Mars Had Beaches Like Earth’s, But What Happened To Its Ocean?

For years, planetary scientists have debated whether Mars once had an ocean. The northern lowlands of the planet resemble an ancient seabed, and early orbiter images suggested the presence of what looked like shorelines, wrapping around the hemisphere like a forgotten coastline.

But there was a problem—those supposed shorelines weren't level. Unlike Earth's beaches, which hug the sea at a consistent elevation, Mars' "shoreline" markers were scattered across elevations that varied by as much as 10 kilometers. Was this proof that an ocean never existed, or was there another explanation?

New evidence from China's Zhurong rover, which explored Utopia Planitia, is now tipping the debate in favor of the ocean hypothesis. And not just any ocean—a long-standing, stable body of water with wave-formed beaches eerily similar to those on Earth.

NASA's Curiosity and Perseverance rovers have found evidence of ancient lakes on Mars, but their discoveries were limited to small, enclosed bodies of water. What makes Zhurong's findings different is that they suggest something much larger—an actual ocean.

Unlike orbiters that rely on surface images, Zhurong used ground-penetrating radar (GPR) to peer up to 80 meters (260 feet) beneath Mars' surface. As it traveled 1.9 kilometers (1.2 miles) across Utopia Planitia, it found something remarkable:

Thick, sloping layers of buried sediment—exactly what you would expect from a beach environment shaped by waves over time.

Uniform angles of deposition—suggesting material was carried and deposited in a consistent manner, not randomly scattered by wind or meteor impacts.

Sand-sized particles, rather than coarse rock fragments—similar to beach sands on Earth.
"The structures don't look like sand dunes. They don't look like an impact crater. They don't look like lava flows," said Michael Manga, a planetary scientist at UC Berkeley. "That's when we started thinking about oceans."

The radar detected layers arranged parallel to the suspected ancient shoreline. Not only was the orientation correct, but so was the slope, strengthening the idea that these structures were formed by water over a long period.

Mars Had Beaches Like Earth's—But What Happened to Its Ocean?
If these are true beaches, it means Mars had persistent open water for an extended period—not just fleeting, localized puddles, but a stable ocean capable of shaping sediment for potentially millions of years.

This fits with previous studies suggesting Mars once had a denser atmosphere, warm enough to sustain liquid water. However, over billions of years, most of that water:
- Escaped into space as Mars lost its atmosphere.
- Froze beneath the surface, potentially forming vast underground ice reserves.
- Became trapped in Martian rocks through chemical reactions.
Even so, Zhurong's findings suggest something even bigger—that Mars had tides and waves.

Tidal movement requires either an atmosphere or a gravitational force acting on the water. On Earth, the Moon drives our tides, but Mars' two small moons (Phobos and Deimos) are too weak to do the same. If tides existed, it implies Mars had strong atmospheric winds or some other unknown mechanism driving wave action.

The idea of an ancient ocean on Mars isn't new. In the 1970s, NASA's Viking orbiters spotted what appeared to be a continuous shoreline encircling much of the northern hemisphere. But then a major problem emerged—the supposed shoreline wasn't level.

Unlike Earth, where water always finds a flat equilibrium, Mars' shoreline seemed warped by up to 10 kilometers.

Theories to explain this included:
- Mars' spin axis shifted over time—meaning what was once a level shoreline is now distorted.
- Volcanic activity in the Tharsis region (home to Olympus Mons, the largest volcano in the solar system) caused massive uplift, reshaping the terrain long after the ocean had dried up.
Zhurong's findings don't necessarily solve this debate, but they confirm that wave-shaped coastal deposits do exist, meaning some kind of large, persistent body of water was once there.

The most intriguing part of all this? Shorelines are excellent places to search for life.
On Earth, some of the oldest known life forms emerged along coastal environments, where shallow water, minerals, and organic compounds mixed in dynamic conditions.

If Mars had beaches, it also likely had:
- Rivers carrying nutrients into the ocean, creating potential zones for microbial life.
- Shallow, sunlit waters, where chemical reactions could have supported biology.
- A stable environment, where life could have developed over long geological timescales.

Benjamin Cardenas, a planetary geologist at Pennsylvania State University, put it this way: "Shorelines are great locations to look for evidence of past life. The earliest life on Earth likely began at locations like this."

While Mars today is cold, dry, and inhospitable, its past may have been far more Earth-like than we previously imagined.

Zhurong, unfortunately, is no longer active, having fallen silent after about a year of exploration. However, its data has left planetary scientists with a treasure map.

Future missions could:
- Target Utopia Planitia's buried beaches for deeper drilling and sample analysis.
- Look for preserved organic molecules that could indicate past microbial life.
- Investigate Mars' underground ice reserves, potentially connected to the remnants of its ancient ocean.

NASA's Perseverance rover and the upcoming ESA Rosalind Franklin rover may not be heading to Utopia Planitia, but their work in Jezero Crater and Oxia Planum—areas also linked to ancient water—will further our understanding of how Mars transitioned from a planet with lakes and oceans to the barren world we see today.
The ocean may be gone, but the story it left behind is only just being uncovered.