Asteroid Shapes Are Always Close To Fluid Equilibrium

Details Asteroid Shapes Are Always Close To Fluid Equilibrium Asteroid Shapes Are Always Close To Fluid Equilibrium

Oct 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010dps....42.6304t&link_type=abstract

American Astronomical Society, DPS meeting #42, #63.04; Bulletin of the American Astronomical Society, Vol. 42, p.1092

Astronomy and Astrophysics

Astronomy

Scientific paper

The simple evidence that asteroid are composed by solid rocks suggests that their shape can be rather far from the theoretical equilibrium for rotating fluid bodies. The possible fragmented ("rubble-pile") nature of most of them has suggested interpretations based on elasto-plastic models (such as the Mohr-Coulomb theory) that take into account the static behavior of a granular structure. However, these approaches did not incorporate explicitly the possible evolution of shapes in time due to external factors such as crater forming impacts or tidal deformations.
We revisited the theory of equilibrium shapes for fluids, quantitatively evaluating - by appropriate metrics - the distance of the observed shapes from fluid equilibrium. This distance turns out to be much smaller than previously expected.
On the basis of this evidence, we simulated numerically the evolution of gravitational aggregates having a small degree of internal friction, consistent with the theoretical findings. Our results offer a global scenario for the evolution of asteroid shapes under the action of gradual stresses due to minor impacts, tidal forces and seismic shaking.
We show that actual asteroid shapes are consistent with the evolution of aggregates tending towards minimum free energy states.
We are able to explain the samples of observed shapes obtained by different techniques. Our findings strongly support a highly porous and fragmented nature for most asteroids, at least in an external layer.
Reference: Tanga et al. 2009: ApJ Letters, 706, 1, L197-L202
Acknowledgments: PT and CC have been supported by the "Programme Nationale de Planetologie" of France; DCR acknowledges support by the NASA (grant no. NNX08AM39G issued through the Office of Space Science) and by the NSF (grant no. AST0708110).

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