeSanctis2012b"/> Consequently, dark material is thought to be largely composed of carbonaceous chondrite, which was deposited on the surface by impacts. Carbonaceous chondrites are comparatively rich in mineralogically bound OH.<ref name="Reddy2012"/>