The formation of lunar mascon basins from impact to contemporary form

Andrew Freed (1) , Brandon Johnson (1, 2) , David Blair (1) , H. Melosh (1) , Gregory Neumann (3) , Roger Phillips (4) , Sean Solomon (5, 6) , Mark Wieczorek (7) , Maria Zuber (2)

1 Purdue University [West Lafayette]
2 EAPS - Department of Earth, Atmospheric and Planetary Sciences [MIT, Cambridge]
3 GSFC - NASA Goddard Space Flight Center
4 Department of Earth and Planetary Sciences [St Louis]
5 LDEO - Lamont-Doherty Earth Observatory
6 Carnegie Institution of Washington
7 IPGP (UMR_7154) - Institut de Physique du Globe de Paris

Positive free-air gravity anomalies associated with large lunar impact basins represent a superisostatic mass concentration or "mascon." High-resolution lunar gravity data from the Gravity Recovery and Interior Laboratory spacecraft reveal that these mascons are part of a bulls-eye pattern in which the central positive anomaly is surrounded by an annulus of negative anomalies, which in turn is surrounded by an outer annulus of positive anomalies. To understand the origin of this gravity pattern, we modeled numerically the entire evolution of basin formation from impact to contemporary form. With a hydrocode, we simulated impact excavation and collapse and show that during the major basin-forming era, the preimpact crust and mantle were sufficiently weak to enable a crustal cap to flow back over and cover the mantle exposed by the impact within hours. With hydrocode results as initial conditions, we simulated subsequent cooling and viscoelastic relaxation of topography using a finite element model, focusing on the mare-free Freundlich-Sharonov and mare-infilled Humorum basins. By constraining these models with measured free-air and Bouguer gravity anomalies as well as surface topography, we show that lunar basins evolve by isostatic adjustment from an initially subisostatic state following the collapse stage. The key to the development of a superisostatic inner basin center is its mechanical coupling to the outer basin that rises in response to subisostatic stresses, enabling the inner basin to rise above isostatic equilibrium. Our calculations relate basin size to impactor diameter and velocity, and they constrain the preimpact lunar thermal structure, crustal thickness, viscoelastic rheology, and, for the Humorum basin, the thickness of its postimpact mare fill.

Domaines: Planétologie

Andrew Freed, Brandon Johnson, David Blair, H. Melosh, Gregory Neumann, et al.. The formation of lunar mascon basins from impact to contemporary form. Journal of Geophysical Research. Planets, 2014, 119 (11), pp.2378-2397. ⟨10.1002/2014JE004657⟩. ⟨hal-02458602⟩