Martian Araneiforms: A Review

Araneiforms are enigmatic dendritic negative topography features native to Mars. Found across a variety of substrates and exhibiting a range of scales, morphologies, and activity level, they are hypothesized to form via insolation‐induced basal sublimation of seasonal CO2 ice. With no direct Earth analog, araneiforms are an example of how our understanding of extant surface features can evolve through a multipronged approach using high resolution change‐detection imaging, conceptual and numerical modeling, and analog laboratory work. This review offers a primer on the current state of knowledge of Martian araneiforms. We outline the development of their driving conceptual hypothesis and the various methodologies used to study their formation. We furthermore present open questions and identify future laboratory and modeling work and mission objectives that may address these questions. Finally, this review highlights how the study of araneiforms may be used as a proxy for local conditions and perhaps even past seasonal dynamics on Mars. We also reflect on the lessons learnt from studying them and opportunities for comparative planetology that can be harnessed in understanding unusual features on icy worlds that have no Earth analog. Plain Language Summary Araneiforms, more colloquially coined “spiders,” are strange branched networks of troughs that are carved in the Martian regolith within the south polar regions, poleward of ∼70°. They have been proposed to form in spring, when sunlight passes through and heats the Martian seasonal CO2 ice layer, causing gas to build up beneath it and crack the ice, scouring squiggly and branched troughs on the surface and depositing the eroded material in the form of a plume. Such a process does not occur on Earth, so since their original detection, scientists have used creative approaches to understand the formation of araneiforms; comprising computational mathematical modeling, small‐scale experiments in planetary chambers to recreate the process, and even citizen science campaigns, where planetary enthusiasts have helped to map their locations on Mars. We review the work that has been conducted to understand the formation of these beautiful and puzzling surface features and discuss how they may help us to understand seasonal change on Mars in the present‐day and even the past. We discuss how our understanding of araneiforms can be applied to other icy planetary surfaces and finally present gaps in our knowledge and ways