17:15 〜 19:15
[PPS09-P10] Reconstructing Ancient Martian Hydrology: Insights from Delta Morphology and Tank Experiments
キーワード：デルタ地形、火星、水槽実験、海水準変動
The existence and duration of an ancient Martian Ocean remain a subject of debate. Substantial evidence suggests that before 3.6 Ga, a vast ocean may have occupied the northern hemisphere. However, its fate and the continuity of aqueous environments beyond this period remain unresolved. Competing hypotheses propose either (1) a once-existing ocean that later disappeared, followed by localized lacustrine environments persisting into the Amazonian, or (2) the absence of a global ocean from the beginning, with only transient and regionally restricted water bodies. To address this uncertainty, we investigate Martian delta morphology through a combination of controlled tank experiments and remote sensing analysis.
We analyzed Martian satellite imagery to identify deltas analogous to those in our tank experiments, examining their distribution, elevation, and morphology. Formation ages were estimated using crater size-frequency distribution (CSFD) analysis, accounting for potential uncertainties such as resurfacing effects. Lobe-extruding deltas (having elongated lobes) occur in plains and open basins at -2077 to -3122 m, having a good correlation with estimated ancient coastlines. CSFD indicates their formation at ~3.8–3.2 Ga, suggesting they developed during ocean regression. Deltaic steps, in contrast, are found in closed basins across a broader elevation range (-5000 to +1950 m), with ages ~3.7–0.5 Ga and a peak around 0.5 Ga, indicating more recent localized water activity. Inflow channel orientations also differ: lobe-extruding deltas predominantly exhibit eastward to northeastward flow, consistent with drainage into a northern ocean, whereas deltaic steps display greater directional variability, often trending southward, suggesting localized hydrological processes.
Based on these results, it is considered that the lobe-extruding deltas were formed at coastlines during the period when the ancient ocean existed and formed due to sea-level fall in the range of -2077m to -2760m at around 3.6-3.3 Ga. On the other hand, deltaic steps were formed by rising water levels in the basin or crater during 1.6-0.5 Ga. In addition, we did not find a delta aged between 3.2 and 1.6 Ga, suggesting that fluvial processes were not active and the oceans most probably did not exist during this period. Therefore, our study revealed ocean disappearance at 3.6-3.3 Ga and localized lacustrine environments at 1.6-0.5 Ga.
We analyzed Martian satellite imagery to identify deltas analogous to those in our tank experiments, examining their distribution, elevation, and morphology. Formation ages were estimated using crater size-frequency distribution (CSFD) analysis, accounting for potential uncertainties such as resurfacing effects. Lobe-extruding deltas (having elongated lobes) occur in plains and open basins at -2077 to -3122 m, having a good correlation with estimated ancient coastlines. CSFD indicates their formation at ~3.8–3.2 Ga, suggesting they developed during ocean regression. Deltaic steps, in contrast, are found in closed basins across a broader elevation range (-5000 to +1950 m), with ages ~3.7–0.5 Ga and a peak around 0.5 Ga, indicating more recent localized water activity. Inflow channel orientations also differ: lobe-extruding deltas predominantly exhibit eastward to northeastward flow, consistent with drainage into a northern ocean, whereas deltaic steps display greater directional variability, often trending southward, suggesting localized hydrological processes.
Based on these results, it is considered that the lobe-extruding deltas were formed at coastlines during the period when the ancient ocean existed and formed due to sea-level fall in the range of -2077m to -2760m at around 3.6-3.3 Ga. On the other hand, deltaic steps were formed by rising water levels in the basin or crater during 1.6-0.5 Ga. In addition, we did not find a delta aged between 3.2 and 1.6 Ga, suggesting that fluvial processes were not active and the oceans most probably did not exist during this period. Therefore, our study revealed ocean disappearance at 3.6-3.3 Ga and localized lacustrine environments at 1.6-0.5 Ga.