The question of water on Mars has driven planetary science for generations, stemming from a curiosity about the Red Planet’s ability to harbor life. Unraveling the story of Martian water, from its ancient past to its current frozen state, is a primary focus of modern space missions. The scientific journey to find this water reveals a planet that was once much more like Earth.
Evidence of Mars’s Ancient Watery Past
Billions of years ago, Mars was a warmer and wetter world. Orbiters have captured images of vast, dried-up river valleys and enormous outflow channels, some hundreds of kilometers wide. These features suggest that cataclysmic floods once carved their way across the Martian landscape. In other areas, intricate valley networks resemble the river systems on Earth, pointing to more persistent, long-term water flow.
More compelling evidence comes from fan-shaped deposits of sediment, known as deltas, found within ancient craters. Features like the delta in Jezero Crater, explored by the Perseverance rover, are strong indicators of where rivers once emptied into large bodies of standing water, such as lakes. The existence of a delta requires a stable body of water over a long period to allow sediments to accumulate, confirming a past Martian water cycle.
On the ground, rovers have provided definitive proof by analyzing the planet’s rocks. Missions like Opportunity, Curiosity, and Perseverance have discovered minerals that form in the presence of water. These include clay minerals (phyllosilicates), which form from the prolonged interaction of rock and water, and sulfates like gypsum, which are left behind as water evaporates. The discovery of these hydrated minerals in ancient lakebeds confirms that liquid water was a significant agent of geological change on Mars.
Where Water Exists on Mars Today
While liquid water is not stable on the Martian surface today, a substantial amount of water remains in various forms. The most visible reservoirs are the polar ice caps. Both the north and south poles are covered by permanent caps of water ice, though they are also topped with a seasonal layer of frozen carbon dioxide, or “dry ice.” This seasonal layer expands and retreats with the Martian seasons. The northern polar cap alone contains an estimated 1.6 million cubic kilometers of ice, enough to cover the entire planet in a shallow sea if melted.
A significant portion of Mars’s water is hidden from view, buried beneath the surface. In the planet’s mid-latitudes, vast sheets of subsurface water ice have been detected. This ice exists as thick, extensive layers, sometimes hundreds of meters deep, covered by a protective layer of rock and dust that prevents it from sublimating into the atmosphere. These deposits, likely formed by snowfall during past climate cycles, represent a massive and accessible reservoir of frozen water.
Water is also present in other, less voluminous forms. The thin Martian atmosphere contains a trace amount of water vapor, which moves between the poles and the equator depending on the season. Additionally, water molecules are chemically bound within the crystal structure of minerals in the Martian soil, or regolith. Rovers have detected these hydrated minerals across the planet, indicating that water is a widespread component of the Martian surface environment.
The Tools of Discovery
Our understanding of Martian water is the result of instruments on orbiters, landers, and rovers. Orbiters provide a global perspective. The Mars Reconnaissance Orbiter (MRO) carries a camera called the High Resolution Imaging Science Experiment (HiRISE). This camera can capture images detailed enough to identify individual boulders and fine layering in rock, revealing geological features carved by ancient water.
To understand surface composition, scientists use spectrometers like the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). This instrument analyzes light reflected from the planet’s surface to create mineral maps, pinpointing the locations of water-bearing minerals like clays and sulfates. To see beneath the surface, orbiters use ground-penetrating radar instruments, such as SHARAD on the MRO. This radar sends radio waves through the ground to detect and map buried ice sheets.
Robotic explorers on the surface provide ground truth for these orbital observations. Landers like Phoenix were equipped with robotic arms that could dig trenches and scoop up soil samples to be analyzed for water ice. Rovers such as Curiosity and Perseverance use drills to collect powder from the interior of rocks. These samples are then delivered to onboard chemistry labs to analyze for minerals that formed in water.
Implications for Exploration and Astrobiology
The discovery of widespread water on Mars has implications for the future of space exploration. The ability to use local resources, a concept known as In-Situ Resource Utilization (ISRU), is part of planning for human missions. Water ice can be mined and melted for drinking and for growing plants. It can also be split into its elements, hydrogen and oxygen, through electrolysis to provide breathable air and rocket propellant, which could fuel a return trip to Earth.
From a scientific standpoint, the presence of water is linked to the search for extraterrestrial life. Because all known life depends on liquid water, the guiding strategy for Martian astrobiology has been to “follow the water.” The evidence of ancient lakes and rivers points to environments that may have been habitable for microbial life billions of years ago. These water-rich locations are now primary targets in the search for biosignatures—the chemical or fossilized traces that past life may have left behind.
Finding water today, even in a frozen or mineral-bound state, provides clues about where to look. While the current surface is hostile, life could have retreated to subsurface niches where it might persist in a dormant state. The existence of subsurface water, whether frozen in ice sheets or as deep underground liquid reservoirs, identifies locations where evidence of past or present life could one day be found.