A European Space Agency (ESA) spacecraft orbiting Mars has found evidence of ice far below ground, helping build a picture of the planet’s water history.

The Mars Express first studied a 100km hilly region near the planet’s equator, known as Medusae Fossae Formation (MFF), in 2015. It found massive deposits up to 2.5 km deep but scientists were unclear if they were formed of dust piles or ice layers.

On revisiting the mysterious formation, the spacecraft found evidence of water ice. If melted, the ice would cover the entire planet in a layer of water 1.5m to 2.7 m deep - the most water ever found in this part of Mars, and enough to fill Earth’s Red Sea.

Thomas Watters of the Smithsonian Institution, USA, lead study author, said, “We’ve explored the MFF again using newer data from Mars Express’ MARSIS radar, and found the deposits to be even thicker than we thought - up to 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’s polar caps, which we know to be very ice rich.”

The MFF consists of several wind-sculpted features measuring hundreds of kilometres across and several kilometres high. Scientists couldn’t rule out the deposits being giant accumulations of windblown dust, volcanic ash or sediment.

Colin Wilson, ESA project scientist for Mars Express, “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.”

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

Co-author Andrea Cicchetti of the National Institute for Astrophysics, Italy “Given how deep it is, if the MFF 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 modelled how different ice-free materials would behave, nothing reproduced the properties of the MFF – we need ice.”

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. The ESA has also found significant stores of water ice elsewhere 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 MFF – couldn’t have formed in the planet’s present climate. They must have formed in a previous climate period.

The extent and location of these icy MFF deposits would also make them potentially very valuable for future exploration of Mars. Missions to Mars would need to land near the planet’s equator, far high-latitude glaciers, and they’ll need a water source – so finding ice in this region is almost a necessity for human missions to the planet.

“Unfortunately, these MFF deposits are covered by hundreds of metres 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’s water has flowed before, and where it can be found today.”

The Mars Express has been investigating the red planet since 2004. In that time, it has built the most complete map of the chemical composition of the atmosphere and traced the planet’s history of water, demonstrating that Mars once harboured environmental conditions that may have been suitable for life.