Vesta seen by Dawn. Copyright: NASA.

Interesting polar craters on Vesta

Hi there! Today’s post is on the paper On the possibility of viscoelastic deformation of the large south polar craters and true polar wander on the asteroid Vesta, by Saman Karimi and Andrew J. Dombard, both at the University of Illinois at Chicago during the study; Saman Karimi is now at Johns Hopkins University. This study has recently been accepted for publication in Journal of Geophysical Research: Planets. It is a study of 2 craters of the small planet Vesta, Rheasilvia and Veneneia, which present two unusual features:

  1. they are located close to the South Pole,
  2. they are shallow with a central peak.

The authors have tried to explain these two properties.

The small planet Vesta

Vesta, or more precisely (4) Vesta, is the second largest object of the Main Asteroid Belt. It has a triaxial shape, i.e. (572.6 × 557.2 × 446.4) km, and is large enough to have a differentiated structure. It orbits at a distance of 2.36 AU from the Sun, i.e. 354 millions km, which implies an orbital period of 3.63 years. However, it rotates much more rapidly, in 5.3 hours. This rapid rotation is responsible for the high polar flattening, i.e. you can see from its shape that one of its axes is much smaller than the other ones. This axis is actually the rotation axis. This rotation around one axis permits to define easily the North and the South Poles, close to which are the 2 craters of interest.

(4) Vesta has been recently the target of the space mission Dawn. Dawn has been launched from Cape Canaveral in September 2007. It has orbited Vesta between July 2011 and September 2012, and is orbiting Ceres since March 2015. Dawn permitted invaluable progress on our knowledge of Vesta. It gave us an accurate cartography of the surface, which resulted in a count of the craters, measurements of its shape, of its gravity field, of its rotation… All of these data permit to constrain the interior. Many papers on Vesta followed, the paper I am presenting you is one of these.

Impacts in the Solar System

The Solar System bodies are impacted since the beginning of their formation. During the early ages of the Solar System, the impacts were more frequent than now, because of the presence of a protoplanetary disk composed of small objects before they accrete into larger ones. For instance, the Late Heavy Bombardment (LHB) is known as an episode of intense bombardment which occurred approximately 4 billion years ago. Some models consider that it could have been triggered by a gravitational interaction between giant planets and a former asteroid belt, which has destabilized it. For instance a previous version of the Nice model stated that the LHB could have been the consequence of a former 2:1 mean-motion resonance between Jupiter and Saturn during their migration. That resonance would have raised the orbital oscillations of these planets, which would have favored the destabilization of the asteroid belt and the bombardment of the terrestrial planets.

Meteorites are signatures of impacts on the Earth. Actually, many small objects are destroyed when they enter our atmosphere, this is why we get these small meteorites on the surface. Atmosphereless bodies usually present signatures of bombardment, for instance the Moon is known for its craters. When such a body does not present evidence of craters, it could mean that its surface has been recently renewed by some internal processes, due to tectonic or volcanic activity. So, counting the impacts is a way to age the surfaces.

When large enough, impacts can be responsible for dramatic events such as: the creation of the Moon, which has probably been split from the Earth by an impact, the creation of the rings of Saturn, which could be made of a large impactor, the destruction of the outer envelope of the proto-Mercury, or the extinction of the dinosaurs.

The study I present here deals with two impact basins at the South Pole of Vesta: Rheasilvia and Veneneia, with diameters of 505 and 395 km, respectively. You can compare these numbers with the dimensions of Vesta, and you understand how significant the impacts creating these craters should have been in the history of Vesta.

A viscoelastic rheology

The issue is: how does the surface respond to a large impact? It depends on its structure, of course. Basically, when you hit the surface, you create a crater, ejecta being expelled. After that, the surface of the asteroid tends to relax, i.e. the deformation due to the impact is kind of damped, but the final aspect will not be the initial one, since some material has been displaced, some other ejected, and the heating due to the impact tends to molten the surface. During the process of relaxation, the material tends to converge to the center of the basin, while it was pushed to the edges when the impact occurred, this can result in a central peak. Measuring the topography of the crater, i.e. its width, its depth, and the height of its central peak, can give constraints on the way the surface responds. This response characterizes the rheology of the surface, which is basically viscoelastic. Elastic would mean that the surface would recover its initial shape without any energy loss, and viscous means that you have actually some energy loss, which results in a permanent deformation once the surface is relaxed.

This study

The study first points out the two peculiarities of the two craters, and test the hypothesis that the impacts occurred close to the equator As a consequence Vesta would have been reoriented, this would explain why the impacts are now located close to the South Pole. This would mean that the surface is molten enough to result in the current topography of the craters and in the present polar flattening of Vesta.

To try to understand these facts, the authors assumed that the impactors hit Vesta close to its equator, and ran numerical simulations to check whether Vesta was able to reach its current state, which implies reshaping and reorientation. The numerical simulations consist to propagate the response to the impact not only in time, but also on the surface of Vesta. For that, the surface is discretized on a mesh, and finite elements modeling is used. This is a classical way to integrate Partial Derivative Equations (PDE). A key parameter is the temperature: if the impact is energetic enough, then Vesta heats enough to be molten enough to create the central peak, relax the crater, and reshape according to its new orientation state.

The reader should be aware that such simulations require high computation facilities, and take a long time. This is the reason why the authors ran only 8 of them, with different assumptions to cover most of the physically acceptable properties for the lithosphere of Vesta. These properties are in this study ruled by 6 parameters: the crustal thickness, the temperatures of the surface and of the mantle, the crustal thermal conductivity, the background heat flux, and the isostatic compensation. This last parameter characterizes the capacity of the surface to recover its gravity after the shock of the impact, which displaced the internal masses. This particularly affects the height of the central peak.

None of these 8 simulations result in a Vesta which is close enough to the observed one, since it does not heat enough. This means that the shape of Vesta is not a direct consequence of these two impacts, which probably occurred close to the South Pole, even if impacts at this latitude have a low probability.

A question for the authors

I am no specialist of impacts, but I wonder: if we have two tangent impacts instead of perpendicular ones, I guess they would have resulted in craters with a limited depth, but a strong reorientation of Vesta. The authors do not mention this possibility in the paper, and I would be interested in their opinion on this issue.

Some links

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