Unearthing Early Martian Oceans through Topographic Signatures
Researchers compared global elevation and geomorphic datasets for Earth and Mars to locate signs of past shorelines. The work uses elevation, slope, curvature and landscape classification to search for topographic signatures consistent with ancient seas.
Data sources and digital elevation models
The study relied on three primary dataset types. These included global digital elevation and bathymetric grids, mapped fluvial and oceanic features, and cell-by-cell terrain metrics.
For Earth, the ETOPO1 Global Relief Model was chosen. It provides uniform 1 arcminute resolution, about 1.85 km per pixel. The SRTM30_PLUS and GEBCO datasets were evaluated but not selected for global uniformity.
For Mars, the team used the MOLA gridded topography. MOLA offers 463 m per pixel resolution and is based on more than 600 million measurements.
Resampling and resolution
Both planetary DEMs were resampled to 2.5 km, 5 km and 10 km grids. Resampling used the nearest-neighbour method in ArcGIS Pro to preserve original elevation values.
The choice of resolutions aimed to standardize analyses across planets. It also reduces influence from fine-scale, younger Martian landforms.
Fluvial, deltaic and oceanic maps
The researchers compiled maps of major rivers and deltas on Earth. They also assembled maps of valley networks, fluvial ridges, outlet canyons and interpreted deltas on Mars.
Earth seafloor morphologies were included. Key oceanic elements considered were the continental shelf, shelf break, slope, rise, abyssal plains and hadal zones.
Delta selection and classification
The team filtered published Martian delta datasets using specific criteria. They selected deltas open to downstream flow along the dichotomy boundary or showing complex stacking patterns.
The filtering yielded 48 deltas. Those were grouped into single-lobate and stacked deltaic systems.
Search-zone design and data sampling
Shapefiles were converted to point samples for elevation extraction. Zonal statistics were used for polygonal oceanic features.
- Earth river points: 195,022
- Earth deltas: 10,848
- Continental shelf pixels: 14,820,634
- Continental slope pixels: 7,606,463
- Continental rise pixels: 12,144,045
- Abyssal plain pixels: 116,749,407
- Hadal zone pixels: 1,238,491
- Mars valley networks: 3,294,322
- Mars depositional rivers: 16,515
- Mars outlet canyons: 248,865
- Mars deltas: 48
- Arabia shoreline points: 10,192
- Deuteronilus shoreline points: 42,900
On Earth, delta-to-deep-ocean transitions typically occur within the upper 2.5 km below sea level. That interval defined the depth search window for a potential Martian shelf.
Topographic metrics and landscape classification
Elevation values were sampled from resampled rasters using the Add Surface Information tool. Slope was calculated with a 3×3 neighbourhood in ArcMap.
Curvature was derived from the second derivative of elevation using the ArcMap Curvature function. Negative curvature values were multiplied by −1 for comparative plotting.
Geomorphons and flat-terrain thresholds
The Geomorphons algorithm classified landforms into flats, ridges, shoulders, spurs, slopes, pits, footslopes, hollows and peaks. Local Ternary Patterns underlie its classification logic.
Because Mars lacks active plate tectonics, its topographic wavelengths are longer than Earth’s. The team therefore applied different flat-angle thresholds to each planet.
Forty experiments on Earth established detection characteristics. A flat-terrain angle of 1.22° detected the entire mapped continental shelf, but with low precision.
For Mars, analysis concentrated on the northern lowlands. That region preserves 48 deltas and interpreted submarine-channel belts linked to an ancient oceanic margin.
The candidate Martian shelf is bounded roughly between −1,800 m and −3,800 m. At 5 km grid resolution, the median slope there is about 0.31°.
Combining elevation, slope, curvature and geomorphic indicators narrowed the shelf-like zone. The constrained area extends mainly between about 30°S and 70°N.
On Earth, a flat-angle threshold near 0.31° would detect about 69–71% of the continental shelf. That correspondence supports similar landscape-to-seascape detection on Mars.
Statistical results
Median slope and curvature were computed at 200 m elevation intervals. Martian profiles display an intermediate-elevation, low-slope, low-curvature band between −1,800 m and −3,800 m.
Researchers defined three elevation bands: above −1,800 m, between −1,800 m and −3,800 m, and below −3,800 m. A Kruskal–Wallis H test assessed slope differences among these bands.
The test returned H = 27.50 with P = 1.07 × 10−6. This result indicates significant differences in slope distributions across the three elevation bands.
Limitations and sources of uncertainty
Several factors could alter topography since the time of deposition. True polar wander and emplacement of the Tharsis volcanic province likely caused regional uplift and subsidence.
Isostatic rebound from ocean unloading may have modified elevations. Recent estimates for Mars suggest rebound of tens to just over 100 metres.
These values are small compared with the roughly 2 km span of the detected shelf-like zone. Long-term burial, exhumation and erosion add regional variability.
Hesperian-aged outflow floods likely redistributed sediments along the dichotomy. Chryse Planitia was particularly affected, producing local flattening.
Despite this, comparable flat, low-slope surfaces occur at other locations. Aeolis Dorsa and segments of the proposed shelf preserve stacked deposits and deltaic evidence.
Independent delta records at Hypanis show sea-level-related elevation changes. Such deposits support the interpretation of landscape-to-seascape transitions.
The combined approach uses global DEMs, geomorphic mapping and quantitative metrics to identify topographic signatures. These signatures help in unearthing evidence for early Martian oceans and refine search zones for ancient shorelines.
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