Water is present on Mars, existing almost entirely as ice or vapor, not as stable, flowing liquid on the surface. Understanding the history and current state of Martian water is paramount, as its presence is the main indicator for the planet’s potential to have once hosted life and its capacity to support future human exploration. The evidence spans billions of years, from ancient oceans that shaped the planet’s geology to the massive ice reservoirs locked away today.
Evidence of Ancient Martian Oceans
Geological observations confirm that Mars once hosted vast amounts of liquid water on its surface, likely during the Noachian period (about 3.7 to 4.1 billion years ago). High-resolution images from orbiters reveal extensive networks of dried-up river valleys, which branch out in dendritic patterns similar to drainage systems found on Earth. These features indicate prolonged periods of rainfall or snowmelt and sustained surface runoff that carved the landscape over time.
Massive outflow channels provide further support for a wet past, appearing to have been created by catastrophic flooding events that discharged enormous volumes of water onto the northern plains. These floods may have contributed to a primordial ocean that scientists estimate could have covered between 36% and 75% of the planet’s surface. The Chinese Zhurong rover, for instance, found evidence of a potential ancient coastline in the southern Utopia region, suggesting a sea may have existed there roughly 3.5 billion years ago.
Rovers, such as Opportunity and Curiosity, provided concrete evidence by finding specific hydrated minerals within the Martian rocks. Opportunity discovered abundant traces of sulfate salts and minerals like jarosite, which require the presence of liquid water to form. Curiosity’s study of the Gale Crater confirmed the existence of an ancient freshwater lake that could have provided a hospitable environment for microbial life.
Where Water Exists Today
While surface water is gone, substantial amounts remain locked within the planet. The most visible reservoirs are the polar ice caps, which contain a mix of water ice and frozen carbon dioxide (dry ice). The North Polar Cap, for example, is composed primarily of water ice and is massive enough that if melted, it would cover the entire planet in a shallow layer of water.
Beyond the poles, massive amounts of water ice exist just beneath the surface as permafrost, particularly in the mid-latitudes. NASA’s Phoenix lander directly observed this subsurface ice when it dug into the Martian soil and exposed white material that sublimated over several days. A vast reservoir of water ice, estimated to hold a volume equivalent to Lake Superior, has been found buried in the Utopia Planitia region.
In the extremely thin and cold Martian atmosphere, a small amount of water exists as vapor, forming thin cirrus clouds and occasional fog. Low atmospheric pressure and cold temperatures prevent stable liquid water from existing on the surface; water would either freeze instantly or immediately sublime. However, transient liquid water exists as brines, or extremely salty water, which have a lower freezing point. These briny flows are suspected to cause the recurring slope lineae (RSL), which are dark streaks that appear seasonally on steep crater walls, although sustained liquid water is still not possible.
Why Mars Lost Its Water
The transition of Mars from a warm, wet world to a cold, arid one is attributed to the loss of its global magnetic field. Unlike Earth, Mars is a smaller planet, and its internal heat engine cooled quickly, causing the core’s churning motion (dynamo) to stop about four billion years ago. This cessation resulted in the collapse of the magnetosphere, the magnetic bubble that once shielded the planet.
Without this magnetic protection, the solar wind—a stream of charged particles constantly emanating from the sun—began to interact directly with the upper layers of the Martian atmosphere. The high-energy particles from the solar wind collided with atmospheric gases, stripping them away molecule by molecule in a process known as atmospheric escape. The MAVEN mission has confirmed that this process was highly effective over geological time in removing a significant portion of Mars’s early atmosphere.
The loss of a dense atmosphere caused a catastrophic drop in surface pressure and temperature. The remaining surface water could no longer be stable as a liquid, leading it to either freeze into the subsurface permafrost or sublime directly into space. This combination of atmospheric stripping and global freezing transformed Mars into the desiccated planet observed today.
Implications for Habitability and Human Exploration
The discovery of water, both historical and contemporary, is significant for two reasons: the search for life and the practicality of human missions. The evidence of ancient lakes and long-lived water cycles informs the search for past microbial life, suggesting that Mars was once a habitable environment. Current scientific efforts focus on subsurface regions where life might be preserved, or where pockets of liquid water could potentially exist away from the harsh surface radiation.
For future human exploration, the presence of vast reserves of water ice is a game-changer. This water is considered a crucial resource for In-Situ Resource Utilization (ISRU). Martian water ice can be melted and purified for drinking and agriculture, providing astronauts with a sustainable supply of life support. More importantly, water can be split through electrolysis into its component elements: hydrogen and oxygen. Oxygen can be used for breathable air, and the two elements together form high-performance rocket propellant, allowing for a more cost-effective return journey to Earth.