The Acheron Dorsum on Mars: A novel interpretation of its linear depressions and a model for its evolution

- Acheron Dorsum is a ridge on Mars with unique characteristics.
- Fossae on Acheron Dorsum could result from gravitationally-induced self-spreading of the ridge, rather than endogenous upwelling.
- Numerical simulations indicate viscosity within ranges compatible with ice-rock mixture.

The Acheron Dorsum north of Olympus Mons on Mars is a unique 800 km-long arcuate ridge apparently unrelated to other adjacent morphologies. Acheron Dorsum is crossed by deep and wide fractures (“fossae”, here referred to also as Linear Depressions or “LDs”), commonly interpreted as grabens formed by local lithosphere stress in response to endogenous mantle upwelling. We suggest an alternate view that the linear depressions may result from gravitational self-adjustment of a loose deposit similar to the Deep-Seated Gravitational Slope Deformations (DSGSD or Sackung), a kind of slow mass movement well documented on Earth. Numerical simulations based on a finite difference numerical model devised for terrestrial DSGSDs reproduce the timing and position of LDs reasonably well provided that the viscosity of the viscoplastic layer is between ≈71015 and ≈1017 Pas. While rock masses typically exhibit much higher values, based on comparison with glacial examples, we find that these viscosities could be compatible to a rock-ice mixture.This leads to the hypothesis that the rock underneath Acheron Dorsum may be highly fragmented and that ice might have been involved in its deformation. Our hypothesis could shed light on the puzzling stress pattern on Acheron Dorsum, which follows approximately the ridge, without requiring the presence of hypothetical small-scale, endogenous currents.