New results show that the Medusae Fossae Formation, a set of broad and enigmatic plains, provides most of the fine dust found everywhere on the Martian surface.

As the recent planet-wide dust storm on Mars dramatically demonstrates, the Martian atmosphere might be thin (its surface pressure is only 0.6% that at sea level on Earth), but its winds can quickly mobilize incredible amounts of fine particles. By one estimate, some 3 billion tons of dust cycles between the planet's surface and atmosphere each year.

Yet to become airborne, these particles must be extremely small, microscopic motes no more than 50 to 100 microns across, and for decades researchers have struggled to find geologic processes on Mars that operate efficiently enough, and for long enough, to generate all that silty grit.

Now a quartet of researchers believes they have pinpointed the main source of Martian dust. It's called the Medusae Fossae Formation (MFF), a patchwork of plains straddling the planet's equator that have intrigued geologists for decades.

Map of Medusae Fossae Formation
The extensive Medusae Fossae Formation appears to involve a layer of fine dust, hundreds of meters deep, emplaced by gas-driven volcanic eruptions about 3 billion years ago.
Nature Communications / Lujendra Ojha et al

If this name seems familiar, it's because a recent study argues that MFF's thick blanket of sediment likely resulted when a Martian supervolcano erupted about three billion years ago.

And ever since, say Lujendra Ojha (Johns Hopkins University) and his colleagues, the gradual but incessant erosion of the MFF has created most of the present-day dust on Mars.

Medusae Fossae Formation HiRISE 480p
Streamlined features and isolated mesas that show the extent of wind erosion in the Medusae Fossae Formation. The High Resolution Imaging Science Experiment (HiRISE) onboard the Mars Reconnaissance Orbiter recorded this image in 2013.
NASA / JPL / Univ. of Arizona

The MFF covers an area that is equivalent to about 20% of the continental U.S. However, disconnected parts of it can be found all over the Red Planet, and when created it might have covered 2½ times more of the Martian landscape. "We do see isolated sections of MFF that are spread out, and that's how we know it was a much bigger unit," says Ojha. "A large part of it has gotten eroded."

Ojha's team explores this possibility in July 20th's Nature Communications. You can read a popular summary of this geologic detective story in a JHU press release.

Unlike on Earth, where much of the dust is trapped by bodies of water, Martian dust floats in atmosphere or settles on the ground. Data collected by Mars rovers show that all this dust, if spread out evenly around the planet, would create a layer about 3 meters thick.

So how much erosion has occurred in MFF? Ojha's group finds that over time the region has lost, through wind-driven erosion, enough dust to mantle the whole planet to a depth of 2 to 12 meters, so the actual 4-meter thickness fits well with this result.

Medusae Fossae Formation abundances
Martian dust contains relatively high abundances of sulfur and chlorine. These two maps show that S and Cl are also abundant in the Medusae Fossae Formation (black outlines near center).
Lujendra Ojha et al

In addition, previous rover data show that Martian dust is extremely rich in sulfur and chlorine, and has a distinct sulfur-to-chlorine ratio. When Ojha's team used a gamma-ray spectrometer on NASA's Mars Odyssey orbiter to deduce the chemical composition of the MFF, they found that MFF is also extremely rich in sulfur and chlorine and has the same sulfur-to-chlorine ratio as Martian dust. It appears that no other region on Mars has these same chemical characteristics.

Ojha thinks that the Medusae Fossae Formation made a significant contribution to the total current amount dust on Mars — perhaps as much as half of it. Jim Zimbelman (National Air & Space Museum), who was not involved in the study, agrees. "There isn’t another significantly large deposit that matches the [spectrometer] signal that comes from Medusae," he says, which means that other sources are unlikely to have produced as much dust as the MFF.


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