Decades of orbital and surface missions have confirmed that water was once abundant on Mars and remains a significant component of the planet today. The presence of water fundamentally dictates a planet’s geological and climatic history, shaping its surface and determining its potential for life. The discovery of water in its various forms is central to planetary science, connecting Mars’s past habitability with the potential for future human exploration.
The History of Water on Mars
Observations of the Martian landscape reveal a past dominated by the flow of vast quantities of surface water. During the Noachian and Hesperian periods, over three billion years ago, Mars was significantly wetter than it is now. Evidence for this ancient past includes expansive geological features such as dendritic valley networks, which closely resemble terrestrial river systems. These networks suggest precipitation and sustained surface runoff once occurred across the planet’s rugged southern highlands.
Specific formations like delta deposits and ancient lakebeds, notably Jezero Crater, demonstrate that standing bodies of water persisted for long periods. Mineralogical evidence supports this picture, with the detection of clay minerals (phyllosilicates) that only form through prolonged interaction with non-acidic water. As the planet’s magnetic field faded and its atmosphere thinned, much of the surface water was lost to space or became locked away. The subsequent Hesperian period saw the climate transition, characterized by massive, episodic outflow channels formed by catastrophic floods from subsurface reservoirs.
Current Locations and States of Martian Water
Today, water on Mars exists primarily as solid ice, locked within several major reservoirs. The most visible are the polar ice caps, which contain immense deposits of water ice, partially covered by frozen carbon dioxide. Beneath the surface, vast deposits of water ice are mixed with soil, forming a layer of permafrost that extends across the mid-latitudes.
This subsurface ice is a major inventory, estimated at more than five million cubic kilometers near the surface. This volume is enough to cover the planet to a depth of 35 meters. The Phoenix lander, for example, directly confirmed this shallow ice when it uncovered white patches that sublimated upon exposure to the atmosphere. Small amounts of water vapor are also present in the thin Martian atmosphere, mostly concentrated near the polar regions during the summer.
Transient liquid water may appear under specific, rare conditions. Dark streaks, known as Recurring Slope Lineae (RSL), seasonally appear on steep slopes during warmer periods. These are caused by briny water, where dissolved salts lower the freezing point, allowing it to flow briefly before it evaporates or freezes again. This process, involving hydrated salts, represents the closest thing to liquid water existing on the Martian surface today.
The Search for Life: Water and Habitability
The presence of water, both past and present, is the most important factor for astrobiology, as liquid water is a fundamental requirement for all known life. The early Martian environment, with its rivers and lakes, provided a potentially habitable window approximately 3.5 billion years ago. Scientists now focus on environments protected from the harsh surface radiation and temperature swings that followed the loss of the atmosphere.
Subsurface habitats, such as those near ancient hydrothermal systems or in deep, water-saturated fractures, may have shielded microbial life. Modern missions target areas where water-rock interactions were intense and prolonged, like the ancient lakebed of Jezero Crater, explored by the Perseverance rover. The rover is collecting rock cores containing minerals such as gypsum, which are excellent at trapping and preserving potential biosignatures. Returning these samples to Earth for detailed analysis will be the most direct way to determine if Mars ever hosted life.
Water as a Resource for Future Exploration
Beyond its scientific implications, Martian water is an invaluable resource for future human missions and permanent settlement. The concept of In-Situ Resource Utilization (ISRU) relies on using local materials to reduce the massive launch costs associated with carrying everything from Earth. Water ice is the most significant resource for ISRU, as it can be converted into life support consumables and rocket propellant.
Converting Water into Resources
Electrolysis, the process of splitting water molecules, yields oxygen for breathing and for use as an oxidizer in rocket fuel, and hydrogen for propellant. Water can also be combined with atmospheric carbon dioxide in the Sabatier reaction to produce methane and oxygen, the fuel combination planned for many return-trip rockets. The accessibility of water ice is therefore a major consideration when selecting landing sites, ensuring that future explorers can “live off the land” and achieve mission sustainability.