Delta morphology reveals timings of early ocean regression and hydrologic shift on ancient Mars

Abstract Mars’ ancient hydrologic history remains a subject of debate, with competing interpretations suggesting either a large ocean or scattered lacustrine environments. Here, we show that delta morphology on Mars records a shift between these hydrologic regimes. Through controlled tank experiments, we identify two distinct delta types: “lobe-extruding deltas” formed during water-level falls and “stepped fan-deltas” formed during rising water levels. The lobe-extruding deltas occur near proposed paleo-coastlines and are dated to ~3.8–3.2 Ga, consistent with the existence of a Late Noachian–Hesperian ocean and its subsequent regression in the Late Hesperian. In contrast, stepped fan-deltas are found at a wider range of elevations and exhibit significantly younger ages (~2.1–0.1 Ga), suggesting episodic, localized lake formation during the Amazonian. Our findings constrain the timing of the planetary-scale transition from a global hydrosphere to localized aqueous environments and refine the understanding of Mars’ paleoclimate evolution and potential habitability.