Data from ESA's Mars Express spacecraft show that there may be underground ice up to 2.3 miles (3.7 kilometers) thick at the equator of Mars. The water here is enough to fill the Red Sea on Earth and is a huge resource for future Mars colonies. The environment on modern Mars is quite dry. Scientists often use the Atacama Desert on Earth as a stand-in for experiments, but although the Atacama Desert is one of the driest places on Earth, compared with Mars, it is considered a tropical rainforest in a monsoon climate.
That's not to say there's no water at all on the Red Planet. Mars has ice caps at its poles, and geology shows there was abundant flowing water on Mars billions of years ago, including the shallow seas that now line the Great Plains.
There are also deposits of water ice beneath the surface of Mars. The question is, where is the water ice and how much is there? This is not just of academic interest, because if humans one day decided to colonize Mars, humans would undoubtedly need to find a source of water, in addition to transporting water down from the poles as Perceval Lowell speculated about the Martians.
Of particular interest is the Medusae Fossae Formation (MFF), an equatorial sedimentary deposit covering an area the size of India. It's the largest source of dust on Mars, and now it's discovered it may also be a reservoir of water.
The evidence comes from ESA's Mars Express orbiter, which is equipped with ground-penetrating radar. Fifteen years ago, the spacecraft discovered sedimentary layers deep within Martian lava. The mystery is, what are these sedimentary layers made of?
One possibility is that these deposits are made of dust or volcanic dust that is transparent to radar, like ice. However, this dust is denser than ice because it compresses under its own weight, while ice does not. Computer models show that dust or ash would not match, but layers of ice and dust would.
If it were indeed water ice, it would be the largest deposit of water on Mars. If it melted, it would be enough to flood the entire planet, with water reaching depths of up to 8.9 feet (2.7 meters). Such vast amounts of water would be a boon to future colonists, but only if they could be mined with considerable effort.
Colin Wilson, project scientist for ESA's Mars Express and ESA's ExoMars Trace Gas Orbiter (TGO), said: "This latest analysis challenges our understanding of the Medusae Fossae formation. There are as many questions as there are answers. How long ago did these ice deposits form, and what was Mars like back then? If confirmed to be water ice, these giant deposits will change our understanding of Mars' climate history and make them a fascinating target for human or robotic exploration."
"Unfortunately, these water ice deposits are covered in hundreds of meters of dust and will be inaccessible for at least the next few decades. However, every bit of ice we find helps us better understand where Martian water once flowed, and where it can be found today."