Mars Express finds evidence of a large water deposit in the Medusae Fossae Formation

Windswept piles of dust or layers of ice? ESA’s Mars Express has revisited one of Mars’ most mysterious features to clarify its composition. His findings suggest layers of water ice extending several kilometers underground – the largest amount of water ever found in this part of the planet.

More than 15 years ago, Mars Express studied the Medusae Fossae Formation (MFF), revealing massive deposits up to 2.5 km deep. From these early observations, it was unclear what these deposits were made of, but new research now has an answer.

“We explored the MFF again using more recent data from Mars Express’ MARSIS radar, and found that the deposits were even thicker than we thought: up to 3.7 km thick,” said Thomas Watters of the Smithsonian Institution, USA, lead author of both. the new research, published in Geophysical research lettersand the initial study from 2007. “Interestingly, the radar signals match what we expect from ice layers and are similar to the signals we see from Mars’ polar caps, which we know are very rich in ice.”

If it melted, the ice locked in the MFF would cover the entire planet with a layer of water 1.5 to 2.7 m deep: the largest amount of water ever found in this part of Mars, and enough to fill the Red Sea on Earth.

Alternating layers of ice

The MFF consists of several wind-sculpted features measuring hundreds of kilometers in diameter and several kilometers high. Located on the boundary between the highlands and lowlands of Mars, these formations are probably the largest source of dust on Mars and one of the most extensive deposits on the planet.

Early Mars Express observations showed the MFF to be relatively radar transparent and low density, two features we would observe in icy deposits. However, scientists cannot rule out a more serious possibility: it could actually be giant accumulations of dust, volcanic ash or wind-blown sediment.

“This is where the new radar data comes in. Given its depth, if the MFF were just a huge pile of dust, we would expect it to be compacted under its own weight,” explains Andrea Cicchetti, co-author from the National Institute of Astrophysics, Italy. “That would create something much denser than what we actually see with MARSIS. And when we modeled the behavior of different materials without ice, nothing replicated the properties of MFF: we need ice.”

The new results instead suggest layers of dust and ice, all topped by a protective layer of dry dust or ash several hundred meters thick.

Future exploration and collaboration

Although Mars now appears to be a barren world, the planet’s surface is full of signs that water was once abundant, including dry river channels, ancient ocean and lake bottoms, and valleys carved by water. water. We also discovered significant reserves of water ice on Mars, such as the huge polar ice caps, buried glaciers closer to the equator, and near-surface ice that cuts through the Martian soil.

Massive reserves of ice near the equator, like those believed to lurk beneath the dry surface of the MFF, could not have formed in the planet’s current climate. They must have formed in an earlier climatic era.

“This latest analysis challenges our understanding of the Medusae Fossae formation and raises as many questions as answers,” says Colin Wilson, ESA project scientist for Mars Express and the ESA ExoMars Trace Gas Orbiter (TGO). “How long ago did these ice deposits form and what did Mars look like at that time? If confirmed to be water ice, these massive deposits would change our understanding of the The climatic history of Mars. Any reservoir of ancient water would be a fascinating target for human or robotic exploration.

The extent and location of these icy MFF deposits would also make them potentially very valuable for our future exploration of Mars. Missions to Mars will need to land near the planet’s equator, far from ice-rich polar caps or high-latitude glaciers. And they will need water as a resource. Finding ice in this region is therefore almost a necessity for human missions on the planet.

“Unfortunately, these MFF deposits are covered in hundreds of meters of dust, making them inaccessible for at least the next decades. However, each piece of ice we find helps us get a better idea of ​​where the Water from Mars has flowed before and from where it flowed it can be found today.

While Mars Express maps water ice down to a depth of a few kilometers, a view of near-surface water is provided by the TGO Mars orbiter. This orbiter carries the FREND instrument, which maps hydrogen – an indicator of water ice – in the uppermost meter of Martian soil. FREND spotted a hydrogen-rich area the size of the Netherlands within Valles Marineris on Mars in 2021 and is currently mapping the distribution of shallow water deposits on the Red Planet.

“Together, our Mars explorers are revealing more and more about our planetary neighbor,” adds Colin.