Was There a "Sea" in the Northern Hemisphere of Mars? — New Geological Evidence Suggests an Ancient Watery Planet

"Rivers become calm before reaching the sea"—Traces of ancient seas revealed by topography

The claim that Mars once had rivers is not particularly novel. The question is, "where did those rivers flow into?" Recent research suggests that they might have flowed into a vast body of water—essentially a "sea"—spanning the northern lowlands, as indicated by the geometric behavior of the river systems. The research team examined the "backwater zone" seen in large Earth rivers (e.g., the Mississippi), where the flow velocity decreases as it approaches the mouth, and the meander width (channel belt width) systematically narrows. They applied this fingerprint to Martian deltas and found evidence of long backwater lengths and regular narrowing of channel belts. This strongly suggests the presence of a "calm, vast body of water" beyond the river mouths.Phys.org

Observed "Shape Law"

The study normalized and compared the widths of deltas and upstream deposits using satellite images and topographic models. On Earth, a "window" where the channel belt width falls between 2 and 6 emerges within the backwater length from the river mouth. A similar window was observed in the Martian regions, indicating that the same "formula" applies to both planets. If the topographic equations hold across planets, the underlying fluid—water—must have been common as well. The initial report is published in 'Geophysical Research Letters (GRL)'.AGU Publications

Inverted Topography Depicts Lost River Channels

On Mars, "inverted channel belts," where the coarse gravel layers (sandstone) of former riverbeds are more erosion-resistant than their surroundings and remain as ridges, are widely found. A similar phenomenon is observed on Earth, such as the "inverted delta" seen in the Weddington Sandstone in Arkansas. The research team used the "textbook" geometry of inverted deltas visible on Earth to interpret Martian ridges as "blueprints of ancient rivers." The reconstructed river network depicts a flow over long distances, ultimately draining into a broad, calm water area.University of Arkansas News

Other Independent Evidence Supporting the Ocean Hypothesis

In February this year, China's Mars rover "Zhurong" reportedly detected inclined sandy layers resembling "beaches" (suggesting wave action) with its ground-penetrating radar. This may indicate an ancient coastal environment in the northern lowlands (Utopia Planitia), aligning well with the ocean hypothesis. Multiple independent lines of evidence—the backwater fingerprint of deltas and beach sediment structures—are converging on the same conclusion.Reuters

The Remaining "Invisible Shoreline" Issue

It's intuitive to think that if there was an ancient sea, a clear shoreline should remain. However, model studies suggest that countless meteor impacts over billions of years have fragmented the coastlines, making it rare to find continuous contours. Estimates suggest that over 70% of the oldest shorelines could be directly destroyed, meaning "discontinuous shorelines ≠ no sea." The strength of this study lies in focusing on the "physics of the estuarine zone" as an area rather than a line of "shore."arXiv

Implications for Life Exploration and Landing Site Selection

"Large rivers flowing into the sea" carry nutrients and fine organic particles to the coast. On Earth, deltas tend to become "hotspots of life," balancing high productivity and preservation. On Mars, such delta and beach deposits might have been advantageous for preserving biomolecules and metabolic traces. The identified backwater zones and inverted deltas will likely become increasingly important as future landing and sample collection candidates. The primary authors of the study also emphasize the presence of "mature deltas."Phys.org

The "Essence" of the Study is Scale and Comparison

This study uses real-world examples like the Mississippi Delta as "standards" from Earth to read Martian topography through scale-aligned comparisons (normalization). The notion that the backwater zone's length reaches hundreds of kilometers, making it detectable from orbit, is reasonable. In Mars geomorphology, which primarily relies on satellite observation, introducing scaling laws of dynamics as a "ruler" is a method that can be expanded to future studies of ice deposits and lakes.Phys.org

Consistency with the "Already Known" Theory

The assertion that "Mars was wet is already known" is valid. In July this year, a report on an ancient river network spanning about 16,000 km in the southern hemisphere further reinforced the image of a "surprisingly wet Mars." However, the leap from "there were rivers" to "there was a sea" is significant. The existence of a large-scale calm water body would greatly alter the constraints on climate, crust, and atmospheric escape history. This study is a step forward in quantitatively demonstrating the "intermediate (river mouth—coast—sea)" that bridges this leap.The Guardian

What Can Be Said Now

In summary, "The possibility of a long-lasting sea in the northern lowlands has increased, but final confirmation requires ground core verification." While the "beach" structure detected by ground-penetrating radar, the backwater fingerprint of deltas, and the continuity of inverted topography all support the ocean hypothesis, the continuous detection of wide shorelines is physically challenging. Remaining tasks include in-situ measurements of sediment grain size, layering, deposition direction, age determination, and evaluation of organic preservation potential.Reuters

Summary of Reactions on Social Media

• "This makes the 'northern sea' a strong contender" (numerous shared posts of the Phys.org article. For example) On X, the article URL is shared widely (original text: "Geological evidence points to ancient ocean…")X (formerly Twitter)

• "It's exciting how the Zhurong's 'beach' report aligns. Independent evidence is reinforcing each other" (shared with news links. The Zhurong report has been a topic since the beginning of the year)The Guardian

• On Reddit's r/space and r/Mars, threads citing the difficulty of shoreline detection due to collisions, such as "Shorelines are shredded by impacts—it's natural they are hard to see," indicate the presence of cautious voices.arXiv

• Some say, "The 'sea' is romantic, but first, on-site verification of age and depositional facies is needed" (a common comment in science-oriented communities).

*Note: Individual social media posts are primarily focused on rapid sharing, with the tone in expert communities leaning towards "promising as supporting evidence, but awaiting on-site and age confirmation."*

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