Radial gravity anomalies associated with the ejecta of the Orientale basin

Abstract

GRAIL gravity data shows small-scale gravity anomalies radiating out from the Orientale basin between distances of ∼550 km (average distance to the Cordillera ring) and ∼1000 km. These radial gravity lineations are sometimes associated with secondary crater chains or catenae, of which the specific details of formation and their relation to the basin ejecta remain unclear. However, many of the radial gravity anomalies have no clear topographic signature at all. Typical anomalies have an average width of about 20 km, with a magnitude of ± 20 mGal, and a length of 50–200 km. Here we use gravity inversions, hydrocode modeling, and observations to investigate the radial gravity anomalies in more detail. Density inversion models show that the gravity can be matched by solutions ranging from broad low-amplitude anomalies with density contrasts of ± 20 kg/m3 extending to depths of tens of kilometers, to shallow high-amplitude anomalies with density contrasts of about ± 200–400 kg/m3 confined to the top ∼1–2 km of the crust. Hydrocode models of the low-velocity secondary impacts of basin ejecta show that the impacting material does not disperse – as occurs for hypervelocity impacts – but instead remains as a thin layer within the secondary craters. This remaining secondary projectile material derived from Orientale ejecta can explain the linear gravity anomalies associated with secondary crater chains and catenae. However, this does not explain the majority of linear gravity features that are not associated with secondary crater chains. Therefore, we conclude that the majority of radial gravity anomalies represent the structure of the ejecta deposit. This density variability within the ejecta of basins likely contributes substantially to the density variability of the shallow upper crust as a whole.