Three billion years ago, Mars looked nothing like the cold, dusty world we see today. According to new research from Swiss and Italian scientists, the Red Planet once hosted a vast ocean that covered nearly half its surface, spanning almost the entire northern hemisphere. The findings add some of the strongest evidence yet that Mars was once a “blue planet,” with long-lasting bodies of liquid water and environments that may have supported life.

The study goes beyond earlier hints of ancient rivers and lakes. Using high-resolution orbital imagery, researchers say they have identified what appears to be an ancient Martian coastline and river deltas, geological fingerprints that are difficult to explain without a large, stable ocean.

What did scientists discover about Mars’ ancient ocean?

The new study, led by researchers from the University of Bern and the INAF–Osservatorio Astronomico di Padova, reconstructs the presence of a massive ocean that existed roughly three billion years ago. The ocean is estimated to have been comparable in size to Earth’s Arctic Ocean and to have covered most of Mars’ northern lowlands.

This is not the first time scientists have suggested that Mars once had oceans. What makes this research stand out is the quality of the evidence. Instead of relying largely on indirect clues, the team identified physical landforms that closely resemble coastal and delta structures seen on Earth.

Where was this ocean located?

The ocean appears to have occupied Mars’ northern hemisphere, a region that sits at a much lower elevation than the southern half of the planet. This topographic contrast has long intrigued scientists and made the idea of a northern ocean plausible.

Key evidence was found near Valles Marineris, the largest canyon system in the solar system. Although Valles Marineris lies close to the equator, researchers identified structures nearby that look like ancient river mouths, places where rivers once emptied into a large body of water.

Today, the region is covered by dust and dunes. But according to the researchers, the underlying shapes are still visible in high-resolution images, preserving the story of Mars’ watery past.

How did researchers find evidence of a Martian coastline?

The team relied on data from several orbiting spacecraft, combining observations to build a detailed geological picture.

The role of advanced space cameras

A key instrument was the Colour and Stereo Surface Imaging System, or CaSSIS, aboard the European Space Agency’s ExoMars Trace Gas Orbiter. CaSSIS provides exceptionally sharp, color images of the Martian surface, allowing scientists to distinguish subtle landforms.

These images were supplemented with data from:

• ESA’s Mars Express orbiter
• NASA’s Mars Reconnaissance Orbiter

Together, these datasets allowed the team to identify features that strongly resemble river deltas on Earth, fan-shaped deposits of sediment formed where rivers slow down and spill into oceans or large lakes.

Fritz Schlunegger, a professor of exogenous geology at the University of Bern, said the structures they observed are best explained as the mouths of rivers flowing into an ocean, rather than isolated lakes or short-lived floods.

Why is this evidence stronger than earlier claims

Scientists have debated the idea of a Martian ocean for decades. Earlier studies pointed to valley networks, mineral deposits formed in water, and the planet’s topography as clues. However, critics argued that these features could have formed through episodic flooding or ice-related processes rather than a long-lived ocean.

The new study addresses those doubts.

Clearer data, clearer conclusions

According to Schlunegger, earlier claims were often based on lower-resolution imagery or indirect reasoning. This time, the reconstruction of sea level is grounded in identifiable coastal features that match what geologists see on Earth.

The researchers argue that these landforms indicate a stable shoreline that persisted long enough for rivers to transport sediment and build deltas. That implies not just the presence of water, but a climate that allowed liquid water to exist over extended periods.

This ocean, they say, may be the deepest and largest body of water ever identified on Mars.

Was Mars once capable of supporting life?

The discovery has major implications for the long-running question of whether Mars was once habitable.

A warmer, wetter Mars

Ignatius Argadestya, a doctoral researcher involved in the study, said the findings reinforce the idea that Mars once had environmental conditions similar to early Earth. Liquid water is considered one of the essential ingredients for life as we know it, and a planet-wide ocean dramatically increases the chances that life could have emerged.

A stable ocean would have:

• Moderated Mars’ climate
• Enabled long-term chemical interactions between water and rock
• Provided potential habitats for microbial life

Even if life never arose, the conditions appear to have been far more favorable than those on Mars today.

Why did Mars lose its ocean?

One of the biggest mysteries is not whether Mars once had water, but where it all went.

The role of atmosphere and gravity

Mars is much smaller than Earth and has weaker gravity. Scientists believe this made it harder for the planet to hold onto a thick atmosphere. Over time, solar wind may have stripped away much of Mars’ atmosphere, causing surface water to evaporate or freeze.

Without sufficient atmospheric pressure, liquid water could no longer remain stable on the surface. What remained likely ended up as ice locked in the poles, buried beneath the surface, or lost to space.

How this discovery fits into Mars exploration today

The idea of an ancient ocean is not just an academic curiosity. It directly informs where scientists look for signs of past life.

Implications for future missions

River deltas and ancient coastlines are considered prime targets in the search for biosignatures — chemical or structural signs of past life. On Earth, such environments are excellent at preserving organic material.

NASA’s Perseverance rover, for example, is already exploring Jezero Crater, an ancient lakebed with a preserved delta. Findings like those from the Swiss-led study help scientists refine where future landers and sample-return missions should go.

Why does this change how we think about Mars

For many people, Mars is defined by its present appearance: cold, dry, and hostile. Discoveries like this challenge that perception.

From red planet to blue planet

The study strengthens the case that Mars underwent a long and complex environmental evolution. Rather than a planet that was always barren, Mars may have been Earth-like for a significant portion of its early history.

That raises deeper questions:

• How common are Earth-like conditions in the universe?
• How often do planets lose their habitability?
• Could Earth face similar risks over geological timescales?

In that sense, Mars becomes not just a target of exploration, but a cautionary tale.

What still needs to be confirmed

While the evidence is compelling, researchers emphasize that further work is needed.

What future studies could reveal

Scientists will want to:

• Date the identified coastal features more precisely
• Analyze mineral compositions to confirm long-term water interaction
• Compare findings with climate models of early Mars

Future missions, especially those capable of returning samples to Earth, could provide the definitive proof needed to fully reconstruct Mars’ oceanic past.

TL;DR

• New research suggests Mars had a massive ocean covering most of its northern hemisphere about three billion years ago.
• Scientists identified coastline and river delta-like structures using high-resolution orbital imagery.
• The ocean may have been as large as Earth’s Arctic Ocean and could have supported life.
• Mars likely lost its water after its atmosphere thinned and solar radiation stripped it away.
• The findings reshape where scientists search for signs of past life on Mars.