Gravity field expansion in ellipsoidal harmonic and polyhedral internal representations applied to Vesta

Abstract

A 20th degree ellipsoidal harmonic gravity field of Vesta is determined by processing radiometric Doppler and range data from the Dawn mission. The gravity field shows sensitivity up to degree 18 and the coefficients are globally determined on average to degree 15. Gravity anomalies are mapped to the Brillouin ellipsoid (304×289×247-km), which is a substantially closer fit to the surface than the reference ellipsoid (290×290×265-km) used to map the conventional spherical harmonic series, especially near the poles. Two models of internal structure are subsequently explored, in which density variations are permitted in the uppermost layer (i.e., crust) in order to explain Vesta’s local gravitational signature. These models include the case of a two-layer model with an average crustal thickness of 55.5km and a three-layer model with an average crustal thickness of 22.4km. For both two-layer and three-layer scenarios, the Bouguer gravity anomaly is minimized for a crustal density of 2970kg/m3. The remaining Bouguer anomalies can be explained by lateral crustal density variation of 2310–3440kg/m3 and 2660–3240kg/m3 for the 22.4km and 55.5km crustal thickness models, respectively. The general trend of the estimated lateral crustal densities for the two cases is very similar, with a wider range for the 22.4km case due to a thinner crust. This indicates that a thick crust (e.g., 55.5km) would be more favorable for minimizing the range of lateral crustal density variations. Consideration of independent geochemical and petrological constraints suggests that a three-layer model is a more appropriate representation of Vesta’s internal structure, despite the fact that two-layer models provide a satisfactory fit to gravity data alone. In detail, it is found that densities derived from gravity data assuming three-layer models and those derived from the howardite–eucrite–diogenite meteorites and estimates of plausible bulk-Vesta composition show an excellent level of mutual consistency.