Scientists Finally Crack The Mystery Of The Medusae Fossae Formation On Mars

A new study reveals that the bizarre Martian rock formation may have been born out of explosive volcanic eruptions.

Western Medusae Fossae formation
Mars Reconnaissance Orbiter / NASA/JPL-Caltech/University of Arizona

A new study reveals that the bizarre Martian rock formation may have been born out of explosive volcanic eruptions.

The Medusae Fossae formation is one of the largest deposits of sedimentary rock on Mars. First observed in the 1960s with NASA’s Mariner spacecraft, the bizarre Medusae Fossae formation has puzzled scientists for decades.

Its unusual soft rock deposits, about one-fifth the size of the continental United States, stretch for more than 620 miles (1,000 kilometers) along the Red Planet’s equator and represent one of the largest rock formations not only on Mars, but in the entire solar system.

The origin of the mysterious Martian rock formation has remained unclear until now, Phys.org reports, detailing the findings of a recent study which suggest that the Medusae Fossae formation was shaped by the plane’s volcanic activity.

According to the new research, published in the Journal of Geophysical Research: Planets, this strange, easily-eroded formation, which boasts undulating hills and abrupt mesas, was most likely created more than three billion years ago as a result of explosive volcanic eruptions.

These eruptions pelted Mars with jets of hot ash, rock, and gas, depositing rock dust and debris on the planet’s surface. After cementing over the years, such deposits are the most plausible explanation for the rise of the Medusae Fossae formation, note the study authors.

In the past, the origin of the famous Martian rock deposits has been attributed to the action of either wind, water, ice, or volcanic eruptions.

“The easily eroded nature of the Medusae Fossae formation suggests that it is composed of weakly cemented particles and was most likely formed by the deposition of wind-blown dust or volcanic ash,” NASA officials wrote in 2002.

But the new study shows that the porous structure of the Medusae Fossae formation can only be explained if the rock deposits were created by volcanic eruptions.

Using gravity data from NASA’s Mars Reconnaissance Orbiter, the scientists performed the first-ever measurement of the formation’s density and discovered that this peculiar sedimentary rock is two-thirds as dense as the rest of the Martian crust.

A combination of radar and gravity data revealed that the Medusae Fossae formation is unusually porous and that its high density couldn’t be accounted for by the presence of ice, which is much less dense than rock.

“When combined with previous radar measurements, our density constraint rules out the presence of ice as the cause of unusual radar permittivity. Rather, we find the joint radar and gravity constraints imply a dry and highly porous rock unit,” the study authors wrote in their paper.

This would make the Medusae Fossae formation, the largest known explosive volcanic deposit in the entire solar system, 100 times more massive than the largest similar deposit found on Earth.

“This is a massive deposit, not only on a Martian scale, but also in terms of the solar system, because we do not know of any other deposit that is like this,” said study lead-author Lujendra Ojha, a planetary scientist at Johns Hopkins University in Baltimore.

Western Medusae Fossae formation.
Dark dunes in the western Medusae Fossae formation. Mars Reconnaissance Orbiter / NASA/JPL-Caltech/University of Arizona

Since a volcanic rock deposit of this magnitude would have needed a massive amount of volatile gases stirring in the planet’s interior to fuel the explosive volcanic eruptions, the new findings provide a more in-depth look at the heart of Mars, notes Ojha.

“If you were to distribute the Medusae Fossae globally, it would make a 9.7-meter (32-foot) thick layer. Given the sheer magnitude of this deposit, it really is incredible because it implies that the magma was not only rich in volatiles and also that it had to be volatile-rich for long periods of time.”

The discovery that the Medusae Fossae formation is most likely volcanic in origin brings up more details about Mars’ past. The researchers believe that the explosive volcanic eruptions that gave rise to this sedimentary rock formation may have changed the planet’s history forever, blasting staggering amounts of gases into the atmosphere of the Red Planet and altering its climate.

According to Ojha, these volcanic eruptions could have ejected enough water to cover Mars in a 4-inch-thick (more than nine centimeters) global ocean. At the same time, the eruptions would have changed the chemistry of both the Red Planet’s surface and its atmosphere, by spewing out toxic volcanic gases, such as hydrogen sulfide and sulfur dioxide.

As Ojha explained, the consequences of the explosive volcanic eruptions in Mars’ distant past may have influence the planet’s potential to be habitable.