Black-and-white image of the Martian landscape feature Medusae Fossae
Medusae Fossae is an enigmatic region of Mars whose origin has puzzled researchers for decades. Credit: Generated by Peter Mouginis-Mark (University of Hawai’i at Mānoa) from a CTX image draped over a digital elevation model created by Harold Garbeil (also at University of Hawai’i)

Pumice rafts can “travel 5–8 kilometers on Earth. There’s no conceptual reason why they couldn’t do that on Mars.”

Rafts of pumice-like material, similar to rafts found floating in Earth’s Pacific Ocean, may be responsible for some of the most enigmatic terrain on Mars. A new theory suggests that low-density rocks could have slid down the slopes of the Red Planet’s largest volcano to create giant rocky rafts in the planet’s early ocean. Ultimately, researchers suggest, the material washed up on shore to create the expansive region known as Medusae Fossae.

Geologists have been trying to understand how Medusae Fossae formed since NASA’s Mariner spacecraft first observed the region in the 1960s. A variety of mechanisms have been suggested, but each mechanism comes with its own set of challenges that keep it from fully explaining the deposit.

Now a new idea has surfaced. Pumice rafts can “travel 5–8 kilometers on Earth,” said Peter Mouginis-Mark, a planetary scientist at the University of Hawai’i at Mānoa and lead author on the new paper. “There’s no conceptual reason why they couldn’t do that on Mars.”

Photo of a pumice raft created by an underwater volcano in August 2019
In August 2019, a massive pumice raft was spotted in the Pacific Ocean. Credit: NASA Earth Observatory/Joshua Stevens/U.S. Geological Survey

When molten rock from underwater volcanoes interacts with ocean water, the exploding fragments of lightweight rock float to the surface as pumice rafts. These extensive formations can be quite large; last summer, a pumice raft roughly 200 square kilometers in size caught worldwide attention as it traveled toward Australia. Mouginis-Mark and his colleagues suspect that similar volcanic processes could create a pumice-like raft on Mars by interacting with ice locked beneath the surface.

The new theory brings its own challenges. Mouginis-Mark admits that there is “a lot of hand waving” in the discussion. But he said the potential problems are no worse than those found in other contending theories for the origin of the unusual feature.

“If [the process] works in the same way that it does on Earth, it is a good possible explanation for how these deposits—[for] which no one’s come up with a definite way to produce—could have formed,” Mouginis-Mark said.

The new study was published this month in the journal Icarus.

A “Very Different Mechanism”

The Medusae Fossae Formation covers 2.2 million square kilometers near the Martian equator. Previous observations revealed that it is an unusually porous region, about two thirds as dense as the rest of the planet’s crust. Those same observations revealed that ice beneath the surface was insufficient to account for the shockingly low density. Instead, some researchers suggest that explosive volcanism could have created such porous rocky material.

But getting ash and rocky debris from the volcanoes closest to Medusae Fossae poses a problem, according to Mouginis-Mark. In the thin Martian atmosphere, the material would have fallen out of the eruption plume before it reached the formation.

According to Bruce Campbell, a planetary scientist at Smithsonian’s National Air and Space Museum in Washington, D.C., the sheer size of Medusae Fossae adds an additional challenge. “It’s very difficult for any of these models…to easily explain the vast volume of the final deposit,” he said. Campbell was not part of the new study.

An ocean could potentially carry material from the giant volcano Olympus Mons to the distant shores of Medusae Fossae.

One of the intriguing features around the formation is a proposed shoreline for an ancient ocean. Early in its evolution, Mars is thought to have hosted such an ocean, although that theory is also subject to ongoing debate. Mouginis-Mark realized that an ocean could potentially carry material from the giant volcano Olympus Mons to the distant shores of Medusae Fossae. The researchers targeted Olympus Mons as the source of the volcanic material because its base is at a much lower elevation than other Martian volcanoes, Mouginis-Mark said.

Unlike pumice rafts on Earth, which burble up from underwater and immediately begin their travels, Martian rafts likely sat on the slope for some time and wound up covered by material from later eruptions of denser material. Eventually, a landslide would drag them all the way to the base of the volcano, where an ocean would allow them to drift long distances.

“The basic processes which we think we can see on Mars at Olympus Mons are known to happen on Earth in Hawaii and at other volcanic islands,” Mouginis-Mark said.

As time passed, the rafts traveled to shore, where they piled up in a giant mass of pumice-like material. Again, this happens frequently on Earth.

“There are lots of places on Earth where there is evidence of repeated deposition of pumice from the breakup of floating rafts,” said researcher Patrick Nunn, an oceanographer at the University of the Sunshine Coast in Australia who studies pumice rafts. Pumice rafts on Earth are usually split into pieces by waves and offshore reefs long before they reach land. “What you find on the coast is often layers of pumice from broken-up rafts at different places and elevations, signaling the repeated deposition of pumice,” he said. Nunn was not part of the new study.

According to Campbell, “it’s a very different mechanism from the more classic air fall deposits” also proposed as solutions to the Medusae Fossae mystery.

No Smooth Sailing for Any Theory

Like other models hoping to solve the mystery of Medusae Fossae, the pumice raft idea brings its own challenges. One of the biggest problems comes from measurements of the feature, which reveal that it is too dense to float on water. Campbell said it’s possible that the porous material compacted after it sat on the shoreline.

“It’s a problem, but there are probably ways of getting around it,” he said.

Another issue is a lack of landslide scars on Olympus Mons, as well as an explanatory mechanism for what could have triggered them. While Mouginis-Mark agrees that this is a problem, he points out that other researchers have invoked giant landslides as well.

The final pileup of pumice-like material also brings its own challenges. On Earth, pumice rafts pile up in collections that reach around 10 meters high at a maximum, far more than would be required to build up Medusae Fossae. Mouginis-Mark points out that the low gravity and the million-year timescale, as well as the unknown volume of original material and the uncertain state of an ocean that could have been at least partially covered in ice, make an apples-to-apples comparison with Earth a challenge.

The new theory comes with a handful of ways to disprove it. If researchers determine that the Red Planet never had an ocean, or if the Medusae Fossae feature is found to have formed while lava from Olympus Mons was still warm, the idea of pumice rafts can clearly be thrown out.

“Yes, our model has problems,” Mouginis-Mark said. “But none are any more challenging than the alternative explanations for the origin of the Medusae Fossae feature, and people have been thinking about these ideas for the last 40 years with no luck. We therefore think it is worth proposing a new idea for the origin of Medusae Fossae, particularly one that combines many aspects of the geology of Olympus Mons into a coherent picture.”

—Nola Taylor Redd (@NolaTRedd), Freelance Science Journalist


Tillman, N. T. (2020), Scientists float a new theory on the Medusae Fossae Formation, Eos, 101, Published on 19 May 2020.

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