The honeycomb terrain on the Hellas basin floor, Mars: A case for salt or ice diapirism. Issue 4 (28th April 2016) - Bernhardt, H.
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 km 2 . 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 km 3 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 km 3 ) 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. Key Points: Previously favored formation models for the honeycomb terrain are incompatible with detailed observations and terrestrial analog studies Upwelling of a ~2 km thick salt layer could have formed the honeycombs and is in agreement with the geological context Upwelling of a ~1 km thick water ice layer could have formed the honeycombs but then their uniqueness on Mars needs to be explained