The polar layered deposits are the headline reservoir: kilometres-thick stacks of water ice interleaved with dust, capped seasonally by carbon dioxide frost. They are unambiguous, enormous, and inconveniently located at latitudes where solar power is poor and the cold is at its worst.
The more useful discovery is the mid-latitude ice. Radar sounding found a buried sheet in Utopia Planitia comparable in volume to one of the North American Great Lakes, and ice-detection surveys have mapped shallow deposits across broad swathes of both hemispheres, in places sitting under only a metre or two of dust. Those are the landing sites everyone argues about, because accessible ice sets where a base can exist.
Water is not one resource but four. It is drinking water and life support. Split by electrolysis it is breathing oxygen and rocket fuel. Combined with atmospheric CO2 through the Sabatier reaction it is methane propellant. And in bulk it is radiation shielding, because hydrogen stops cosmic-ray secondaries better than almost anything else.
What Mars cannot do today is keep it liquid. At 610 pascals, water at the surface sits below its triple point: ice sublimes straight to vapour rather than melting. Liquid water on the surface needs both pressure above about 610 Pa and temperature above 273 K simultaneously — which is precisely the pair of conditions every warming method on this site is trying to deliver.
The polar caps alone would cover Mars twenty to thirty metres deep if melted. Unlike nitrogen, water is not a resource Mars has to import.
Radar surveys have found ice under as little as a metre or two of dust at mid-latitudes, within reach of a modest excavator rather than a polar expedition.
Life support, breathing oxygen, methane propellant via Sabatier, and hydrogen-rich radiation shielding all come out of the same ice deposit.
Accessible ice is the strongest single criterion for site selection, which turns an open question into a mapped, surveyable one.
At 610 Pa, water is below its triple point. Ice sublimes directly to vapour, so open water is impossible anywhere on Mars until pressure rises.
The largest reservoirs sit where sunlight is weakest and temperatures lowest — the worst place to operate the machinery needed to extract them.
Excavating frozen regolith at 210 K, then melting and purifying it, is a continuous heavy power draw on a planet where power is the scarcest commodity.
Shallow ice is exactly where any extant Martian life would most plausibly survive, which puts the best resource sites under the strictest planetary-protection rules.
| Extraction | Excavate + melt | Frozen regolith at ~210 K |
| Power | Continuous kW-scale | Melting dominates the budget |
| For liquid surface water | >611 Pa and >273 K | Both, at once |
| Siting | Shallow mid-latitude ice | Radar-mapped from orbit |
The polar caps alone contain enough water ice to cover the entire planet in a layer roughly twenty to thirty metres deep, and substantial additional ice is buried at mid-latitudes, including a deposit in Utopia Planitia comparable in volume to a Great Lake.
Because atmospheric pressure is about 610 pascals, right at the triple point of water. Ice sublimes directly into vapour instead of melting. Liquid water needs pressure above roughly 611 Pa and temperature above 273 K at the same time.
Yes. Electrolysis splits it into hydrogen and oxygen, and the Sabatier reaction combines that hydrogen with atmospheric carbon dioxide to make methane — the propellant combination most Mars return architectures assume.
