Billions of years ago, a giant landslide cascaded down the slopes of the largest mountain in the solar system—Mars’ Olympus Mons. When all this material fell into the water of Mars’ (probable) ancient ocean, it created a towering tsunami stretching between 25 and 43 miles long that crashed against the shore of the planet’s northern hemisphere.

A new study in Planetary and Space Science identifies the remnants of this long-ago event. The landslide-induced tsunami would have required a large body of water, yet more evidence for the case in favor of the existence of a long-disappeared Martian ocean.

Martian tsunamis are not a new idea. In 2015, researchers showed that impactors from space had splashed into the planet’s ancient ocean and kicked up giant waves. The newest findings could also help planetary scientists pin down how big the ocean might have been.

Mega-waves on Mars

Landslide-generated tsunamis are common on Earth, says Fabio Vittorio De Blasio, a scientist at Italy's University of Milan and the author of the new research. To show the same thing could’ve happened on Mars, De Blasio studied satellite images of the planet’s topography, more specifically the remains of the enormous landslide. At 370 to 430 miles long, the scar is "probably the largest single landslide deposit on Mars," he says, and possibly the longest landslide in the solar system.

When these rocks tumbled down the mountain billions of years ago, de Blasio argues, they slammed into the water to create an enormous wave that rushed across the landscape. As the tsunami progressed, the rocks scraped along solid ground beneath. The sandy, muddy waves left their fingerprints on the shoreline far from Olympus Mons. De Blasio identified the fingerprints, then used models to verify that the waves could’ve traveled several hundred miles.

Olympus Mons towers over the Martian landscape, reaching a staggering 15 miles high. Landslides mar its slopes. Scientists have identified at least ten enormous slides stretching longer than 310 miles. The largest is the western aureole, a ring of fine-grained rock around the volcano that covers land to the north as well as to the west. The region was created by a single massive landslide.

When studying the region at the outer edge of the extensive landslide, De Blasio noticed that unusual deposits marred the ridge. Using images from the NASA Mars Reconnaissance Orbiter and Mars Odyssey satellites, he studied the surface to better identify the features at the edge of the landslide. The rough surface suggests the presence of ridges created as the rocky landslide slowed. As the flow of material encountered a ridge known as Acheron Dorsum, the material began to rise, creating pressure ridges that are pushed closer together until eventually, the landslide grinds to a halt.

As the tsunami crashed against Acheron Dorsum, the sandier and more coarse components fell out of the wave first, creating a smooth layer. Small grains of what was most likely clay-like particles traveled farther, creating mud lobes. According to De Blasio, these deposits are typical of those created by terrestrial tsunamis and are also replicated in experiments.

"If you stood safely on top of Acheron… you would probably observe a muddy wave invading the shore at a high speed of perhaps 50 meters per second, rising up the gentle slope for tens of kilometers before retreating after some hours," he says.

De Blasio then simulated the process to verify that a wave created by the underwater landslide could travel so far. Such long distances are unlikely on Earth. But on Mars, where gravity is weaker, he found landslide waves could travel farther, especially combined with the gentle slope of the ridge.

"It is a complex topic," says Alexis Rodriguez, a researcher at the Planetary Science Institute in Arizona who was not part of De Blasio's team. Rodriguez authored the 2015 research , the first of several papers on impact tsunamis. "The recent discoveries add to the potential longevity of the Martian ocean as well as to the mechanisms that produced tsunamis," he said. "In a way, we approximate the histories of Mars and Earth."

A controversial issue

If the features De Blasio has identified were sculpted by tsunamis, then they make a strong case for the presence of an ocean roughly 3 billion years ago, when the solar system was only 1.5 billion years old. But even with the enormous waves, an ocean isn't a certainty. A giant lake or a network of lakes could have provided the water necessary to send the waves to shore. According to Rodriguez, an impact-generated tsunami would not have required a full ocean, "just enough surface and depth to allow for the wave to grow and propagate."

De Blasio, however, points out that such a network would have had to be precisely placed to generate the tsunami. "Moreover, I believe it is even more problematic to invoke large lakes to avoid admitting the presence of an ocean, as an ocean is thermodynamically and hydrologically more stable than a lake," he says.

Another possibility is that the landslide melted ice laying at the base of the volcano as it traveled across the icy terrain. However, if the region was made up of ice, De Blasio says, then ice should be present around the western edge of the aureole. It is not. Instead, he thinks that the tsunami deposit inconsistent with the hypothesis of a large Martian ocean in the northern hemisphere. "The existence of a Martian ocean is a controversial issue which is extremely relevant to Mars evolution," De Blasio says. "The possibility of an entire ocean driving the hydrology of Mars, with its corollary of lakes, rivers, and a moist atmosphere, is of great importance."