Earth gravity up to second order in topography and density

The gravity potential of a planet is usually expanded up to first order only as a linear function of topography and lateral variations of density. In this article, we extend these expressions up to second order and we estimate the magnitude of the new non-linear terms. We find that they are not negligible when compared to observed values: tens of metres for height anomalies and tens of milligals for gravity anomalies. Therefore, second-order expressions should be taken into account when inverting global gravity data.