Scientists found a 'bathtub ring' on Mars. Could it be evidence of an ancient Red Planet ocean?

A huge geological structure on Mars resembling a bathtub ring may be evidence of an ocean that once covered a third of the Red Planet's surface, a new study finds.

Decades of previous research have suggested that although Mars is now famously the "Red Planet," it once possessed liquid water on its surface. However, it remains uncertain whether that water was limited to lakes and streams, or if there was enough to form long-lasting oceans. Solving this mystery could shed light on whether ancient Mars once hosted life as we know it.

Past missions to Mars have discovered many geological features that resemble shorelines. However, the shorelines are found at varying elevations across the planet. If they were truly signs of a stable ocean, they would ideally all line up at the same elevation in much the same way sea levels are consistent on Earth.

In the new study, researchers instead searched for other geological features that might serve as signs of lost Martian oceans. Using computer simulations, they essentially dried up Earth's oceans to see which details remained.

The scientists discovered the most distinct feature of Earth's oceans, besides their water, are flat bands of land up to several hundred miles wide and ranging in elevation from about 50 to 1,345 feet (15 to 410 meters) below sea level. These bands, known as the coastal plains and continental shelves, wrap the contours of where land meets ocean like a ring that remains around a drained bathtub.

Sea levels on Earth and the locations of shorelines have fluctuated over many years. In contrast, the researchers found that coastal shelves kept relatively stable over time.

The scientists then analyzed Martian topographic data captured by orbiters. They discovered a flat zone on Mars that was suggestive of a coastal shelf of an ocean in the Martian northern hemisphere that covered a third of the planet's surface. It would have existed about 5,900 to 12,470 feet (1,800 to 3,800 m) below Martian sea level.

A coastal shelf that size would have taken a lot of time to form, and would not be found around lakes. These new findings suggest that an ocean on Mars must have survived stably for possibly millions of years.

"Mars possibly had a coastal shelf, which adds a simple new piece of evidence for the presence of an ocean," study lead author Abdallah Zaki, a planetary geologist at the University of Texas at Austin, told Space.com. "The possible existence of an ocean suggests that a large body of water may have persisted for a long time. That could have been an important ingredient for life."

In addition, the researchers saw evidence that river deltas — the triangle-shaped plains of sediment created when rivers spill into oceans — lined up with the coastal shelf. On Earth, deltas cluster on continental shelves as well.

Future missions can analyze this coastal shelf on Mars. If there was once life on the red planet, sedimentary deposits in the coastal shelf might have preserved evidence of it, just as Earth's coastal sediments at times hold fossils from the continents, the researchers noted.

A question that remains "is what formed the Martian coastal shelf," Zaki said. "Even on Earth, we do not have a definitive answer to that question."

Zaki and Caltech professor of geology Michael Lamb detailed their findings online April 15 in the journal Nature.