Viewing lobate patterns on Earth and Mars as climate-modulated fluid-like instabilities

Earth and planetary surfaces are home to some of the most complex soft materials in existence; for example, typical soil or river sediment is composed of a heterogeneous mixture of fluids, organic matter, and sticky sediment grains with different shape and size. Sediment transport dynamics result in striking landscape patterns that exist at time and length scales well beyond those typically studied in Soft Matter Physics. For example, hillslopes in cold regions commonly feature downslope-oriented wavelike patterns known as solifluction terraces, which often break into cross-slope fingers or solifluction lobes. Similar lobate patterns are observed on Martian crater walls in high latitude regions. While these features seem to form only in the presence of frost-heave processes, the specific ingredients required for their formation are not known, and it remains unclear why we only observe these patterns in cold places. We present a case for viewing solifluction patterns as climate-modulated, fluid-like granular instabilities. Drawing on findings in soft matter and granular physics, fluid mechanics, and periglacial geomorphology, we use a combination of theory, remote sensing, numerical modeling, published field data and preliminary laboratory experiments to explore the nature of these enigmatic hillslope features on both Earth and Mars. We find that lobate patterns on both planets obey a simple theoretical first-order morphologic scaling similar to that of surface tension-induced fluid instabilities at flow fronts, such as paint dripping down a wall. We explore several alternative working hypotheses for the conditions needed to initiate these non-inertial instabilities, including rheological origins, buckling instabilities, frost heave dynamics, and cohesion controls. Our work highlights the importance of drawing from different fields to tackle challenging problems in hillslope geomorphology, and outlines many remaining open questions about the nature of soil creep in cold regions.