Efficient early global relaxation of asteroid Vesta

Abstract

The asteroid Vesta is a differentiated planetesimal from the accretion phase of solar system formation. Although its present-day shape is dominated by a non-hydrostatic fossil equatorial bulge and two large, mostly unrelaxed impact basins, Vesta may have been able to approach hydrostatic equilibrium during a brief early period of intense interior heating. We use a finite element viscoplastic flow model coupled to a 1D conductive cooling model to calculate the expected rate of relaxation throughout Vesta's early history. We find that, given sufficient non-hydrostaticity, the early elastic lithosphere of Vesta experienced extensive brittle failure due to self-gravity, thereby allowing relaxation to a more hydrostatic figure. Soon after its accretion, Vesta reached a closely hydrostatic figure with <2 km non-hydrostatic topography at degree-2, which, once scaled, is similar to the maximum disequilibrium of the hydrostatic asteroid Ceres. Vesta was able to support the modern observed amplitude of non-hydrostatic topography only >40-200 My after formation, depending on the assumed depth of megaregolith. The Veneneia and Rheasilvia giant impacts, which generated most non-hydrostatic topography, must have therefore occurred >40-200 My after formation. Based on crater retention ages, topography, and relation to known impact generated features, we identify a large region in the northern hemisphere that likely represents relic hydrostatic terrain from early Vesta. The long-wavelength figure of this terrain suggests that, before the two late giant impacts, Vesta had a rotation period of 5.02 hr (6.3% faster than present) while its spin axis was offset by 3.0• from that of the present. The evolution of Vesta's figure shows that the hydrostaticity of small bodies depends strongly on its age and specific impact history and that a single body may embody both hydrostatic and non-hydrostatic terrains and epochs.

Power Laws of Topography and Gravity Spectra of the Solar System Bodies

“…Due to its fast rotation and large size, Vesta's shape is substantially oblate (Ermakov, Zuber, Smith, Raymond, Balmino, et al, ). Fu et al () showed that early in its history Vesta likely achieved hydrostatic equilibrium. Later, when Vesta cooled, it experienced two large‐scale collisions responsible for the formation of the two giant impact basins in the southern hemisphere: Rheasilvia and Veneneia, with diameters of ≈500 and ≈400 km.…”

Multiple and Fast: The Accretion of Ordinary Chondrite Parent Bodies

“…For now, it seems highly unlikely that one could (1) hide seven additional such basins on Vesta's surface and, at the same time (2) avoid bringing lots of the diogenites to the surface. Note that the relaxation of Vesta's surface may have occurred during the first ∼100 Myr of the solar system implying that primary basins may be no longer observable (Fu et al 2013). As such, Vesta's actual surface may thus not be fully incompatible with an early period of more intense bombardment.…”

Surface Roughness and Gravitational Slope Distributions of Vesta and Ceres

“…The difference between the polar and equatorial axes of the best fit ellipsoids is ≈56 km for Vesta and ≈36 km for Ceres. The long‐wavelength content of the shape models has been used along with the gravity field models (Konopliv, Park, et al, , ; Park et al, , ) to constrain the internal structure of these bodies (see Ermakov, Mazarico, et al, ; Fu et al, , for Vesta and Ermakov, Mazarico, et al, ; Fu et al, , for Ceres). However, the short‐wavelength topography, at scales of 10–100 image pixels (several kilometers‐scale and shorter) from the lowest altitude orbit, has not been characterized in a statistical sense.…”