Miles-thick buried water ice at Mars’ equator?

Windswept piles of dust, or layers of ice? In order to clarify the composition of a mysterious region on Mars, ESA’s Mars Express has revisited it. The new findings suggest layers of water ice stretching miles below ground. It’s the most water ever found in this part of the planet. ESA published this story originally on January 18, 2024. Edits by EarthSky.

Miles-thick buried water ice on Mars?

Over 15 years ago, the European Space Agency’s Mars Express spacecraft studied the Medusae Fossae Formation on Mars, revealing massive deposits up to 1.5 miles (2.5 km) deep. It was unclear what the deposits were made of, from these earlier observations. But new research now has an answer: it’s ice.

Thomas Watters of the Smithsonian Institution in Washington D.C. is lead author of both the new research and the initial 2007 study. He said:

We’ve explored this region again using newer data from Mars Express’s MARSIS radar. And we found the deposits to be even thicker than we thought: up to 2.2 miles (3.7 km) thick. Excitingly, the radar signals match what we’d expect to see from layered ice, and are similar to the signals we see from Mars’ polar caps, which we know to be very ice rich.

The total area of the Medusae Fossae Formation is equal to 20% the size of the continental United States. These scientists said that, if melted, the ice locked up in this region would cover Mars in a layer of water about 5 feet to about 9 feet deep (1.5 to 2.7 m deep).

It’s the most water ever found in this part of Mars, and enough to fill Earth’s Red Sea.

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Alternating layers of buried water ice

The Medusae Fossae Formation consists of several wind-sculpted features measuring hundreds of kilometers across and several kilometers high. Found at the boundary between Mars’ highlands and lowlands, the features are possibly the biggest single source of dust on Mars, and one of the most extensive deposits on the planet.

Initial observations from Mars Express showed the Medusae Fossae Formation to be relatively transparent to radar and low in density, both characteristics we’d see from icy deposits. However, scientists couldn’t rule out a drier possibility: that the features are actually giant accumulations of windblown dust, volcanic ash or sediment.

The new study’s co-author Andrea Cicchetti of the National Institute for Astrophysics in Italy explained:

Here’s where the new radar data comes in! Given how deep it is, if the Medusae Fossae Formation was simply a giant pile of dust, we’d expect it to become compacted under its own weight. This would create something far denser than what we actually see with MARSIS. And when we modeled how different ice-free materials would behave, nothing reproduced the properties of the Medusae Fossae Formation.

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 an arid world, the planet’s surface is full of signs that water was once abundant, including dried-up river channels, ancient ocean and lake beds, and water-carved valleys. We’ve also found significant stores of water ice on Mars, such as the enormous polar caps, buried glaciers nearer the equator, and near-surface ice laced through martian soil.

Massive stores of ice near the equator – such as those suspected to lurk below the dry surface of the Medusae Fossae Formation – couldn’t have formed in the planet’s present climate. They must have formed in a previous climate epoch. Colin Wilson, ESA project scientist for Mars Express and the ESA ExoMars Trace Gas Orbiter, said:

This latest analysis challenges our understanding of the Medusae Fossae Formation, and raises as many questions as answers. How long ago did these ice deposits form, and what was Mars like at that time? If confirmed to be water ice, these massive deposits would change our understanding of Mars climate history.

Any reservoir of ancient water would be a fascinating target for human or robotic exploration.

A resource for future Mars explorers?

The extent and location of these icy Medusae Fossae Formation 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 the ice-rich polar caps or high-latitude glaciers. And they’ll need water as a resource. So finding ice in this region is almost a necessity for human missions to the planet. But Colin Wilson said:

Unfortunately, these Medusae Fossae Formation deposits are covered by hundreds of meters of dust, making them inaccessible for at least the next few decades. However, every bit of ice we find helps us build a better picture of where Mars’ water has flowed before, and where it can be found today.

While Mars Express maps water ice to a depth of a few kilometers, a view of near-surface water is provided by Mars orbiter TGO. This orbiter is carrying the FREND instrument, which is mapping hydrogen – an indicator of water ice – in the topmost meter of martian soil. FREND spotted a hydrogen-rich area the size of the Netherlands within Mars’ Valles Marineris in 2021, and is currently mapping how shallow water deposits are distributed across the Red Planet. Wilson added:

Together, our Mars explorers are revealing more and more about our planetary neighbor.

Bottom line: Planetary scienetists knew, from an earlier study, that the Medusae Fossae Formation near Mars’ equator had massive buried deposits of some material. But it was unclear what the deposits were made of, from these early observations. New research now has an answer: it’s ice.