Basin evolution and crustal structure on Mercury from gravity and topography data

Introduction:

Available gravity and topography data derived from MESSENGER mission provide a great opportunity to investigate the surface and the interior structures of Mercury’s impact basins. In contrast to previous studies, which focused on image data, topography, or gravity alone, we use the complementary data sets to obtain a more comprehensive picture of basins and possibly their related subsurface structures.

Methods:

In this study we use image, gravity and topography data obtained by the MESSENGER spacecraft, from the Mercury Dual Imaging System (MDIS), the Mercury Laser Altimeter (MLA) as well as a radio science experiment for gravity field modelling. Digital Terrain Models from stereo images (150m/px) [1] were used in combination with mosaiced image data (166m/px) [2] to support identification of the basins. Using the most recent gravity model [3], combined with a topography model [4], we calculated Bouguer anomalies [5] and determined a crustal thickness model[6].

Results:

We created an inventory of 319 impact basins (>150 km) classifying their morphological and gravitational characteristics, including measurements of gravity disturbance, Bouguer anomaly, crustal thickness and morphometrical measurements (Fig 1). Basins tend to undergo relaxation processes over time, which would explain the high number of modified basins.

Fig 1: A classification scheme was chosen according to rim preservation state, appearance of terraces, filling of the basin floor, depth and diameter.

 

With increasing diameter, basins were found to show more complex gravity signatures (Fig 2).  In both gravity anomalies, gravity disturbance as well as Bouguer anomaly, strong centred anomalies reflect high mass and/or density concentrations inside the impact basins, that were caused by an uplift of mantle material after the crater excavation phase [8]. The negative collar of the Bouguer anomaly profile suspected to be a consequence of depression of crust-mantle boundary, i.e. thickening of the crust. Consequently, profiles of Bouguer anomaly reflect profiles of the crust-mantle boundary.  With increasing diameter, the crustal thickness is showing a decrease in rim and centre proving a link between crustal thinning and impact basin formation (Fig 3). 

Fig. 2: [a]Gravity disturbance are mostly negative for small basins, but become positive for larger basins. [b] Bouguer anomaly showing positive centre and negative rim area (bullseye pattern).

 

Fig. 3: Bouguer anomaly contrast and crustal thickness ratio from centre and rim area. 

References:

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