Water is everywhere on Mars. It is more accessible in some places than others, but future Mars-tronauts are going to need that alien water, and now they might know where to find it.

Mars may be hiding a massive reservoir of water that would actually be usable if it really exists. Observations from the ExoMars Trace Gas Orbiter suggest that there are subsurface reserves of water under a huge system of canyons known as Valles Marineris, sometimes referred to as the “Grand Canyon of Mars.” The soil of this Martian “oasis” could be holding water, either as ice or bound to minerals, just a few feet beneath the surface.

When the Trace Gas Orbiter’s FREND neutron telescope revealed an unusually large amount of hydrogen over Valles Marineris, it revealed this ultracold canyon is packed with frozen water under dry soil, much like Earth’s permafrost. A team of researchers found that nearly half of the material near the surface of this region could be some form of water. Does it mean life? Maybe some sort of hypothetical micro-extremophiles like tardigrades are crawling in there. Researcher Alexey Malakhov led a study recently published in Icarus to (at least virtually) dig deeper.

"We detected 40% WEH or water equivalent hydrogen, a standard measure of H when we assume that all of the H is contained in H2O molecules," Malakhov told SYFY WIRE. "Our data alone does not say whether this is water or some other form of H, but it's possible that such an amount of H is contained in hydrated minerals or in the form of water."

FREND (Fine-Resolution Epithermal Neutron Detector) seeks out neutrons. Built by Roscosmos and installed on the Trace Gas Orbiter, the hi-res instrument picks up on fluctuations in neutrons escaping Mars and keeps a proverbial eye on radiation in the Martian orbit. Hydrogen had previously been found in permafrost at both poles, but the instruments that had discovered it were not capable of finding areas rich in hydrogen that were hidden in geological structures like Valles Marineris. FREND was better able to detect neutron emissions that may be evidence for subsurface water. There are fewer, less energetic neutrons where there is more hydrogen.

It isn’t so much that neutrons were being released, but being held back near the equator of Mars. That was the a sign of high hydrogen content in the planet’s reddish regolith. Nobody really expected something like this to show up in an equatorial region, but this is a planet that has surprised us over and over again. What we still don’t know is how much is water ice and how much is contained in hydrated minerals. Hydrated minerals are actually difficult to get to the level of hydration they have reached in Valles Marineris. Extremely specific conditions are needed for that.

While water can easily explain the presence of so much water-equivalent hydrogen, it's difficult to prove that it doesn't evaporate considering the pressure and temperatures at the Martian equator. However, someone may eventually event a device that can act as a next-gen water filter by the time we can meet all the safety requirements to get humans to Mars. Water is still water. Though it would take more effort to separate water from hydrated minerals, Malakhov believes the amounts FREND is suggesting would make that worth it.

"What our study does show is that this place is unique, not only because it's the largest relief feature on the planet, but also because of its hydrology. Really unique conditions required for water and/or heavily hydrated minerals are there," he said. "Regardless of the form, it is still an exploitable water reservoir."

Near-infrared spectrometers, which detect infrared light in the part of the spectrum that is closer to visible light (but still slightly too long for our eyes to process) have detected water that was chemically bound to some hydrated minerals before. There is also probably adsorbed water — water that is a thin film on the outside or inside of other materials — having condensed on grains of Martian soil. Maybe there will be some sort of contraption, like a more complex water filter, that will be able to purify that water by the time humans are ready to take off to Mars.

Neutron data has been able to unearth what looks like an immense amount of extraterrestrial water without even touching the surface of Mars. Even more specific methods of detecting hydrogen and H2O will be needed to differentiate exactly how much water is chemically bound, and adsorbed as opposed to free water ice that can be accessed. Not all hydrated minerals can be seen with infrared spectroscopy, either. Maybe conditions that allowed ice to be preserved for billions of years mean there really is a treasure trove of water ice under Valles Marineris.

"If you ask 'the human' in me whether I think it is water or minerals, I'd say that it's water, or a mixture of both," Malakhov said. "I can hardly imagine a huge territory consisting exclusively of heavily hydrated minerals, but there's also 'the scientist' in me reminding me to wait for in-situ analysis from the canyon floor or from some other similar region on Mars."

Who will be the first human to leave boot prints on Mars? Maybe the more interesting question is who will be the first to put Martian water to their lips.