The Origin of Non-mare Mascon Gravity Anomalies on the Moon

Introduction: Although a significant fraction the mascon gravity anomalies can be explained by the flexural support of the mare basalts within the basins, in some cases this explanation does not suffice. A number of the lunar basins exhibit mascons in excess of that which can be accounted for by the maria, and some basins possess central gravity anomalies yet lack mare fill altogether [1]. An example of a prominent non-mare mascon is found in Orientale, in which the central gravity anomaly reaches a value of ~180 mGal relative to the field outside the basin, but the mare partially covering the basin floor has a thickness less than 600 m [2]. Neglecting the flexural response to the mare loading, the maximum gravity anomaly that can arise from the mare alone is ~80 mGal. These non-mare mascons were previously interpreted as arising from the instantaneous super-isostatic rebound of the basin floor and underlying mantle plug following the impact [1, 3]. However, this mechanism would require a sufficiently thick lithosphere to exist in the moments following the impact, which would seem at odds with the excavation of the majority of the lithosphere and extreme thermal and shock weakening of the remainder. This study suggests instead that the non-mare mascons are a secondary effect of the gradual flexural rebound of a thickened annulus of sub-isostatic crust surrounding the basins [4]. This is supported by gravity observations and finite element models. Observations: In the free air gravity, those basins that possess non-mare mascons are also surrounded by annuli of strongly negative gravity anomalies (Figure 1). These annuli likely correspond to rings of thickened yet sub-isostatic crust, originating as basin ejecta [3]. This is consistent with models of basin formation, which predict the basins and their surroundings to be left in a strongly sub-isostatic state in the after-math of the impacts [4]. These over-thick crustal roots would have acted as upward-directed loads on the lithosphere, causing flexural uplift of the annuli, which would uplift the basin center as well. Though it could be suggested that the negative gravity rings surrounding the basins arise as a result of the flexural subsidence beneath the maria, this is inconsistent with the relative magnitudes of the anomalies. The total present-day loads associated with the central mascons and surrounding negative annuli can be quantified by integrating the gravity anomaly relative to the surroundings over the area of the mascon/annulus. For three of the basins that possess significant non-mare mascons, the magnitude of the integrated negative gravity anomaly over the annulus exceeds the magnitude of the integrated positive anomaly over the mascon by factors of 5-25 (Table 1). Thus, the central mascon could plausibly be explained as an effect of the flexural uplift of the annulus together with the basin center, but the opposite scenario is not viable. In contrast, basins such as Imbrium and Serenitatis possess sufficient mare fill to explain the observed gravity anomalies and lack significant negative gravity annuli.