Reconciling Mars InSight Results, Geoid, and Melt Evolution With 3D Spherical Models of Convection.

We investigate the geodynamic and melting history of Mars using 3D spherical shell models of mantle convection, constrained by the recent InSight mission results. The Martian mantle must have produced sufficient melt to emplace the Tharsis rise by the end of the Noachian–requiring on the order of 1–3 × 109 km3 of melt after accounting for limited (∼10%) melt extraction. Thereafter, melting declined; however, abundant evidence for limited geologically recent volcanism necessitates some present‐day melt even in the cool mantle inferred from InSight data. We test models with two mantle activation energies and a range of crustal Heat Producing Element (HPE) enrichment factors and initial core‐mantle boundary temperatures. We also test the effect of including a hemispheric (spherical harmonic degree‐1) step in lithospheric thickness to model the Martian dichotomy. We find that a higher activation energy (350 kJ mol−1) rheology produces present‐day geotherms consistent with InSight results, and crustal HPE enrichment factors of 5–10‐times produce localized melting near or up to present‐day. The 10‐times crustal HPE enrichment is consistent with both InSight and geochemical results and also produces present‐day geoid power spectra consistent with Mars. However, calculations that match the present‐day geoid power spectra require more than 60% melt extraction to produce the Tharsis swell. The addition of a degree‐1 hemispheric dichotomy, as an equatorial step in lithospheric thickness, does not significantly improve upon melt production or the geoid. Plain Language Summary: Mars' mantle needed to produce an extremely high volume of melt by ∼3.7 billion years ago in order to build the immense volcanic plateau of Tharsis. There is also abundant evidence for small volumes of geologically recent volcanism, yet the InSight mission results are consistent with a relatively cool present‐day mantle. We use 3D numerical models of the Martian mantle to determine what properties can produce a melting history and present interior temperatures consistent with InSight results and Mars' volcanic history. We test sets of models with two different sensitivities of the mantle viscosity to changes in temperature (i.e., activation energy), and a range of enrichment factors in Heat Producing Elements in the crust. We also test the effect of including a simplified version of the Martian hemispheric dichotomy. Our models with the higher activation energy and 10‐times crustal enrichment in HPEs (consistent with Mars' crustal composition) produce melt near the present‐day as well as temperature profiles consistent with InSight. However, it is very difficult to produce sufficient melt to form Tharsis in such a cool mantle, without assuming most of the melt produced in the mantle reaches the surface. Addition the simplified dichotomy does not significantly improve our results. Key Points: Our results favor a higher activation energy mantle rheology and 10‐times crustal heat producing element enrichment factorEven with the cool mantle consistent with InSight results, we obtain models having melt up to present‐dayProducing sufficient melt to form Tharsis requires a degree of melt extraction 6‐times as efficient as Earth's mid‐ocean ridges [ABSTRACT FROM AUTHOR]