Global Surface Modification of Asteroid 4 Vesta Following the Rheasilvia Impact

Timothy J Bowling, Purdue University

Abstract

4 Vesta, the second most massive asteroid in the Solar System, was the recent target of NASA’s Dawn mission, which spent many months orbiting and analyzing the body. A geologically young (~1 Ga) giant impact basin, Rheasilvia, which has a diameter nearly equal to that of Vesta, dominates Vesta’s southern hemisphere. In addition to opening a massive crater, the impact that formed the Rheasilvia basin produced a shockwave that was sufficiently strong to modify Vesta’s surface morphology on a nearly global scale. Like a small planet, Vesta is internally differentiated, with an iron core, an ultramafic mantle, and a mafic crust. Because the speed of sound in the core is lower than in the mantle, the core acts like a convex lens to the shockwave emanating from the Rheasilvia forming impact, focusing stresses to the antipode and disrupting terrain there. We investigate the amount of deformation expected at the Rheasilvian antipode using numerical models of sufficient resolution to directly observe terrain modification and material displacements following the arrival of impact stresses. We find that the magnitude and mode of deformation expected at the impact antipode is strongly x dependent on both the sound speed and porosity of Vesta’s mantle, as well as the strength of the Vestan core. Observations by the Dawn spacecraft cannot provide definitive evidence that large amounts of deformation occurred at the Rheasilvia antipode, largely due to the presence of younger large impact craters in the region. However, a deficiency of small craters near the antipodal point suggests that some degree of deformation did occur. A set of prominent linear grooves, interpreted as a graben system named ‘Divalia Fossae’, circle much of Vesta’s equator and have been spatially correlated with the Rheasilvia basin. Using numerical impact models, we show that the crust of Vesta in the region where the Fossae are located undergoes considerable extension following the passage of the Rheasilvia impact induced shockwave. The magnitude of this extension is dependent on both the porosity and strength of Vesta’s crust and mantle, suggesting that Vesta is heavily fractured throughout it’s interior. We also show that both the major graben systems on Vesta are localized by the surface curvature of the asteroid.

Degree

Ph.D.

Advisors

Freed, Purdue University.

Subject Area

Planetology

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