The massive Olympus Mons volcano on Mars—one of the solar system’s highest peaks—may have towered above a Martian ocean in the distant past, a new study suggests.

The research identifies an escarpment at the base of the giant volcano that looks similar to those found on volcanic islands here on Earth, such as Hawaii and the Azores. These features are caused when molten lava flows into the sea, and the researchers argue that Olympus Mons may have formed a volcanic island roughly 3.8 billion years ago.

Hawaii and Olympus Mons have “kind of a similar morphology, but Olympus Mons is much bigger,” says volcanologist Anthony Hildenbrand of the French National Center for Scientific Research. “By itself, Olympus Mons has more than the total volume of all the Hawaiian island chain.”

Hildenbrand is the lead author of the study, which was published recently in Earth and Planetary Science Letters. In addition to the escarpment around Olympus Mons, Hildenbrand and his colleagues report signs of a similar escarpment on another Martian volcano, Alba Mons, which lies about 1000 miles to the northeast—suggesting this, too, was caused by hot lava flowing into the sea.

But their claims are questioned by other experts who suggest the escarpments could also have been made by lava flows that did not encounter water, forming terraces that were much too high to be ancient shorelines.

To address this problem, the authors suggest what were once lava shorelines were raised to their present height by volcanic uplift. But planetary scientist and geophysicist Patrick McGovern of the Lunar and Planetary Institute in Houston, who wasn’t involved in the study, says there’s no sign in data from NASA orbiters that this occurred.

“That sort of thing would have a fairly immense signal in the gravity field, and I really can’t discern it in the gravity field data that we have,” he says.

Massive Martian volcano

Olympus Mons today covers an area about the size of Arizona. Scientists think it’s so big because the gravity on Mars is only about a third of that on Earth and because the volcanic plume that created it has been very active over the eons. Mars has no tectonic plates that could have moved the mountain away from this source of magma, allowing it to grow and grow.

The volcano has never been seen to erupt, but studies suggest it might have as recently as two million years ago—which suggests it could erupt again.

Seen from above, Olympus Mons is roughly circular, with vast overlapping craters from ancient calderas visible on its peak—a shield volcano built up from layers of lava, like many of Earth’s volcanic islands. The escarpment around its base is clearly visible on the northwest and southeast of the mountain, where the slope suddenly plunges down for several miles.

“A plan view from the top of Olympus Mons shows the escarpments are concave towards the center,” Hildenbrand says. “And the sharp variations in the slope of about 15 degrees are highly consistent with what we observe around terrestrial volcanic islands.”

He says that what are interpreted as ancient shorelines in parts of the northern highlands could be evidence of an ocean there in the distant past, or maybe two oceans at different times: the first about 3.8 billion years ago, and another as recently as three billion years ago.

Other experts are skeptical of this idea, however. The escarpments on Olympus Mons stretch roughly four miles above the surrounding plains—about twice the estimated maximum depth of the ancient ocean that’s thought to have once filled the northern hemisphere of Mars, where the volcano is located.

Geologist Julia Morgan of Rice University in Houston, who studies the evolution of volcanic islands like Hawaii, says the escarpments might instead be “benches” of lava that develop on the lower flanks of volcanoes due to outward spreading as they grow, unrelated to whether any shorelines are present.

Changing Martian landscapes

The authors of the study suggest the escarpment formed at sea level when Olympus Mons was lower than it is now, and that its present height was caused by volcanic uplift.

“We do not say that there was a global ocean that was six thousand meters deep,” Hildenbrand says. Instead, they suggest that the great weight of the volcano pushed the surrounding seafloor down and that it rose again with the later uplift.

He notes that a similar escarpment on the north side of Alba Mons, which the authors think also may have been caused by molten lava flowing into the sea, is less than three miles above the nearby plain, lower than the escarpment on Olympus Mons. In that case, it may be that the initial depression or the subsequent uplift was not as great, he says.

Alba Mons has very different structure than Olympus Mons. It’s relatively flat—just four miles high—but is surrounded by vast lava outflows that cover an area almost the size of the United States.

It lies within the northern volcanic highlands of the Tharsis region, while Olympus Mons stands apart from them, in the west. That suggests Alba Mons may not have been a complete island but a volcanic cape, Hildenbrand says.

Planetary volcanologist Lionel Wilson, a professor emeritus at Lancaster University in the United Kingdom, says the idea that the cliffs around Olympus Mons were formed by water has been proposed before, but the great height of the escarpment was not completely explained. The new study suggests Olympus Mons grew from volcanic uplift, but the authors need to find more evidence of the sequence of events, he says.

McGovern adds that other geological processes could have created the escarpments as well, and he’s glad to see such questions being researched. “I’m not convinced by the overall scenario,” he says. “But it’s an interesting hypothesis … Olympus Mons is always fascinating to study.”

Future radiometric dating on the rocks in the Olympus Mons escarpments could reveal exactly when and how they formed. At the moment their age can only be estimated by studying craters left by meteorite impacts across the region.

Hildenbrand hopes such rock samples could be taken by future Mars probes, and either returned to Earth or remotely measured on the red planet itself. “Then we could date with the actual numerical ages, rather than indirectly by crater counting,” he says. “Samples from these two volcanoes could show us where the ocean was, and when it was.”