Mars Water Mystery Solved: Up to 99% May Be Locked in Crust, Not Lost to Space

The fate of water on Mars, once abundant and shaping its surface, now has a compelling dual answer: loss to space and storage deep in the crust. This twin resolution, derived from years of orbital imaging, rover exploration, and laboratory experiments, fundamentally reshapes how we understand the planet’s history and its potential to have harbored life. For the first several hundred million years, until roughly 3.7 billion years ago, Mars was a wet world marked by rivers, lakes, valley networks, and perhaps a northern ocean. Sediment fans and deltaic deposits, like those Perseverance is now studying in Jezero Crater, testify to sustained liquid water. Then the planet’s climate collapsed. The mechanism of that collapse begins deep inside Mars. Early in its history, a molten core generated a global magnetic field that shielded the atmosphere. Around 4 billion years ago, as the smaller planet cooled faster than Earth, the internal dynamo stalled and the shield vanished. Without it, the solar wind began stripping the atmosphere directly, carrying off water molecules dissociated by UV light. NASA’s MAVEN mission, which arrived in 2014, measured this ongoing escape and confirmed that hydrogen loss to space accounts for a volume of water equivalent to a global layer meters to tens of meters deep—enough to explain a significant fraction of the missing water. But not all of it. The second fate is chemical and geological. Rock-forming minerals like olivine and pyroxene react with water to form hydrous phases, locking H₂O into their crystal structures. This process, known as serpentinization on Earth, can immobilize vast quantities of water. In 2021, a study led by Eva Scheller at Caltech modeled the bulk chemistry of Mars and concluded that between 30% and 99% of the planet’s original water could now reside in crustal minerals. The wide range reflects uncertainties in the early water inventory, but even the lower bound implies that a majority of water never left the planet—it simply sank underground, bound into rocks. This dual-resolution answers the long-standing “water budget” puzzle and unifies disparate data sets: the D/H ratio measured by MAVEN, the clay and sulfate deposits mapped from orbit, and the hydrological features on the surface. It also brings new questions. If vast stores of water are mineral-bound at depth, accessing them for future human missions becomes a complex engineering challenge. The water is not in liquid aquifers or ice lenses but chemically bonded, requiring high-energy processing to release it. Yet, the presence of any accessible hydrated minerals could provide life-support consumables and propellant feedstock for return journeys. The richer implication concerns habitability. Surface water clearly existed for a geologically prolonged time. The crustal water, though mineral-locked, may have interacted with geothermal heat to create transient subsurface habitats for microbial life long after the surface turned arid. This shifts the astrobiological target from surface fossils to deep chemical environments. Meanwhile, evidence from neutron spectrometry suggests that even today, some ice persists in the shallow subsurface at mid-to-high latitudes, offering a more accessible but spatially limited resource. The convergence of these two enormous water sinks—atmospheric escape and crustal hydration—paints a picture of a planet that transitioned through a catastrophic atmospheric collapse but retained its internal water in a form we are only now learning to detect. For planetary scientists, this dual model provides a framework to understand Mars’s transition from a potentially habitable world to the desert we see. It also feeds into comparative planetology: Earth, with its persistent dynamo and plate tectonics, kept most water on the surface, while Venus, with a runaway greenhouse, lost its water to space. Mars sits in between, a cautionary example of a planet that could not hold onto its magnetic shield and yet held onto its water in a hidden way. Looking forward, upcoming missions like the Mars Sample Return, the ExoMars rover, and advanced orbital radar sounders will further constrain the crustal water inventory and perhaps pinpoint zones where mineral hydration meets heat flow. Such intersections could become prime drilling sites for both science and resource utilization. The decades-old mystery of Mars’s water is yielding to a synthetic picture: some went up, but even more may have gone down, hidden in the rocks beneath our landers.