Jigsaw Puzzle Polygons in the Western Qaidam Basin

Polygons are natural geometric patterns on the ground of Mars with various shapes and a wide range of sizes. The morphological features of polygons can reveal their forming environments, the role of water and the ancient Martian climates. Polygons are also widespread on Earth that, comparatively, can help to understand the forming mechanism of Martian polygons. The low-rim polygons on Mars have many corresponding terrestrial analogs, which have been widely investigated that indicate the freezing and thawing genesis of low-rim Martian polygons. However, the genesis of the raised-rim Martian polygons is still unknown because the terrestrial analogs of them are rare.

The western Qaidam Basin was suggested to be the largest terrestrial analog of Mars, in which the raised-rim polygons of diverse shapes have recently been reported. Here, we report a unique type of raised-rim polygons in the playa area of the western Qaidam Basin, which is called jigsaw puzzle polygons. This type of polygons shows unique boundaries as they have tabs vertically out the sides and blanks vertically into the sides. The XRD measurements and grain size distribution analyses signify that those jigsaw puzzle polygons were formed by the combination of several processes including the deposition of salts, wind erosion and re-deposition of salts. We suggest that the formation of jigsaw puzzle polygons is closely related to the evolution of playas, and wind erosion has been the major force in shaping the jigsaw-puzzle structure.

The raised-rim polygons on Mars might be developed by the similar processes inclusive of the dissolution of episodic brines or recharged water from water-rich evaporites and a following evaporation. Although raised-rim polygons are rarely observed on Mars, e.g., in the Roddy Crater reported by Xiao et al. (2016), they may indicate the activity of brine water on the early Mars.