Inferring Airflow Across Martian Dunes From Ripple Patterns and Dynamics

D R Hood,K P Roback,K Runyon,R C Ewing,J.-P Avouac,M Mcenroe

doi:10.3389/feart.2021.702828

Abstract

Large ripples form striking patterns on the slopes of martian sand dunes which can be mapped and tracked using high-resolution optical images. The ripples vary in orientation, wavelength, plan-view morphology, and rates of migration. The variations in the ripple patterns are recognized to signal the effects of the regional and local winds and feedbacks between winds and dune topography. We examine the ripple patterns and the motion of these ripples to interpret airflow dynamics around dunes in the dune field at Nili Patera. We find that coincident changes in ripple patterns and migration rates in dune wakes indicate reattachment lengths of 4–7 brink heights. This reattachment length is similar to length scales of flow reattachment for airflow over dunes measured on Earth despite the differences in aeolian environment. Furthermore, ripples on dune flanks are shown to behave according to terrestrial models for ripple development on steep slopes. Compensating for these slope effects allows them to act as indicators of dune-modified and regional wind directions. Changes in ripple patterns and migration rates also signal the response of dunes and airflow during dune collisions. Collectively, we find that differences in ripple patterns connected to changes in migration rate provide information on airflow over and around dunes. This detailed assessment of ripple measurement and ripple migration rates advances the use of ripples on martian dunes and sand sheets to infer dune- and field-scale wind dynamics. These measurements also indicate that the low density atmosphere on Mars does not significantly modify the behavior of wind-topography interactions compared to Earth. Such observations provide targets for computational fluid dynamic and large-eddy simulation models seeking to reveal complex airflows across dune fields both on Earth and on Mars.

Highlights

The spatial changes in dune patterns in the dune field and the abundance and nature of the interactions provide a means to assess how ripple patterns and migration rates are modified by airflow moving over and around dunes
We examine how the dominant, regional airflow is deflected by dune topography by measuring 2-D ripples that exist on dune flanks (Wind Orientation)
There is no apparent relationship between crest-brink separation distance and reattachment length as seen on Earth (Baddock et al, 2011), though there is a lack of sampling diversity as most transects show a close crest-brink spacing

Summary

Introduction

Meter-scale ripples cover sand dunes on Mars (Schatz et al, 2006; Bridges et al, 2007; Ewing et al, 2010; Silvestro et al, 2010; Chojnacki et al, 2011; Bridges et al, 2012; Liu and Zimbelman., 2015; Lapotre et al, 2016; Runyon et al, 2017; Roback et al, 2020). Unlike on other bodies, both martian dunes and ripple patterns are visible in satellite images, which allows detailed observation and mapping of ripple patterns across dunes This affords the opportunity to assess wind flows across dunes and through dune fields in a manner not feasible via satellite remote sensing on Earth. These observations can improve understanding of meteorological patterns on Mars (Ayoub et al, 2014) and reduce uncertainty associated with winds affecting landing or launch crafts

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Discussion

Conclusion