Asteroidal Beauty Pageant: Comparing Vesta to the Rest

The Cratering Record, Chronology and Surface Ages of (4) Vesta in Comparison to Smaller Asteroids and the Ages of HED Meteorites

Authors:

Schmedemann et al

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 3.5±0.1 Ga and for the Veneneia formation a lower limit of 3.7±0.1 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.

How Vesta's Asymmetrical Craters Formed

Asymmetric craters on Vesta: Impact on sloping surfaces

Authors:

Krohn et al

Abstract:

Cratering processes on planetary bodies happen continuously and cause the formation of a large variety of impact crater morphologies. On Vesta whose surface has been imaged at high resolution during a 14 months orbital mission by the Dawn spacecraft we identified a substantial number of craters with an asymmetrical shape. These craters, in total a number of 2892 ranging in diameter from 0.3 km to 43 km, are characterized by a sharp crater rim on the uphill side and a smooth one on the downhill side. The formation of these unusual asymmetric impact craters is controlled by Vesta's remarkable topographic relief. In order to understand the processes creating such unusual crater forms on a planetary body with a topography like Vesta we carried out the following work packages: (1) the asymmetric craters show various morphologies and therefore can be subdivided into distinct classes by their specific morphologic details; (2) using a digital terrain model (DTM), the craters are grouped into bins of slope angles for further statistical analysis; (3) for a subset of these asymmetric craters, the size-frequency distributions of smaller craters superimposed on their crater floors and continuous ejecta are measured in order to derive cratering model ages for the selected craters and to constrain possible post-impact processes; (4) three-dimensional hydrocode simulations using the iSALE-3D code are applied to the data set in order to quantify the effects of topography on crater shape and ejecta distribution. We identified five different classes (A to E) of asymmetric craters. Primarily, we focus on class A in this work. The global occurrence of these crater classes compared with a slope map clearly shows that these asymmetric crater types exclusively form on slopes. We found that slopes, especially slopes>20°, prevent the deposition of ejected material in the uphill direction, and slumping material superimposed the deposit of ejecta on the downhill side. The combination of these two processes explains the local accumulation of material in this direction. In the subset of asymmetric craters which we used for crater counts, our results show that no post-impact processes have taken place since floors and continuous ejecta in each crater show comparable cratering model ages within the uncertainties of the cratering chronology model. Therefore the formation, or modification, of the asymmetric crater forms by processes other than impact can be excluded with some certainty.

Vesta in a 3:1 Resonance with Jupiter?

Capture Probability in the 3:1 Mean Motion Resonance with Jupiter

Authors:

Folonier et al

Abstract:

We study the capture and crossing probabilities into the 3:1 mean motion resonance with Jupiter for a small asteroid that migrates from the inner to the middle Main Belt under the action of the Yarkovsky effect. We use an algebraic mapping of the averaged planar restricted three-body problem based on the symplectic mapping of Hadjidemetriou (1993), adding the secular variations of the orbit of Jupiter and non-symplectic terms to simulate the migration. We found that, for fast migration rates, the captures occur at discrete windows of initial eccentricities whose specific locations depend on the initial resonant angles, indicating that the capture phenomenon is not probabilistic. For slow migration rates, these windows become narrower and start to accumulate at low eccentricities, generating a region of mutual overlap where the capture probability tends to 100%, in agreement with the theoretical predictions for the adiabatic regime. Our simulations allow to predict the capture probabilities in both the adiabatic and non-adiabatic cases, in good agreement with results of Gomes (1995) and Quillen (2006). We apply our model to the case of the Vesta asteroid family in the same context as Roig et al. (2008), and found results indicating that the high capture probability of Vesta family members into the 3:1 mean motion resonance is basically governed by the eccentricity of Jupiter and its secular variations.

A Shallow Ocean of Magma on Vesta?

Differentiation of Vesta: Implications for a shallow magma ocean

Authors:

Neumann et al

Abstract:

The Dawn mission confirms predictions that the asteroid 4 Vesta is differentiated with an iron-rich core, a silicate mantle and a basaltic crust, and confirms Vesta as the parent body of the HED meteorites. To better understand its early evolution, we perform numerical calculations of the thermo-chemical evolution adopting new data obtained by the Dawn mission such as mass, bulk density and size of the asteroid. We have expanded the model of Neumann et al. (2012) that includes accretion, compaction, melting and associated changes of material properties and partitioning of 26Al, advective heat transport, and differentiation by porous flow, to include convection and effective cooling in a magma ocean. Depending on the melt fraction, the heat transport by melt segregation is modelled either by porous flow or by convection and heat flux of a magma ocean with a high effective thermal conductivity. We show that partitioning of 26Al and its transport with the silicate melt is crucial for the formation of a magma ocean. Due to the accumulation of 26Al in the sub-surface (for formation times t0 less than 1.5 Ma), a shallow magma ocean with a thickness of 1 to a few tens of km (depending on the silicate melt viscosity) forms. The lifetime of the shallow magma ocean is O(10^4)-O(10^6) years and convection in this layer is accompanied by the extrusion of 26Al at the surface. The interior differentiates from the outside inward with a mantle that is depleted in 26Al and core formation is completed within ~0.3 Ma. The lower mantle experiences melting below 45% suggesting a harzburgitic to dunitic composition. Our results support the formation of eucrites by the extrusion of early partial melt and cumulative eucrites and diogenites may form from the crystallizing shallow magma ocean. Silicate melt is present for up to 150 Ma, and core convects for ~100 Ma, supporting the idea of an early magnetic field.

The Origin of Vesta's Dark Material

The contamination of the surface of Vesta by impacts and the delivery of the dark material

Authors:

Turrini et al

Abstract:

The Dawn spacecraft observed the presence of dark material, which in turn proved to be associated with OH and H-rich material, on the surface of Vesta. The source of this dark material has been identified with the low albedo asteroids, but it is still a matter of debate whether the delivery of the dark material is associated with a few large impact events, to micrometeorites or to the continuous, secular flux of impactors on Vesta. The continuous flux scenario predicts that a significant fraction of the exogenous material accreted by Vesta should be due to non-dark impactors likely analogous to ordinary chondrites, which instead represent only a minor contaminant in the HED meteorites. We explored the continuous flux scenario and its implications for the composition of the vestan regolith, taking advantage of the data from the Dawn mission and the HED meteorites. We used our model to show that the stochastic events scenario and the micrometeoritic flux scenario are natural consequences of the continuous flux scenario. We then used the model to estimate the amounts of dark and hydroxylate materials delivered on Vesta since the LHB and we showed how our results match well with the values estimated by the Dawn mission. We used our model to assess the amount of Fe and siderophile elements that the continuous flux of impactors would mix in the vestan regolith: concerning the siderophile elements, we focused our attention on the role of Ni. The results are in agreement with the data available on the Fe and Ni content of the HED meteorites and can be used as a reference frame in future studies of the data from the Dawn mission and of the HED meteorites. Our model cannot yet provide an answer to the fate of the missing non-carbonaceous contaminants, but we discuss possible reasons for this discrepancy.