Latest Paper: Dune grainflows on Earth and Mars

This paper was part of (the now) Dr. Carin Cornwall’s Ph.D undertaken at the University of Ulster, Coleraine.

Cornwall, C., M. C. Bourke, D. W. T. Jackson and J. A. G. Cooper (2018). Aeolian slipface dynamics and grainflow morphologies on Earth and Mars. Icarus 314: 311-326.

Highlight 1: Grainflow morphologies on Earth are similar to those images on the Namib dune slipface suggesting similar aeolian morphodynamics on lee slopes.

Highlight 2: Grainflows on the Namib dune on Mars are thinner than Earth grainflows. This may be related to the amount and delivery mechanism of sediment to the slipface.

Highlight 3: When summer wind directions are incident to the Namib dune lee slope, dune advancement may result from large stoss ripple migration which regularly introduces sediment to the slipface.

Top: CTX/HRSC/Viking composite mosaic showing the location of the Namib dune on Mars (black rectangle below the Curiosity landing site oval) in relation to the Bagnold dune field.
Bottom: HiRISE image ESP_044172_1755 of the Namib dune and neighboring High dune with the curiosity rover in front of the Namib dune slipface.

Abstract

In 2015, an active dune field on Mars was visited up close by the Curiosity rover in Gale Crater providing the first high resolution ground images of fine scale windblown features not previously resolved from orbital-based imagery. For the first time, these images allow for direct comparison with terrestrial aeolian dynamics and provide critical ground truth data to bridge the gap between model predictions and satellite observations. The image data from the slipface on the Namib dune within the Bagnold dune field shows grainflow morphologies that are similar to dunes on Earth. Quantitative estimates of flow thickness, based on shadow length are presented for the grainflows on the Namib dune slipface and compared to grainflow characteristics measured by terrestrial laser scans from the Maspalomas dune field located in Gran Canaria, Spain.

Grainflow morphologies identified in the Maspalomas dune field, Gran Canaria, Spain, including hourglass grainflows accompanied by the formation of an alcove, smaller funnel and lobe flows, and slab flows that affect large areas of an entire slipface. Slipface height is approximately 2.2 m.

Using observations from Maspalomas to support interpretations of martian slipface dynamics, we discuss implications for the local wind regime, style of grainflow, seasonal activity, and dune migration.

The presence of multiple large-magnitude grainflows on the Namib slipface suggests an active aeolian environment, capable of delivering enough sediment to the slipface to initiate these flows and transport sediment to the bottom of the lee slope. However, the thinness of grainflows on the Namib dune, the formation of smaller grainflows directly below the dune brink and limited grainfall suggest a lower wind energy environment, at least for the most recent slipface activity. Large, actively migrating stoss ripples obliquely oriented to the dune crest regularly deposit sediment on to the upper slipface and may be a mechanism in which larger grainflow occur during seemingly low energy wind events.


Map of the grainflow morphology and slipface activity on the Namib dune on Mars (A). Mapped morphologies include: tensional cracks (yellow), ripples (blue), hourglass-shaped grainflows (red), and secondary grainflows (magenta). Panels B-D show magnifications of some of the secondary flows identified on the slipface.

This mechanism of sediment delivery may also explain the existence of a variety of slipface morphologies, both young and old, which are otherwise quickly erased on Earth due to sediment redistribution and grainfall.

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