Mars' Hourglass Landforms as Local Source-to-Sink Fluvial Systems

Mars' landscapes offer enigmatic clues about a once wetter climate. Particularly puzzling are hourglass landforms, distinguished by two small (10–100 s km²) branching ridge systems connected by a narrow neck. This geometry resembles a source-to-sink fluvial system, but occurs on relatively flat terrain without a clear drainage direction. Here, we characterize 13 hourglass landforms and branched ridge networks that occur near the crustal dichotomy boundary and compare them with flume experiments and terrestrial analogs. We find that hourglass landforms are composed of branching and sinuous fluvial ridges, indicating that they are ancient river deposits exposed in positive relief due to substantial differential erosion. Typically, one side of the hourglass is composed of a ridge network with larger and more distinct ridges (type 1), whereas the other network has smaller cross-cutting ridges (type 2). In some cases, a remnant crater rim divides the two sides, with the type 1 network eroded into the crater wall, indicating a drainage network, and the type 2 network bounded by the crater, indicating an alluvial fan. Results indicate hourglass landforms are eroded remnants of small catchment-fan drainage systems that have experienced major climate change. They formed following impact cratering in a wet climate by runoff or seepage erosion where the crater breached the groundwater table. Subsequent wind erosion in a dry climate created ridge networks and completely removed the antecedent catchment-fan topography. Our findings on the distinction between different types of hourglass networks may help differentiate distributary from tributary networks in fluvial ridge systems elsewhere on Mars.