Isidis basin: Site of ancient volatile-rich debris layer

Abstract

The interior plains of the Isidis impact basin on Mars contain landforms which have previously been interpreted as volcanic features. These landforms can be classified into two broad units: hillocky terrain and ridged terrain. Hillocky terrain, the most central unit, consists of isolated mounds commonly arranged in arcuate chains. Ridged terrain contains systems of parallel, curvilinear ridges and occurs in a narrow band almost completely encircling the hillocky terrain. Ridges range from approximately 10 to 40 km long, 0.5 to 1.0 km wide, and 0.15 to 0.3 km high. Ridge properties and their distribution within Isidis appear inconsistent with characteristics of typical volcanic fields on Earth and Mars. Alternatively, we suggest that the Isidis landforms reflect the deposition and subsequent removal of a thick layer of material within the basin. Crater statistics and the morphology of additional features within Isidis provide further evidence that a major resurfacing event occurred within the basin, although depositional and erosional processes are currently minimal. Terrestrial ice-cover disintegration results in the development of landforms such as moraines, kames, and eskers, and provides possible analogs for the features which occur in Isidis. The Isidis cover, however, probably contained a vast amount of debris, remained stationary throughout most of its existence and eventually sublimated, thus differing from most terretrial glaciers. A global survey of Viking Orbiter images reveals ridges elsewhere on Mars with similar characteristics or spatial distribution, thereby indicating that the Isidis layer may have been part of a more widespread epoch of deposition. This epoch possibly coincided with the period of major outflow channel formation when subsurface volatiles were released.