Bestandsaufnahme von Einschlagbecken aus Schwerkraft und Topographie

Gravitational data in combination with the surface morphology of the basins may help improve our understanding of their formation processes and alterations with time. Gravity anomalies hint at complex interior structures, such as mass and density distributions in the upper crust of the planet, which is beneficial for identifying highly degraded or buried basins. We create an inventory of large basins, for which we introduce a classification scheme according to morphological and gravitational signatures.

For this, we use topographic digital terrain model (DTM) that was derived by the Mercury Dual Imaging System (MDIS), involving a narrow-angle- as well as a wide-angle camera. The gravity data of the Mercury Laser Altimeter (MLA) resulted in a gravity field model with a resolution of degree and order 160, equivalent to approximately 28 km in the spatial domain. 

A clear correlation is noticed between the topography and gravity data, in particular impact basins are well detectable in both data sets. Their global distribution is non-uniform. Around 60% of basins are located on the western hemisphere. This asymmetric pattern may be caused by (I) lateral thermal variances of the crust, (II) synchronous spin- and orbital periods of the planet in its early history, which later changed to its presently observed 3:2 spin-orbit resonance , (III) resurfacing events that includes the northern smooth plains and following flooding of existing basins on the eastern hemisphere, which would eventually bury smaller complex basins.

High positive gravity signals were also recognized in the Bouguer anomaly map. Some basins (>350 km) possess a positive Bouguer anomaly in the basin centre surrounded by a negative anomaly annulus (bullseye pattern). Gravity data are reflecting mass/density deficits and excesses in the planet’s subsurface structures. The high mass and density concentrations may be caused by an uplift of mantle material after the crater excavation phase. The excavation was followed by an isostatic adjustment caused by cooling and contraction of the melt pool. Subsequently, crust is expected to be thinner.

We investigated the amplitude of Bouguer anomalies in the basin centers and surroundings as a measure of crustal thinning, which may hint at basin relaxation state. Lunar observations showed, that young basins with large diameters should contain strong positive anomalies because of limited relaxation due to high viscosity of a cooler planet. In contrast, a less pronounced gravity anomalies hint at higher relaxation due to lower viscosity and a hotter crust in the planet’s earlier history.