Beneath Isidis Planitia

Abstract

Isidis Planitia is the third-largest impact basin on Mars, located on the north-south dichotomy boundary. It is marked by major geological events that have occurred both globally and regionally. The basin shows two distinct faces. The first is the presence of a high-density subsurface mass concentration, the strongest on Mars, outside of the major volcanic provinces. The second is a clustering of kilometre-scale pitted cone chains of unknown origin, hypothesised to be of volcanic, glacial, or sedimentary origin. This thesis investigates whether and to what extent the subsurface structure of Isidis Planitia controls the surface formation of the pitted cones.

To investigate the interior structure, this thesis combines topographic and gravitational data to construct subsurface models. The models are constrained against observed gravitational anomaly to fit plausible layer geometries and densities. The best-fit model is used to assess the basin’s thermal and stress conditions. The relationship between this subsurface structure and the distribution of pitted cones is then evaluated by correlating with both surface and interior features.

The best-fit structure contains a sedimentary layer that is significantly thicker than previous estimates, at approximately 1.3 km. It also contains a central melt sheet (combined with lava deposits) up to 19 km thick, extending across the plains. Most importantly, the model requires a large volume (approximately 1.7 x 10^6 km³) of mantle-like, high-density materials in the inner basin. This element reaches the near-surface and is identified as a significant plutonic intrusion. The pitted cone distribution shows a poor direct correlation with this subsurface, but aligns strongly with the surface topography, conforming to pre-existing wrinkle ridges. Two distinct sets of wrinkle ridges are observed, with the latter one linked to pluton-driven deformation and subsequent cone emplacement.

Findings support the formation of pitted cones as volcanic rootless cone analogues, resulting from the interaction of a lava deposit with near-surface volatiles. The lava deposit is likely sourced in the north-west, flowing out to the north-east and south-east. The evidence suggests the source is located on the Syrtis Major Planum complex, or derived from the basin floor linked to the plutonic intrusions. Although limited by the available topographical and gravitational data, this first complete synthesis of Isidis Planitia is capable of explaining all its key characteristics in a coherent timeline.

Related dataset 4TU.ResearchData: https://doi.org/10.4121/7f02991f-daac-44c4-8f86-1deb5236b1ec