‘Bathtub ring’ offers new evidence for Mars ocean billions of years ago

A view from the "Kimberley" formation on Mars taken by NASA's Curiosity rover. One-third of the red planet might have once been covered by an ocean. (NASA/JPL-Caltech via CNN Newsource)

Mars may have once had an ocean so vast that it covered one-third of the planet before evaporating billions of years ago and leaving behind a telltale sign: a flat band of land, outlining the former ocean — similar to the ring left behind in a drained bathtub.

If confirmed by direct observations, this “coastal shelf,” as researchers call it, would contribute crucial evidence to a long-standing scientific debate, according to a study describing the new evidence. While dried-up river networks, deltas and lakebeds offer proof that Mars had a watery past, there is no consensus among experts on whether it also had a large ocean, which would have made Mars look much more similar to Earth than it does today.

“The question is: If there was an ocean on Mars and it dried up, what signs would it have left?” said Michael Lamb, senior author of the study published last week in the journal Nature. “What we’ve looked for is a band that would wrap around where the shoreline would have been, like a flat bench — because that’s essentially what we see on Earth, which we know as the continental shelf.”

Lamb, a professor of geology at the California Institute of Technology, and lead author Abdallah Zaki, a distinguished postdoctoral researcher at the University of Texas at Austin, ran computer simulations to dry up the oceans on Earth and see what geological traces they would leave behind. The continental shelf emerged as the most distinct feature, enduring through time and changing sea levels.

The research team then searched for an analog on Mars using data from NASA’s Mars Orbiter Laser Altimetre or MOLA, a probe that mapped the planet’s surface features from orbit using laser. “We looked for a similar feature on Mars and found some evidence that it could be there,” Lamb said. “It doesn’t look exactly like the continental shelf on Earth, however, so there’s some evidence in support of it, but not all the pieces of the puzzle.”

Mounting evidence

The idea that an ocean might have once existed on Mars originated in the 1970s, when the Viking 1 and Viking 2 missions launched by NASA detected what some researchers believed was a shoreline — a much narrower band than the newly proposed coastal shelf — and a depression in the planet’s northern hemisphere that was suggestive of an ancient seabed.

However, this older evidence was never considered conclusive: “The shoreline has some issues,” Lamb said. “It doesn’t trace constant elevation as you might expect for a shoreline, but it waves up and down.”

One way to explain this elevation change, he noted, is volcanic eruptions that might have shifted Mars’ crust and deformed the shorelines. “But it’s hard to prove that that’s what happened, and so it remains debated whether those are in fact shoreline features or not,” Lamb said

Another issue is that shorelines are very thin. “If you want to look for long-lived oceans, then there must be something bigger than a shoreline, and we think that’s the coastal shelf,” said Zaki, who conducted the research with Lamb when he was a postdoctoral scholar at Caltech.

The coastal shelf is an improvement on the shoreline data in several ways, according to the new study. The sloping feature is easier to see and much larger, at about 200 to 400 metres wide (650 to 1,300 feet), making it relatively resistant to erosion over billion of years. The formation would have resulted from rivers carrying sediments into the ocean, as well as changing sea levels. “On Earth, the continental shelf is the largest sedimentary sink on the planet, due to the material brought by rivers and some added sediment deposition from waves and currents,” Zaki added.

Zhurong, a Chinese rover that landed on Mars in 2021, detected evidence of ancient beaches within underground sedimentary layers in the planet’s northern plains, the same area where Zaki and Lamb think they have found traces of a coastal shelf. Geological remnants of river deltas further bolster that idea.

Mars still holds some water, mostly in its ice caps, but there might be much more underground — enough to fill an ocean, according to data from NASA’s InSight lander. The red planet lost most of its water as its atmosphere thinned over time, allowing water molecules to escape into space. By some recent estimates, the planet could have had surface water until as recently as 2 billion years ago.

The “bathtub ring” could be spotted soon. The European Space Agency’s rover Rosalind Franklin, expected to launch in late 2028 for a landing on Mars in 2030, will explore the northern hemisphere with the ability to probe both the surface and underground. “It will give us a definitive answer,” Zaki said.

Confirming an ancient ocean on Mars would help deepen our understanding of the red planet and could help illuminate why it changed so dramatically over time and whether it ever hosted any kind of life.

“There are lots of indicators that Mars had liquid water on its surface, but what we really don’t know is how long that liquid water was stable,” Lamb said. “The climate of Mars is now very cold and dry, so it’s changed substantially from the past and it remains a scientific challenge to understand how and why Mars was warm and wet and for how long, and what happened to cause the planet to have such a catastrophic change to its current state.”

A very testable hypothesis

The study offers an intriguing new approach to the question of whether Martian oceans once existed, according to James W. Head, a professor of Earth, environmental and planetary sciences at Brown University who was not involved with the study. However, he added in an email, due to the lack of plate tectonics on Mars, the formation of such a sharp boundary — the coastal shelf — is debatable. Further observations are required to see if this analog to Earth’s continental shelf is a reliable marker. “In the final analysis, two problems remain,” he concluded. “Where did so much water come from? And where did it all go? Neither has yet been adequately accounted for.”

Brian Hynek, a professor in the department of geological sciences at the University of Colorado Boulder, agrees that while the study bolsters the evidence for a past ocean, the appearance of Earth’s continental shelves is partly due to the role of plate tectonics, which Mars never had. “This, plus expected differences in ocean tides and currents, leads to a bit of an apples to oranges comparison,” Hynek said in an email. He was not part of the new research.

Lamb acknowledges the differences in formation between the Martian coastal shelf and Earth’s continental shelf but maintains that some of the elements that operated to make the shelf on Earth — rivers, waves and sea level changes — also likely were present on Mars.

Bryony Horgan, a professor of Earth, atmospheric and planetary sciences at Purdue University in West Lafayette, Indiana, said in an email that whether or not Mars ever had a large ocean is a critical question because it has major significance for the ancient climate, geology and habitability of the red planet. “But it remains one of the longest lived controversies in Mars science, to the point where many younger scientists are hesitant to even discuss the hypothesis,” Horgan said.

The controversy stems from the fact that Mars seems to have had temperate climate periods with rain that deeply altered the rocks, as well as widespread rivers and lakes that persisted for potentially millions of years. “It’s hard to imagine such a well-developed water cycle existing without a large ocean filling the deepest and largest basin in the northern lowlands, but we have yet to see rigorous direct evidence of such an ocean, so the debate rages on,” added Horgan, who also did not participate in the latest work.

Using Earth as a comparison is a good approach, and ESA’s Rosalind Franklin rover will help settle the debate, she concluded. “I appreciate that the study generates a very testable hypothesis — we can now go investigate in detail the mineralogy and geology of the landscapes above, within, and below the proposed marine shelf to see if they support the presence of an ancient ocean.”

By Jacopo Prisco, CNN