A freezing ocean on late-stage Mars, recorded by wave-built foresets at the Zhurong landing site

Whether Mars ever sustained a long-lived northern ocean remains contested, and the wave climate of any such ocean has never been constrained by direct observation. The Zhurong rover's ground-penetrating radar (RoPeR) imaged 76 sub-parallel reflectors at 10-35 m depth beneath southern Utopia Planitia, dipping northward at 14.5° ± 2.9° and interpreted as coastal foresets. Here we invert this geometry for the wave conditions that built them. Reconstructed shoreline geometry places the site on a bay open to a 4.1-12 × 103 km fetch toward the northern ocean, yet the foreset dip constrains the maximum nearshore orbital velocity to 0.003-0.005 m s-1, two and a half to three orders of magnitude below the 1-15 m s-1 band synthesized from general circulation model constraints at any plausible atmospheric pressure. The inconsistency resolves if the ocean was freezing while the foresets accumulated: climate-model freezing timescales (complete freeze ≤104 yr) and our construction timescale (~50 yr) are compatible, and the low-wave ceiling is the expected signature of a decaying wave field as sea ice advanced. The host strata date to ~750 Ma (0.76-1.8 Ga), implying an Amazonian ocean whose final state, a freezing sea, is preserved in coastal sedimentary structure.