Mega‐ejecta on asteroid VestaMartin Jutzi1and Erik Asphaug1Received 16 September 2010; revised 5 November 2010; accepted 19 November 2010; published 8 January 2011.[1] Asteroid 4 Vesta, sometimes called the “smallestterrestrial planet”, will be orbit ed next July by NASA’sDawn mission. This will be the first time a small planet isvisited by a spacecraft, and novel geological structures areexpected. A key issue regarding Vesta (mea n diameter530 km) is to what extent its geology is domi nated bythe ∼ 460 km diameter impact basin on its southernhemisphere. We model the basin’s formation using a ver yhigh resolution 3D smooth‐ particle hydrodynamicssimulation to establish some of the major impact‐relatedaspects of Vesta’ s geology. The goal is to provide aframework for interpreting anticipated observations oflandforms. A collision of this magnitude (a ∼ 50 kmdiameter impactor at ∼5 km/s) exposes man y deep stratafrom within the planet and offsets the center of massby ∼10 km from the center of figure. Vesta s pins every5.3 hr, so that a hemispheric‐scale impa ct evolves in anon‐inertial frame, and deposits variably‐shaped, multiply‐folded and abruptly‐ terminated ejecta sequences ofregional scale. Since little of this ejecta would have beenmolten, these massive sequenced deposits could bemistaken in images of Vesta for other geologic forms suchas thrusts and folds. Detailed mapping, and the piecing‐together of mega‐ejecta via impact mode ls, will enable aninformed understanding of the interior geology of Vesta.Citation: Jutzi, M., and E. Asphaug (2011), Mega‐ej ecta onasteroid Vesta, Geophys. Res. Lett., 38, L01102, doi:10.1029/2010GL045517.1. Introduction[2] Although 20,000 times less massive than the Earth,asteroid 4 Vesta has a similar uncompressed bulk density(∼4 g cm−3)[Kovačević, 2005] and a basaltic crust [McCordet al., 1970; Drake, 1979], indicating a global compositionthat sets Vesta apart from known asteroids [Keil, 2002].Vesta’s spectral type is rare, something that is surprisinggiven the expectation that many early asteroids were dif-ferentiated silicate bodies with basaltic surfaces. With theexception of 17 km diameter Magnya (which probablyoriginated on a different parent body [Lazzaro et al., 2000])nearly all other asteroids with basaltic‐dominated surfacescan be dynamically linked to Vesta [Binzel and Xu, 1993;Marzari et al., 1996]. Vesta’s giant crater, its morphology,its global‐scale deposits, its ejected V‐type asteroids, andthe basaltic and ultramafic meteorites which presumablysample Vesta’s crust and upper mantle, lead to a fascinatinglarge‐scale collisional experiment [Asphaug, 1997] to bestudied using astronomical, dynamical and impact modelingtechniques – and soon in the context of detailed geologicimages.[3] While it is not unusual to see planet‐sized impactcraters on asteroids [Thomas, 1999; Asphaug, 2008] thisstructure is truly gigantic – a depression ∼460 km diameterand ∼13 km deep [Thomas et al., 1997]. Because it wrapsaround almost half the circumference of the planet, thedepression is actually convex at the longest wavelength.There may be no better place in the Solar System to see theexposure of the interior structure of a terrestrial planet, sinceeven after removing most of the crust from the impactedhemisphere the giant crater did not collapse to the extent thatit would on a Mars‐sized body [e.g., Marinova et al., 2008;Nimmo et al., 2008]. Beyond the impacted hemisphere,ejecta from tens of km deep, and the tectonic aftermathof global‐scale crater collapse, are expected to dominateVesta’s landscape. Since the event, spin relaxation hasreoriented the planet so that the depression aligns with thesouthern pole [Schmidt and Moore, 2010]. The originalorientation of the impact with regard to the spin axis ispresently unknown, but as we shall see, can probably bededuced from images of mega‐ejecta distribution.[4] An earlier numerical simulation of Vesta’s impactstructure [Asphaug, 1997] used gravity‐regime scaling toderive an impactor diameter of 42 km striking head‐on at avelocity of 5.4 km/s, as being responsible for the crater. Thisassumed a rocky impactor with density 2.7 g/cm3. Adoptingthe same impactor in the case of a head‐on impact, weachieve good agreement using our code for the size andstructure of the final crater. Because the present study is in3D it allows for the more probable case of off‐axis impacts,which we consider here. Since the impact contributes rathersmall net angular momentum to Vesta (no more than 5% ofthe present angular momentum) we are to assume that thetarget was spinning at close to the present period of 5.3 hrs.There being no hard and fast scaling rule for mega‐crateringcollisions (and certainly not into rapidly spinning targets)we use a slightly larger impactor of 50 km diameter fortypical 45° impacts and show that the model finishes with acrater of similar depth, profile and diameter.[5] In a mega‐ impact there is a gradation from craterformation in the impacted hemisphere, to contiguous ejectadeposition over the rest of the planet. The discontinuousejecta in this case escaped to become V‐type asteroids andassociated meteorites. Because of the rapid spin rate, thedynamical deposition of ejecta on Vesta is not hemi-spherically symmetric, but takes place over one or morerotations, with the planet rotating underneath the fallback.Sheets of ejecta tens of km thick are sheared in the non‐inertial frame, forming massive overturned flaps (even adoubly‐overturned flap in the example below) and othercomplex stratigraphic structures. Given that this ejecta is inthe solid phase (a regolith), the final layer structure is not1Earth and Planetary Sciences Department, University of California,Santa Cruz, California, USA.Copyright 2011 by the American Geophysical Union.0094‐8276/11/2010GL045517GEOPHYSICAL RESEARCH LETTERS, VOL. 38, L01102, doi:10.1029/2010GL045517, 2011L01102 1 of 5expected to undergo much further evolution, other thanoverturn and excavation by subsequent impacts. Clearly thestratigraphic record of Vesta is going to be complex, andpossibly ambiguous owing to this level of redistribution andoverturning. Our future work shall be to change the impactangle and impactor diameter to obtain the best possiblematch to the observed crater and these resultant structures,in order to study in great detail the global processes