The cratering record, chronology and surface ages of (4) Vesta in comparison to smaller asteroids and the ages of HED meteorites

Abstract

We derived model functions for the crater production size-frequency distribution and chronology of the asteroids 951 Gaspra, 243 Ida, 21 Lutetia and 4 Vesta, based on a lunar-like crater production function and a lunar-like chronology with a smooth exponential decay in impact rate for the first $ 1 Ga of Solar System history. For Gaspra, Ida and Lutetia we find surface ages roughly in agreement with published data. Using the same approach for Vesta leads to results with high correlation to Ar-Ar reset ages of HED meteorites, for which a strong dynamical and spectroscopic connection to Vesta has been found. In contrast to recently published young formation ages of the Rheasilvia and Veneneia basins of about 1 and 2 Ga, respectively, we find for Rheasilvia a formation age of 3.57 0.1 Ga and for the Veneneia formation a lower limit of 3.7 70.1 Ga. For comparison we also give surface model ages for a preliminary version of a chronology (pers. comm. D.P. O'Brien) based on the Late Heavy Bombardment theory. Error bars presented in our work stem only from statistical analysis of measured crater distributions and do not include the uncertainty of the used chronology model.

The chronostratigraphy of protoplanet Vesta

“…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.…”

“…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). Nevertheless, in recent years, our understanding of the main asteroid belt has greatly improved, both in terms of its past dynamical evolution and the current size-frequency distribution (e.g., Bottke et al, 2005;Morbidelli et al, 2010Morbidelli et al, , 2012.…”

“…First, the production and chronology functions differ in both methods Schmedemann et al, 2014). Therefore, even where functions of both chronology systems are applied to the same crater size-frequency distribution, they will produce different results.…”

“…For presumably younger (<$3 Ga) terrains (based on accumulated impact craters) both chronology functions are virtually identical and differences in the production functions dominate, depending on whether the counted craters are larger or smaller than the reference diameter of 1 km, to which the chronology functions are calibrated. If crater diameters >1 km are used, the steeper lunar-like production function results in higher crater frequencies and older model ages compared with the asteroid flux-derived chronology, and vice versa: lower crater frequencies and younger model ages for crater diameters <1 km (Kneissl et al, this issue;Schmedemann et al, 2014).…”

“…It is therefore likely that crater retention ages from the floor of Veneneia reflect the age of the Rheasilvia resurfacing event rather than the formation of Veneneia itself (Schenk et al, 2012;Schmedemann et al, 2014). It is conceivable that Veneneia may be responsible for one of the measured 40 Ar/ 39 Ar ages, but more work is needed to investigate this assertion.…”