An ocean the size of the Arctic once covered half of Mars, new images hint

New evidence of ancient rivers suggests Mars may have been a “blue planet,” thanks to an ocean spanning its entire northern hemisphere.

Cameras from several Mars orbiters captured the dusty remnants of apparent river deltas, which were described in research published Jan. 7 in the journal NPJ Space Exploration.

The team, led by researchers at the University of Bern, peered at the famed Valles Marineris, which is the largest valley system on Mars and five times longer than the Grand Canyon. Around this Red Planet region, scientists spotted “structures near the canyon system that resemble river deltas on Earth,” University of Bern representatives wrote in a statement.

“These structures represent the mouth of a river into an ocean,” the statement added. “The new study thus provides clear evidence of a coastline, and consequently, of an earlier ocean on Mars.”

Water, water everywhere

Although Mars is dry and dusty today, there are many signs that the planet hosted water in the ancient past. For example, Mars rovers have spotted “blueberry stones” that may include iron oxide minerals containing water. NASA’s Curiosity rover imaged possible “ripples” of an ancient riverbed in 2025, and some orbital missions have spotted what may be vast underground stores of water.

The new study focused on Martian geomorphology — the study of the surface and its processes — and employed several spacecraft, including the ExoMars Trace Gas Orbiter, Mars Express and the Mars Reconnaissance Orbiter (which recently shared its 100,000th photo).

“The unique high-resolution satellite images of Mars have enabled us to study the Martian landscape in great detail by surveying and mapping,” Ignatius Argadestya, lead author of the study and a doctoral student at the University of Bern Institute of Geological Sciences as well as the university’s Physics Institute, said in the statement.

“When measuring and mapping the Martian images, I was able to recognize mountains and valleys that resemble a mountainous landscape on Earth,” Argadestya said. “However, I was particularly impressed by the deltas that I discovered at the edge of one of the mountains.”

The team saw possible “fan deltas,” which form when debris and sand pile up in still water. The deposits in the Martian imagery appear very similar to active fan deltas on Earth, according to the team; on our planet, these deltas pile up at entry points where rivers flow into the ocean.

All of the “deposits” were mapped at an elevation of between 11,975 and 12,300 feet (3,650 to 3,750 meters) and formed roughly 3.37 billion years ago. Given that all of the deposits are at roughly the same elevation and in a region in the northern lowlands of Mars and Valles Marineris, the researchers argue that these structures mark the boundaries of an ancient shoreline. It’s likely the ocean that once flowed there spanned Mars’ entire northern hemisphere, the team added.

In part drawing on past research, the scientists say this ancient Martian ocean was at least as large as the present-day Arctic Ocean.

“We are not the first to postulate the existence and size of the ocean,” Fritz Schlunegger, a geology professor at the University of Bern and co-author of the study, said in the statement. “However, earlier claims were based on less precise data and partly on indirect arguments. Our reconstruction of the sea level, on the other hand, is based on clear evidence for such a coastline, as we were able to use high-resolution images.”