The Violent Collisional History of Asteroid 4 Vesta

Abstract

Vesta is a large differentiated rocky body in the main asteroid belt that accreted within the first few million years after the formation of the earliest solar system solids. The Dawn spacecraft extensively imaged Vesta's surface, revealing a collision-dominated history. Results show that Vesta's cratering record has a strong north-south dichotomy. Vesta's northern heavily cratered terrains retain much of their earliest history. The southern hemisphere was reset, however, by two major collisions in more recent times. We estimate that the youngest of these impact structures, about 500 kilometers across, formed about 1 billion years ago, in agreement with estimates of Vesta asteroid family age based on dynamical and collisional constraints, supporting the notion that the Vesta asteroid family was formed during this event.

The chronostratigraphy of protoplanet Vesta

“…For example, the asteroid-based chronology derived an age for the Rheasilvia basin from a single large area using craters >3 km diameter superposed on the basin's floor (Marchi et al, 2012a(Marchi et al, , 2012b. More recently Marchi et al, in press analyzed proximal ejecta terrains finding similar conclusions.…”

“…2). When Dawn arrived at Vesta, it became clear rapidly, on the basis solely of crater abundances, that the oldest terrains are in the northern hemisphere (antipodal to the southern impact basins), the youngest terrain occurs in the floors of the south polar basins, and that the equatorial latitudes are more complex and probably intermediate in age (Marchi et al, 2012a(Marchi et al, , 2012bYingst et al, in press). Dawn images resolved two large impact structures in the south polar region, the older Veneneia basin ($400 km in diameter) superposed by the younger Rheasilvia basin ($500 km in diameter) (Schenk et al, 2012;Jaumann et al, 2012).…”

“…For example, photogeologic, color-ratio imaging, and stereo image-based topographic analyses using monochrome and color data from Dawn's Framing Camera (FC: Reddy et al, 2012aReddy et al, , 2012b were used in global and regional geologic mapping (Jaumann et al, 2012;Yingst et al, in press;Williams et al, in press-b, this issue). Application of crater statistical techniques were applied to mapped terrains to determine relative and absolute model ages of portions of the surface (Marchi et al, 2012a(Marchi et al, , 2012bO'Brien et al, 2014;Schmedemann et al, 2014). Hyperspectral and thermophysical analyses of multi-wavelength visible and near-infrared data from Dawn's Visible and Infrared Spectrometer (VIR: De Sanctis et al, 2012;Ammannito et al, 2013a), and correlation of geology with geochemical distributions derived from Dawn's Gamma Ray and Neutron Detector (GRaND: Prettyman et al, 2012Prettyman et al, , 2013Yamashita et al, 2013), were used to provide information on the mineralogical and physical composition of surface materials.…”

“…The lunar-derived production and chronology functions, which are tied to the radiometric ages of the Apollo samples returned from the Moon, have been applied to other terrestrial planets (Hartmann and Neukum, 2001) and to other asteroids (Neukum and Ivanov, 1994;Chapman et al, 1996aChapman et al, , 1996bIvanov et al, 2002;Marchi et al, 2012a). Although the extrapolation of the lunar chronology to other terrestrial planets has been commonly accepted since the early work of Shoemaker (1962aShoemaker ( , 1962b) based on both a dynamical justification (see, for instance, the discussion in Marchi et al, 2013 for a recent application to Mercury) and observation of a common projectile population (Neukum and Ivanov, 1994;Ivanov et al, 2002), its extrapolation to the asteroid belt is questionable (Marchi et al, 2012a(Marchi et al, , 2012bO'Brien et al, 2014) because it lacks a quantitative theoretical justification. However, crater distributions observed on asteroids show similarities to the crater distributions observed on the terrestrial planets, leading to the assumption that both types of body were impacted by the same projectile population and with a similar flux (Neukum and Ivanov, 1994;Ivanov et al, 2002;Schmedemann et al, 2014).…”

“…The Dawn Science Team has developed two independent approaches that use crater statistics to derive absolute cratering retention ages for the surface of Vesta: (a) extrapolation of the lunar-derived crater production and chronology functions (Neukum and Ivanov, 1994) to Vesta ; and (b) application to Vesta of crater production and chronology functions derived from models of asteroid belt dynamics (Marchi et al, 2012a(Marchi et al, , 2012b(Marchi et al, , 2013O'Brien et al, 2014). The lunar-derived production and chronology functions, which are tied to the radiometric ages of the Apollo samples returned from the Moon, have been applied to other terrestrial planets (Hartmann and Neukum, 2001) and to other asteroids (Neukum and Ivanov, 1994;Chapman et al, 1996aChapman et al, , 1996bIvanov et al, 2002;Marchi et al, 2012a).…”