Valles Marineris is considered one of the largest canyons in the solar system. It runs across the planet’s surface like a huge scar for 4,000 km (2,500 miles), it’s 200 km (120 miles) wide and in places 7 km (4.3 miles) deep.
At the time of writing, we Earthlings have no piece of hardware inside the canyon, but there are plenty of pieces of technology up in orbit, looking down at the place. One of them, the ESA-Roscosmos ExoMars Trace Gas Orbiter, is responsible for this discovery.
According to ESA, water is not running freely across the canyon floor, but is rather hidden beneath the surface. The Orbiter packs hardware that is capable of mapping hydrogen in the uppermost meter of the Martian surface. And since the presence of hydrogen is a sign of water, hence the excitement in the ESA announcement.
Now, the agency didn’t go into specifics as to at least give us an idea of what exactly “significant amounts” means, but it does say “assuming the hydrogen we see is bound into water molecules, as much as 40% of the near-surface material in this region appears to be water.” Most of it should be either in the form of ice or bound to other minerals in the soil.
The area the Orbiter looked at is the size of Netherlands, and is located in the Candor Chaos region of the canyon.