Limits on polygonal organization of boulders in the Martian northern lowlands

Abstract

Polygonal terrain on Mars can form via thermal contraction and subsequent fracturing of the permafrost layer and covers much of the surface poleward of ∼60°. In similar terrains on Earth, seasonal freeze-thaw processes create a range of diverse landforms, including several in which clasts on the surface congregate into sorted circles and polygons. In the Martian northern lowlands, several investigations into boulder patterns have come to differing conclusions on whether analogous organization of clasts is present on Mars, whether there is an association between boulders and polygonal fracture margins, and what periglacial process may cause such organization in the modern environment that does not support seasonal melt. To address this discrepancy, we identify and measure boulders in the Martian northern lowlands with the Martian Boulder Automatic Recognition System (MBARS) and assess boulder spatial patterns to determine if boulders are organized into the margins of underlying fracture polygons. Sixty (60) Images from the High-Resolution Imaging Science Experiment (HiRISE) camera aboard the Mars Reconnaissance Orbiter (MRO) with priorly identified and measured polygons make up our survey, in which MBARS characterized 20 million boulders. We find that boulder patterns are not random across the northern lowlands and tend to be clustered with varying intensity. However, analysis of boulder pairwise distances shows that boulders are not generally organized into the 5–10 m polygonal patterns expected from an alignment of boulders to fracture margins. The lack of widespread polygonal organization of boulders indicates that processes responsible for the modern fracture polygons cannot organize meter-scale boulders towards their margins. This greatly reduces the likelihood of any terrestrial-like freeze-thaw organization occurring since the formation of the modern polygonal terrain in the Martian northern lowlands. Isolated instances of boulder patterns consistent with polygonal organization are found at the northern end of our survey. These instances could indicate a restriction of boulder-organizing processes only to the near-polar terrains but are better explained as selective preservation of paleo-organization. Plain language summaryWe have abundant evidence that the near-polar terrains on Mars have water ice buried at or very near the surface, much like on Earth. As this ice-rich surface cools in the winter, regularly spaced, roughly hexagonal cracks appear forming so-called “polygonal terrain”. In similar permafrost-dominated terrains on Earth, the presence of liquid water near the surface can cause rocks on the surface to be pushed outward during freezing. This eventually leads to the formation of stone circles or stone piles outlining the polygons, often forming large networks of sorted polygons. Using high-resolution images of Mars, we can see boulders as small as 1 m across, and it has been suggested that these boulders might also be organized into the edges of polygons on Mars. This raised the question: did these patterns form through Earth-like, wet processes? Or is there a way to make these patterns without liquid water? To answer this question, we surveyed sixty (60) images of Mars and used the Martian Boulder Automatic Recognition System (MBARS) to identify and measure the boulders in each image, totaling 20 million boulders. We determined that the boulders are not organized into a polygonal pattern, except in a few rare cases. Because the boulders are not organized, it is unlikely that any Earth-like wet processes or an unknown dry process is pushing boulders towards the edges of the fracture polygons. In the few places we do see this organization, it might be a pattern formed sometime in Mars' past when liquid water could exist at or near the surface.