Mars may once have been covered by an immense ocean, but today it displays on its surface the mark of an “empty bathtub” which suggests a colossal disappearance of water and rekindles one of the greatest mysteries of the red planet.

New analysis of Martian terrain points to a flat band in the northern plains as a possible trace of an ancient ocean, rekindling a scientific debate that has accompanied missions and studies on Mars for decades.

Mars may preserve, in the northern hemisphere plains, a geological clue associated with the possible existence of an ancient ocean.

A study published in Nature magazine identified a wide, flat band of terrain that, according to researchers, may be consistent with the mark left by a body of water that would have covered about a third of the planet billions of years ago.

The formation was compared by the authors to the mark left in a bathtub after the water drains.

The analogy refers to the topographic contour that could have remained after the evaporation or gradual loss of a Martian ocean.

The interpretation, however, still depends on direct observations and remains under debate among planetary science specialists.

The hypothesis adds to already known evidence that Mars had liquid water on its surface, such as ancient river channels, deltas, and dry lakebeds.

The point still under discussion is whether these environments existed in isolation or were part of a larger system, with a stable ocean in the northern lowlands.

The authors call the structure a coastal platform.

It would be different from the possible shorelines observed since the Viking missions, which are narrower and show variations in altitude.

In the new study, the search focused on a wider and flatter band, formed in a prolonged contact zone between land, sediments, and water.

Sign in Martian terrain rekindles debate about ancient ocean

The work was led by Abdallah Zaki, a researcher at the University of Texas at Austin, and Michael Lamb, a professor of geology at the California Institute of Technology.

Before analyzing Mars, the team used computational simulations to observe what geological marks would remain on Earth if the oceans were removed.

In these simulations, continental platforms appeared as persistent features.

On Earth, these areas accumulate sediments transported by rivers and modified by waves, currents, and changes in sea level.

From this reference, the researchers looked for a similar topographic signature in the Martian terrain.

The analysis used data from the Mars Orbiter Laser Altimeter, MOLA, an instrument installed on the Mars Global Surveyor probe.

The equipment measured Mars’ topography using laser pulses and allowed mapping altitude variations on the planet’s surface.

“The question is: if an ocean existed on Mars and it dried up, what traces would it have left?” said Lamb.

According to the researcher, the team looked for a band that would outline the region where the coast might have existed, similar to a flat bench.

The researchers claim to have found signs consistent with this interpretation.

At the same time, they acknowledge that the Martian formation does not exactly correspond to terrestrial continental platforms.

For this reason, the study treats the structure as evidence under analysis, not as a definitive confirmation of the existence of an ocean.

Mars’ coastlines still divide scientists

The possibility of Mars having harbored an ocean began to be discussed more strongly after the Viking missions, launched by Nasa in the 1970s.

Images and measurements from that time indicated features that some scientists interpreted as possible coastlines in the northern hemisphere.

These marks, however, raised questions.

A coastline formed by a stable ocean would tend to maintain a relatively constant elevation, as occurs on Earth.

On Mars, possible ancient beaches appear at different altitudes, which makes interpretation difficult.

One explanation considered by researchers involves subsequent deformations in the Martian crust.

Volcanic processes or changes in topography could have displaced parts of these ancient margins over time.

Still, according to experts, this hypothesis does not eliminate all doubts about the origin of the observed features.

The new study attempts to broaden the focus of the analysis.

Instead of just looking for narrow beach marks, the authors investigated a more extensive structure that could better preserve the effects of rivers, waves, and water level variations.

The proposal is that a coastal platform would have a greater chance of resisting changes accumulated over billions of years.

Rivers, deltas, and sediments enter the Martian ocean hypothesis

The proposed platform was also compared to the location of ancient Martian deltas.

Deltas form when rivers deposit sediments upon reaching lakes, seas, or oceans.

For the authors, the alignment of some of these structures with the identified band strengthens the hypothesis of an ancient land-water boundary.

Other recent research keeps the topic under discussion.

The Chinese rover Zhurong, which landed on Mars in 2021, recorded radar data in Utopia Planitia, in the northern plains.

A study published in the journal PNAS described inclined subsurface layers interpreted by the authors as ancient coastal deposits.

These structures were associated with sediments that could have been accumulated by waves on an oceanic margin.

Even so, researchers treat the data as geological evidence to be compared with other records, as the surface of Mars has undergone long periods of erosion, burial, and alteration.

The dataset reinforces the need to differentiate smaller aquatic environments, such as lakes and rivers, from a broad ocean basin.

This distinction is relevant because it changes how scientists interpret ancient Martian climate, water circulation, and the duration of potentially habitable conditions.

Water on Mars today appears in ice, minerals, and a possible deep reservoir

Mars today is a cold, dry planet with a thin atmosphere.

Known water appears mainly in the polar ice caps, in subsurface ice, and in hydrated minerals.

Part of the ancient surface reservoir may have escaped into space after the planet lost much of its atmosphere, while another part may have been incorporated into the subsurface or rocks.

Data from NASA’s InSight mission was also used in studies on the presence of water in the Martian interior.

Seismic analyses published by researchers indicated that the probe’s measurements are compatible with the existence of liquid water in porous rocks of the middle crust, at estimated depths between 11.5 and 20 kilometers.

This possible subsurface water does not mean that an accessible environment exists on the current surface.

The interpretation points to deep reservoirs, out of reach of operating robotic missions.

Still, the data helps investigate where the water that shaped ancient Martian landscapes went.

The confirmation of an ancient ocean would also have implications for habitability studies.

Sedimentary environments associated with water can preserve important chemical and mineral records, although no direct evidence of life on Mars has been found so far.

Rosalind Franklin mission could help investigate the Martian subsurface

The Rosalind Franklin mission, from the European Space Agency, is one of the next initiatives planned to investigate the Martian subsurface.

ESA anticipates the rover’s launch in 2028, with arrival on Mars in 2030.

NASA announced it will provide services and components to support the mission, including the launch.

The rover is designed to drill up to two meters below the surface.

The goal is to reach materials less exposed to radiation and the extreme conditions of the current environment.

The mission will search for chemical and geological signs related to the possibility of past or present life on Mars.

The planned region for exploration is Oxia Planum, an ancient area rich in minerals formed in contact with water.

Although the site does not represent a direct verification of the entire coastal platform proposed in the new study, the rover’s instruments may contribute to reconstructing the planet’s water history.

Experts also point out limits in the comparison between Mars and Earth.

The red planet does not have active plate tectonics like Earth, and this changes how margins, basins, and sediments evolve.

Differences in gravity, tides, atmosphere, and ocean circulation may also have influenced any body of water that once existed.

Therefore, the coastal platform described in the study is treated as a testable hypothesis.

New orbital data, radar measurements, and rock analyses on the Martian surface will be needed to assess whether the identified strip indeed corresponds to an ancient ocean margin.

If Mars harbored a stable ocean for long periods, the planet’s environmental history would have included a phase very different from the arid landscape observed today.