Was Mars a "Blue Planet"? The Delta Left in the Valley Indicates a "Northern Hemisphere Ocean"

1) "Did Mars Have Oceans?" Takes a Step Forward with "Topography"

The debate over water on Mars has been long-standing. While there is abundant evidence of "water involvement" such as traces of rivers, clay minerals, and ice, it has been challenging to definitively state the existence of oceans. This is because discussing oceans requires clear contours like "shorelines," which are difficult to discern on Mars due to weathering, erosion, and dune coverage.

In this context, a new study by an international team led by the University of Bern in Switzerland has emerged. They have found multiple terrains around Valles Marineris, Mars' largest canyon system near the equator, that closely resemble Earth's river deltas. This could serve as "shoreline-level evidence" indicating that rivers flowed into an ocean (or a large body of water).

2) The Protagonist is "SFD"—Why Scarp-Fronted Deposits Look Like "River Mouths"

The terrain that caught attention this time is scarp-fronted deposits (SFD). These have relatively flat tops and suddenly rise into "cliff-like fronts" towards the ends. The research team interprets these as traces of "fan deltas" formed when rivers carried sediments (sand and gravel) and flowed into calm water bodies.

The point is not just the shape. The top surfaces of SFDs show radial drainage networks, and the ends have a "fan-shaped" contour that bulges downstream. Additionally, the "break-in-slope" from the top to the front aligns with features commonly seen in delta topography—this accumulation of geomorphological and sedimentological evidence supports the claim that "this is a 'river mouth terrain.'"

3) Tools of Observation: CaSSIS, HiRISE, CTX, and Elevation Models

The strength of this study lies in the combination of observational data. In addition to the CaSSIS camera (Color and Stereo) led by the University of Bern and mounted on ESA's ExoMars Trace Gas Orbiter, data from NASA's Mars Reconnaissance Orbiter's HiRISE and CTX, as well as terrain data from HRSC and MOLA, were integrated to create digital elevation models (DEM) from stereo images to "measure the terrain."

If the appearance alone is similar, it's easy to counter, but the discussion changes when elevation is included. Not only is there a delta, but the fact that they are "aligned at the same height" carries significance.

4) Aligned Elevations: A Clue to "Water Levels" at -3750 to -3650m

This is the most striking part of the paper's abstract. The confirmed SFDs are distributed at the same elevation range (-3750 to -3650m) across multiple locations, including Valles Marineris and the northern lowlands. The team considers this as a record of the sea level (or the water level of a large body of water) having "high-stand" at that height.

One reason the "ocean" hypothesis on Mars is contentious is that the assumed altitude of the shoreline tends to vary between studies. However, the current claim provides a specific range of water levels indicated by delta-like deposits (at least in this region). The discussion about oceans seems to be shifting from "atmosphere" to "numerical."

5) When Was the Ocean: Late Hesperian to Early Amazonian

Furthermore, the paper dates the deposition of these SFDs to the "Late Hesperian to Early Amazonian" period. This overlaps with a time in Martian history expected to trend towards more arid conditions, raising new questions about "what conditions allowed for the formation of 'the largest water bodies' during that time?"

6) What This Study Adds to the "Northern Hemisphere Ocean" Debate

Phys.org and the University of Bern's announcements describe these results as suggesting that "Mars was once a 'blue planet,'" emphasizing the possibility of a large ocean covering the northern hemisphere. They further state that its scale could rival Earth's Arctic Ocean, positioning this study as providing "shoreline evidence" based on high-resolution images, unlike previous studies that relied on "coarse data and indirect arguments."

The important point here is not simply to settle whether "there was an ocean or not," but rather that more "checkpoints (elevation ranges)" have been added to verify the existence of oceans. Going forward, the focus will be on whether the terrain, minerals, and age estimates from other regions align with this elevation range.

7) Remaining Questions: Was It an Ocean or a Large Lake, and "Why Did It Disappear"?

Of course, there are caveats. The delta-like terrain indicates "rivers flowed into a calm water body," but that body of water is not necessarily a global ocean. A large lake could form similar terrain. While the paper and commentary articles push the interpretation as a "shoreline," in the geological world, comparisons with alternative hypotheses (such as different sedimentary processes or geomorphological alterations) will inevitably continue.

And the biggest mystery is, "If there was an ocean, why did it disappear?" Atmospheric escape, freezing, infiltration into the ground, mineral fixation... multiple factors are likely involved, and this study's results may rather reignite the debate with "specific water levels" included.

8) Possibility of Life: More Important Than "Presence of Water" is "Sustained Water"

The same applies to life. What matters is not that water appeared momentarily, but how stable its presence could have been. To form fan deltas, there needs to be a coexistence of flowing and accumulating water for at least a certain period. The research team aims to investigate the mineral composition (types of weathering) of ancient Martian soil as the next step, and if "chemical evidence" is added to the topographical evidence, the environmental image of that time will become significantly more three-dimensional.

Reactions on SNS (As Far as Confirmed)

*Note: Due to display restrictions on the platform side, it may not be possible to comprehensively check the text and comments of posts on SNS. Here, we summarize the "trend" based on posts and reactions we were able to confirm.*

X (formerly Twitter): Spread via the Standard Route for Science News

The official Phys.org account posted an article with the gist that "high-resolution images show ancient river deltas, providing evidence for a once vast ocean covering the northern hemisphere," serving as the starting point for its spread.

Additionally, astronomy-related accounts (e.g., AAS Press Office) are seen sharing the University of Bern's release.

LinkedIn: The Research Community Focuses on "Interdisciplinary Collaboration" and "Celebration"

Lead author Ignatius Argadestya emphasized "interdisciplinary teamwork merging physics and geological perspectives" in a post reporting the publication of the paper. The comments section is filled with congratulations and praise, with reactions highlighting "interdisciplinary dialogue unlocking planetary history" (some even include jokes).

Similarly on LinkedIn, posts quoting the article content and reacting to "the strength of shoreline evidence" and "scale comparable to the Arctic Ocean" with "easy-to-understand metaphors" are seen, making it digestible for the general public.  

https://www.linkedin.com/posts/stephaniesoquet_mars-was-once-a-blue-planet-ancient-river-activity-7416548814779654144-cxuN