The structure of the asteroid 4Vesta as revealed by models of planet-scale collisions

Journal name:
Nature
Volume:
494,
Pages:
207–210
Date published:
DOI:
doi:10.1038/nature11892
Received
Accepted
Published online

Asteroid 4Vesta seems to be a major intact protoplanet, with a surface composition similar to that of the HED (howardite–eucrite–diogenite) meteorites1, 2, 3, 4. The southern hemisphere is dominated by a giant impact scar5, but previous impact models6, 7, 8 have failed to reproduce the observed topography. The recent discovery that Vesta’s southern hemisphere is dominated by two overlapping basins9 provides an opportunity to model Vesta’s topography more accurately. Here we report three-dimensional simulations of Vesta’s global evolution under two overlapping planet-scale collisions. We closely reproduce its observed shape, and provide maps of impact excavation and ejecta deposition. Spiral patterns observed in the younger basin Rheasilvia9, about one billion years old10, are attributed to Coriolis forces during crater collapse. Surface materials exposed in the north come from a depth of about 20kilometres, according to our models, whereas materials exposed inside the southern double-excavation come from depths of about 60–100kilometres. If Vesta began as a layered, completely differentiated protoplanet, then our model predicts large areas of pure diogenites and olivine-rich rocks. These are not seen11, 12, 13, possibly implying that the outer 100kilometres or so of Vesta is composed mainly of a basaltic crust (eucrites) with ultramafic intrusions (diogenites).

At a glance

Figures

  1. SPH simulation of the formation of the two giant impact features in Vesta/'s southern hemisphere.
    Figure 1: SPH simulation of the formation of the two giant impact features in Vesta’s southern hemisphere.

    a, Asteroid Vesta as seen by Dawn; b, final result of the simulation; and c, Lambert azimuthal projection (equal area) of the southern hemisphere. Colours in b and c indicate the elevation (in kilometres; see key in c) with respect to a reference ellipsoid of 280×280×230km. Note that the simulation data (size of the body) was scaled by a factor of 1.04. The dotted circle in c corresponds to the older Veneneia basin, which is partly overlapped by the younger and larger Rheasilvia giant impact feature. Overall, our model results are in good agreement with the observations by Dawn19. To model the impact events, we use an SPH impact code specially written to model geologic materials7, 16, 17. We include a tensile fracture model16 in combination with a standard Drucker–Prager yield criterion for rocky materials. Damaged material is modelled using the Coulomb dry-friction law28. The block-model approximation of acoustic fluidization29 is used, as it gives a better match to central peak formation. Self-gravity is computed using a grid-based solver. The initial target is a non-rotating sphere of diameter d = 550km. To simulate the formation of Veneneia, a projectile of diameter d = 64km, impact velocity of 5.4kms−1 and an impact angle of 90° (that is, head-on impact) is used. The outcome of this simulation is then used as initial condition for a second run where we study the formation of Rheasilvia on top of Veneneia. For this, we use a d = 66km projectile impacting head-on with a velocity of 5.4kms−1, offset 40° from the centre of Veneneia, and we include pre-impact rotation with period P = PVesta = 5.3h on an axis that goes through the centre of the Veneneia basin. Photo credit for a, NASA/JPL-Caltech/ UCAL/MPS/DLR/IDA; three-dimensional images were produced using Vapor (www.vapor.ucar.edu).

  2. Velocity field lines in snapshots of the simulation of the Rheasilvia impact.
    Figure 2: Velocity field lines in snapshots of the simulation of the Rheasilvia impact.

    The tangents of the field lines point in the direction of the velocity vectors of the material flow. In the snapshots at times t = 800, 960 and 1,200s, the velocity field is shown in the co-rotating reference frame, whereas the velocities in the t = 7,000s snapshot correspond to the inertial frame. The rotation axis and direction are indicated by the black line and arrow, respectively. During crater collapse (shown in the co-rotating images), the large (global) scale of the material flow and the fast spin rate of the body lead to conditions where the Coriolis force becomes significant (low Rossby number, see main text). As a result, the velocity field has a spiral, clockwise pattern, possibly explaining the spiral fracture pattern observed in the Rheasilvia crater on Vesta9.

  3. Initial provenance (km depth) of the ejecta and the exposed material on the surface.
    Figure 3: Initial provenance (km depth) of the ejecta and the exposed material on the surface.

    Left, southern hemisphere; right, northern hemisphere. The orientation is the same as in the map in Fig. 1. Shown is the final result of the two large impacts forming the Veneneia and Rheasilvia basins at Vesta’s south pole. The outcome of the subsequent SPH simulations is shown in a Lambert azimuthal projection (equal area). Assuming a radially stratified protoplanet, the initial provenance corresponds to the initial depth of the material before the two impacts. The simulations were stopped at t = 2×104s after the impact of the projectile. This time is about equal to Vesta’s rotation period (PVesta = 5.3h) and corresponds to ~10 dynamical times of (Gρ)−1/22,000s; here G is the gravitational constant and ρ is the density.

  4. Vesta interior models and the corresponding petrological/mineralogical maps.
    Figure 4: Vesta interior models and the corresponding petrological/mineralogical maps.

    a, Model 1; b, model 2; and c, model 3. Models 1 and 2 assume globally distinct layers of eucrites and diogenites21 (see diagrams at left). Model 3 assumes a more complex structure24, 25. The colour coding of the material in the diagram at left is used in the maps of ejecta distribution shown centre (southern hemisphere) and right (northern hemisphere). As an initial target, a solid body of diameter d = 550km with a mantle (density ρ = 2.7gcm−3) and a 240-km-diameter iron core (ρ = 7.8gcm−3) was used. The crust is not explicitly modelled, but we track the dynamical evolution and redistribution of crustal layers during the simulation. For each interior model, the compositional maps were then computed using the distribution of ejecta as a function of initial depth (see Fig. 3).

References

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Author information

Affiliations

  1. Physics Institute, Space Research and Planetary Sciences, Center for Space and Habitability, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland

    • M. Jutzi &
    • W. Benz
  2. School of Earth and Space Exploration, Arizona State University, PO Box 876004, Tempe, Arizona 85287, USA

    • E. Asphaug
  3. Institute of Condensed Matter Physics, Ecole Polytechnique Fédérale de Lausanne (EPFL) Station 1, 1015 Lausanne, Switzerland

    • P. Gillet
  4. Université de Bretagne Occidentale, Institut Universitaire Européen de la Mer, CNRS UMR 6538, Place Nicolas Copernic, 29280 Plouzané, France

    • J.-A. Barrat

Contributions

M.J. performed and analysed the numerical simulations and led the research. E.A. and W.B. helped to design the numerical study and its scientific formulation. P.G. and J-A. B. provided the Vesta interior models. P.G. and W.B. initiated the collaboration between the four institutions. All authors contributed to interpretation of the results and preparation of the manuscript.

Competing financial interests

The authors declare no competing financial interests.

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