Gravity fields of the terrestrial planets: Long‐wavelength anomalies and tectonics

Abstract

We present a review of the long‐wavelength gravity fields of the terrestrial planets, earth, moon, Mars, and Venus with particular emphasis placed on the interrelationship between gravity anomalies and tectonic processes. After first summarizing appropriate statistical formulas, we discuss the relevant continuum mechanical solutions for elastic, viscoelastic, and convecting media in terms of the relationship to the gravity field both for predicting gravity anomalies and for the use of gravity as a constraining boundary condition. Gravity data can provide a strong constraint for the first two classes of solutions, but a significant ambiguity exists in convection interpretations because of the competing effects from flow and boundary deformation. The question of finite strength of crustal and mantle materials plays a paramount role in gravity studies because of the possibility of model discrimination based on stress states. A variety of evidence is reviewed, and it is concluded that the finite strength of the terrestrial planets is unlikely to exceed a kilobar when subject to loads over long geologic periods. Isostasy plays a dominant role in any discussion of the lithospheric contribution to planetary gravity fields. Since it tends to minimize stresses, it is a basic state to be expected, and most useful discussions of the gravity fields are centered on departures from the isostatic state, either due to dynamic forces or to finite strength. For the earth the long‐wavelength anomalies arise largely from beneath the lithosphere and are related to mantle flow. Apart from the obvious correlation of gravity to the plate boundaries, the specific relationships between the gravity field and relevant flow parameters are complex, and there are at present no systematic relationships established between geophysical observables in the intraplate regions. On the moon, most of the contributions to the gravity field arise from the lithosphere and are supported by the finite strength of that layer. The mascons are due to a combination of several kilometer thick basalt layers and crustal collapse at the time of impact, forming ring structures and mantle uplift. Mars appears to be intermediate between the earth and moon and is dominated by the Tharsis anomaly, with the possibility that gravity anomalies arise from both lithospheric and sublithospheric regions. If the Tharsis Province is supported entirely by the lithosphere, then stress levels are of the order of 1 kbar, even with isostasy, which may imply partial dynamic support. Preliminary analysis of Venus gravity data suggests a power spectrum grossly similar to that of the earth, but systematic discrepancies among some harmonics may indicate differences in the nature of interior processes for the two planets. In particular, there appears to be a higher correlation of long‐wavelength gravity with topography for Venus.