Longitudinal ridges imparted by high-speed granular flow mechanisms in martian landslides

Giulia Magnarini,Harrison H Schmitt,Liran Goren,Peter M Grindrod,Thomas M Mitchell

doi:10.1038/s41467-019-12734-0

Abstract

The presence of longitudinal ridges documented in long runout landslides across our solar system is commonly associated with the existence of a basal layer of ice. However, their development, the link between their occurrence and the emplacement mechanisms of long runout landslides, and the necessity of a basal ice layer remain poorly understood. Here, we analyse the morphometry of longitudinal ridges of a martian landslide and show that the wavelength of the ridges is 2–3 times the average thickness of the landslide deposit, a unique scaling relationship previously reported in ice-free rapid granular flow experiments. We recognize en-echelon features that we interpret as kinematic indicators, congruent with experimentally-measured transverse velocity gradient. We suggest that longitudinal ridges should not be considered as unequivocal evidence for presence of ice, rather as inevitable features of rapid granular sliding material, that originate from a mechanical instability once a kinematic threshold is surpassed.

Highlights

The presence of longitudinal ridges documented in long runout landslides across our solar system is commonly associated with the existence of a basal layer of ice
The experimental work and stability analysis on rapid granular flows conducted by Forterre and Pouliquen[20,21] showed for the first time the spontaneous formation of longitudinal vortices that manifest as superficial longitudinal ridges and furrows, which initiation depends on the velocity of the flow, roughness of the basal surface, and the consequent strong shear at the base of the flow
We use state-of-the-art Mars Reconnaissance Orbiter imagery (CTX22 and HiRISE23 cameras) in order to analyse a giant martian landslide with some of the best defined longitudinal ridges (Fig. 1) for comparison with the longitudinal morphologies obtained during granular experimental slides, so to investigate the possibility that the same instability may be responsible for the longitudinal ridges and furrows of large planetary rock avalanches, as suggested by Borzsonyi et al.[19]

Summary

Results

Ridges morphometry with distance and with deposit thickness. We identified longitudinal ridges on the surface of the landslide deposit. In the east area (Fig. 2b), profiles P3, P4 and P5 show S/T ratio below 2, which is a consequence of a localised increase in the deposit thickness (Supplementary Table 2). We observed the appearance of new ridges between diverging ridges (Fig. 3a–d), confirming that the number of ridges increases with distance, as found from the morphometric analysis (Supplementary Tables 2 and 3). This is consistent with the observations of Forterre and Pouliquen[21] of transversal vortex drifting accompanied by annihilation and creation of ridges. The en-echelon features that we report in Fig. 3 pointing at faster-flowing ridges is consistent with both the dense regime and with the dilute regime after topography inversion (Fig. 4)

Discussion

Methods

Code availability