The honeycomb terrain on the Hellas basin floor, Mars: A case for salt or ice diapirism

Abstract

We present quantitative plausibility studies of potential formation mechanisms for the "honeycomb" terrain on the northwestern Hellas basin floor. The honeycomb terrain is a unique landscape of ∼10.5 × 5 km wide, mostly cell-shaped depressions that are arranged in a regular, dense pattern covering ∼36,000 km2. We argue against the honeycombs being (peri)glacial landforms (till rings, iceberg imprints, and thermokarst) or the result of igneous diapirism, as terrestrial analogs do not reproduce their key characteristics. Fossilized impact melt convection cells also appear to be an unsuitable interpretation, as melt solidification should not permit such structures to be retained. We present arguments in favor of salt or ice diapirism as honeycomb formation models. Honeycomb-sized diapirs could be formed by a ∼2 km thick salt layer (∼72,000 km3 for the entire honeycomb terrain), which might have been derived from the highlands north of Hellas Planitia - an area of abundant chloride signatures and intense snowfall according to ancient Mars climate models. Nearby volcanic activity ∼3.8 Ga ago potentially enabled recurring phases of (probably salty) meltwater runoff (as indicated by meandering channels) and might therefore have enabled evaporite deposition in the Hellas basin. Being twice as buoyant as salt, water ice would require an only ∼1 km thick layer (i.e., ∼36,000 km3) to form honeycomb-sized diapirs, which would be in agreement with a likely ∼2 km thick ice stability zone beneath the Hellas basin floor. However, it would remain an open question as to why we find only one such ice diapir landscape on Mars.