A new look at Venus gravity and rotation from a reanalysis of Magellan data

Most of the current knowledge regarding the interior of Venus is derived from Magellan gravity and topography datasets collected three decades ago. While this mission provided the first high-resolution global gravity field, the computational limitations of the 1990s necessitated compromises that impacted the accuracy of the gravity solutions (Konopliv et al., 1999). We report on a reanalysis of the Magellan Doppler tracking data, leveraging modern computational capabilities to improve the Venus gravity field, orientation, and rotational dynamics.

Most significantly, modern computing power allows us to achieve a spherical harmonic degree-and-order 180 solution via a single inversion, eliminating the need for the multi-step approach that previously affected the solution. The single-step solution, removes the discontinuities in the uncertainty estimates of the gravity field coefficients, enabling more coherent and robust uncertainty quantification on derived products.

Differently from previous works we do not apply surface acceleration constraints but rather a smooth, global, Kaula regularization. As a consequence of this approach we observe a reduction in high-frequency noise (ringing) in the solution, leading to more coherent spatial structures in the solution which is beneficial for localized analyses of near-subsurface features

Another limitation of previous works, consisted in a strong de-weighting of the tracking data around the poles, which strongly affected the gravity solution in those regions. We adopted a different approach, based on the actual data weight coupled with a Variance Component Estimator (VCE) to properly derive data weights. The spatial resolution of the resulting gravity field is substantially improved over the poles, where we retrieve higher power, and more coherent spatial structures. 

Using this new field, we investigate the global elastic properties of the lithosphere taking advantage of improved polar resolution and robust uncertainty quantification. Notably we apply the RM-1 constrained inversion technique (e.g., Goossens et al., 2017) which improves the estimate of the short-wavelength gravity by prescribing it to follow the signal associated to the topography. Rather than prescribing gravity to follow uncompensated topography as in previous work, here we account for loading of the lithosphere (e.g., Turcotte et al., 1981). This proves to be a powerful technique enabling improved sensitivity to the global elastic properties of the lithosphere.

Additionally, we assess the sensitivity of the dataset to length-of-day variations which were previously not explicitly estimated, but whose magnitudes as observed from Earth (Margot et al., 2021) would have had a measurable influence on the probe.

 

 

 

References

Konopliv, A. S., Banerdt, W. B. & Sjogren, W. L. Venus Gravity: 180th Degree and Order Model. Icarus 139, 3–18 (1999).

Goossens, S. et al. Evidence for a low bulk crustal density for Mars from gravity and topography. Geophysical Research Letters 44, 7686–7694 (2017).

Turcotte, D. L., Willemann, R. J., Haxby, W. F. & Norberry, J. Role of membrane stresses in the support of planetary topography. J. Geophys. Res. 86, 3951 (1981).

Margot, J.-L. et al. Spin state and moment of inertia of Venus. Nature Astronomy 5, 676–683 (2021).