Mars has its own version of the Grand Canyon, and scientists have learned this dramatic feature is home to "significant amounts of water" after a discovery made by an orbiter circling the red planet, according to the European Space Agency.
The ExoMars Trace Gas Orbiter, launched in 2016 as a joint mission between the European Space Agency and Roscosmos, detected the water in Valles Marineris on Mars.
This canyon system is 10 times longer, five times deeper and 20 times wider than the Grand Canyon.
The water is located beneath the surface of the canyon system and was detected by the orbiter's FREND instrument, or Fine Resolution Epithermal Neutron Detector. This instrument is able to map hydrogen in the top metre (3.28 feet) of Martian soil.
Most water on Mars is located in the planet's polar regions and remains frozen as water ice. Valles Marineris is just south of the planet's equator, where temperatures typically aren't cold enough for water ice to remain.
The observations were collected by the orbiter between May 2018 to February 2021.
Previously, other orbiters have searched for water just beneath the Martian surface and detected small amounts under Martian dust. A study detailing the findings published Wednesday in the journal Icarus.
"With (the Trace Gas Orbiter) we can look down to one metre below this dusty layer and see what's really going on below Mars' surface -- and, crucially, locate water-rich 'oases' that couldn't be detected with previous instruments," said study author Igor Mitrofanov, principal investigator of the FREND neutron telescope, in a statement.
"FREND revealed an area with an unusually large amount of hydrogen in the colossal Valles Marineris canyon system: 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."
To put that into perspective, this area is about the size of the Netherlands. It overlaps with Candor Chaos, a network of valleys within the canyon system.
The FREND instrument searches for neutrons to map hydrogen content in the Martian soil.
"We can deduce how much water is in a soil by looking at the neutrons it emits," said study coauthor Alexey Malakhov, a senior scientist the Space Research Institute of the Russian Academy of Sciences, in a statement.


That's because "neutrons are produced when highly energetic particles known as 'galactic cosmic rays' strike Mars; drier soils emit more neutrons than wetter ones," he said in the same statement.
"We found a central part of Valles Marineris to be packed full of water -- far more water than we expected. This is very much like Earth's permafrost regions, where water ice permanently persists under dry soil because of the constant low temperatures."
The instrument's unique observational capabilities allowed the team to detect water that remained hidden before, Malakhov said. This could be water ice or water that is attached to minerals within the soil.
But the scientists believe the presence of ice is more likely because the minerals at this location contain little water.