Scientists have found new evidence that Mars may once have been home to a vast ocean covering about one-third of the planet.
The discovery comes from researchers at California Institute of Technology (Caltech), who identified a large geological feature that looks similar to structures formed by oceans on Earth.
The study, led by former Caltech researcher Abdallah Zaki and geology professor Michael Lamb, was published in the journal Nature on April 15.
Their work focuses on one of the biggest questions about Mars: did it once have enough water to form a long-lasting ocean, or was water only present in smaller lakes and rivers?
Scientists have long known that Mars once had liquid water. Past missions have found features that resemble ancient shorelines.
However, these possible shorelines are scattered at different heights across the planet.
On Earth, ocean shorelines tend to stay at the same level, so this inconsistency has made it difficult to confirm whether Mars really had an ocean.
To solve this puzzle, the researchers looked for a more reliable clue. Instead of focusing on shorelines, they studied a feature known on Earth as the continental shelf. This is a broad, flat area of land that forms where continents meet the ocean. Unlike shorelines, which can shift over time as sea levels rise and fall, continental shelves remain relatively stable.
Using computer models, the team simulated what Earth would look like if all its oceans disappeared. They found that one of the most noticeable features left behind would be a wide, flat band of land wrapping around continents—similar to a “bathtub ring” left after water drains away.
The researchers then examined detailed maps of Mars created by orbiting spacecraft. They discovered a similar wide, flat band in the planet’s northern hemisphere. This feature stretches across a huge area and matches what scientists would expect if a large ocean had once existed there.
What makes this finding especially important is that such a feature takes a long time to form. It would require a stable body of water lasting millions of years, rather than a short-lived lake. The team also found that ancient river deltas—fan-shaped deposits formed where rivers flow into larger bodies of water—line up with this “bathtub ring,” further supporting the idea of an ocean.
Although more research is needed, this discovery provides one of the strongest pieces of evidence yet that Mars once had a large, long-lasting ocean. It also points scientists toward new locations to explore in future missions.
If life ever existed on Mars, sediments left behind along this ancient coastline could hold important clues, much like fossils preserved in coastal areas on Earth today.
Source: Caltech.
