Salt or ice diapirism origin for the honeycomb terrain in Hellas basin, Mars?: Implications for the early martian climate

Abstract

The “honeycomb” terrain is a Noachian-aged cluster of ∼7km wide linear cell-like depressions located on the northwestern floor of Hellas basin, Mars. A variety of origins have been proposed for the honeycomb terrain, including deformation rings of subglacial sediment, frozen convection cells from a Hellas impact melt sheet, a swarm of igneous batholiths, salt diapirism, and ice diapirism. Recent work has shown that the salt or ice diapirism scenarios appear to be most consistent with the morphology and morphometry of the honeycomb terrain. The salt and ice diapirism scenarios have different implications for the ancient martian climate and hydrological cycle, and so distinguishing between the two scenarios is critical. In this study, we specifically test whether the honeycomb terrain is consistent with a salt or ice diapir origin. We use thermal modeling to assess the stability limits on the thickness of an ice or salt diapir-forming layer at depth within the Hellas basin. We also apply analytical models for diapir formation to evaluate the predicted diapir wavelengths in order to compare with observations. Ice diapirism is generally predicted to reproduce the observed honeycomb wavelengths for ∼100m to ∼1km thick ice deposits. Gypsum and kieserite diapirism is generally predicted to reproduce the observed honeycomb wavelengths for ≥ 600–1000m thick salt deposits, but only with a basaltic overburden. Halite diapirism generally requires approx. ≥ 1km thick halite deposits in order to reproduce the observed honeycomb wavelengths. Hellas basin is a distinctive environment for diapirism on Mars due to its thin crust (which reduces surface heat flux), low elevation (which allows Hellas to act as a water/ice/sediment sink and increases the surface temperature), and location within the southern highlands (which may provide proximity to inflowing saline water or glacial ice). The plausibility of an ice diapir mechanism generally requires temperatures ≤ 250K within Hellas in order to reproduce the observed diapir wavelength. Conversely, the viability of the salt diapir mechanism requires sufficiently thick evaporite deposits to accumulate in Hellas (generally ≃1–3km), which requires the emplacement and evaporation within Hellas of a 14–2045m global equivalent layer (GEL) of saline water (∼2×106 km3 to ∼3×108 km3). On the basis of our analysis, we conclude that ice diapirism is more likely due to the thin deposits (∼0.1–1km thick) and low water volumes required (only 0.3–24m GEL water), and the potential for either glacial deposits or a frozen ocean to supply the necessary ice. Salt diapirism requires thick evaporite deposits and high water volumes by comparison, and thus appears less likely.