Irregular polygonal ridge networks in ancient Noachian terrain on Mars

Numerous irregular, polygonal ridge networks have been mapped in the Nilosyrtis and Nili Fossae regions on Mars. Hypotheses ranging from impact-induced breccia dikes to mineralized fractures have been proposed for their formation. In order to constrain the range of potential formation mechanisms, we mapped the distribution of 952 morphologically similar polygonal ridge networks in Arabia Terra, Terra Sabaea, Tyrrhena Terra, Nili Fossae and Nilosyrtis across an area of 2.8 × 107 km2, representing a 100× larger mapping area in comparison with previous studies of these regions. Of these ridge networks, 864 out of 952 (91%) are confined to Noachian-aged ‘etched’, ‘dissected’ or ‘cratered’ terrain, suggesting that they represent ancient records of crustal fracture and fill processes. Previous spectral analyses of selected ridges in Nili Fossae revealed that both the ridges and their host units are phyllosilicate-rich, but found that the ridges might have larger grain sizes in comparison with their host units. Our thermophysical analysis of seven ridge networks studied previously using spectral data indicates that the ridges typically have lower average thermal inertias (409 ± 120 J m−2 s−0.5 K−1) than their host units (477 ± 138 J m−2 s−0.5 K−1). These lower ridge thermal inertias are contrary to what is expected, and could be due to sub-pixel mixing with nearby detrital material. While it is not possible to determine the precise formation mechanism of these polygonal ridge networks from our new data, their formation can be assessed in terms of three possibly separate processes: (1) polygonal fracture formation, (2) fracture filling and (3) exhumation. We find that polygonal fracture formation by impact cratering and/or desiccation of sedimentary host deposits is consistent with our results and previous spectral studies. Once the polygonal fractures have formed, fracture filling by clastic dikes and/or mineral precipitation from aqueous circulation is most consistent with our results. Exhumation, probably by aeolian processes that eroded much of these ancient Noachian terrains where the ridges are present caused the filled fractures to lie in relief as ridges today. If these ridge networks represent ancient, fossilized remnants of aqueous circulation, then our new results suggest that groundwater processes were widespread across Noachian terrain. In-situ studies by the Perseverance rover in these regions might help shed light on this potential former groundwater activity, and better constrain the formation mechanisms and histories of these ridge networks.