New Support for Hypotheses of an Ancient Ocean on Mars

A new analog for the giant polygons in the Chryse-Acidalia area suggests that those features may have formed in a major body of water – likely a Late Hesperian to Early Amazonian ocean. This analog -terrestrial polygons in subsea, passive margin basinsderives from 3D seismic data that show similar-scale, polygonal fault systems in the subsurface of more than 50 terrestrial offshore basins. The terrestrial and martian polygons share similar sizes, basinwide distributions, tectonic settings, and association with expected fine-grained sediments. Late Hesperian deposition from outflow floods may have triggered formation of these polygons, by providing thick, rapidly-deposited, fine-grained sediments necessary for polygonal fracturing. The restriction of denselyoccurring polygons to elevations below ~ 4000 m to 4100 m supports inferences that a body of water controlled their formation. Those same elevations appear to restrict occurrence of polygons in Utopia Planitia, suggesting that this analog may apply also to Utopia and that similar processes may have occurred across the martian lowlands. Introduction: The potential for an ancient martian ocean has been discussed for more than 20 years [1-14] though the subject is still controversial [15-20]. This study adds to that discussion with a new analog for the martian giant polygons [21] that implies their development in a major body of water [22]. Giant polygons are extensively developed in both Chryse/Acidalia and Utopia of the martian lowlands (Figs. 1-2), and although they have been recognized since the 70’s, their origin and significance are still debated [summarized in 22]. These polygons range up to ~10-15 km across, and their large size distinguishes them from a variety of smaller-scale polygons (usually < 250 m) that have been observed on Mars. Until recently there were few recognized examples of such large-scale polygonal features on Earth. However, with the advent of 3D seismic data, kilometer-scale polygonal fracture systems have been recognized in more than 50 offshore basins on Earth (Fig. 3) [23-29]. Terrestrial kilometer-scale polygons: The terrestrial polygons are thought to result from sediment compaction/dewatering in fine grained materials (muds) that have been rapidly deposited in settings lacking strong horizontal stress (passive margins) [2122]. These subsea polygons occur in water depths of tens to thousands of meters and at burial depths from the near surface to ~1000 m below the sediment-water interface. They can have basinal extent (to > 10 km) and are commonly associated with fluid expulsion structures such as mud volcanoes (MV’s) and depressions called pockmarks that result from subaqueous gas release. Giant polygons in Chryse-Acidalia: Giant polygons are widespread in northern Chryse and southern Acidalia (Figs. 1-2) where they cover more than 10 km. Their distribution was mapped in 1986 using Viking data [30-31]. In this region, the giant