Mars, often called the “Red Planet,” has captivated scientists for centuries. Among its many intriguing features, none has stirred more excitement than the evidence suggesting the planet once hosted vast quantities of liquid water—rivers, lakes, and even possible oceans. While Mars today is an arid, frozen world with a tenuous atmosphere, a wide array of geological and mineralogical data points to a dramatically different past. Understanding where and how water once existed on Mars not only helps reconstruct the planet’s climatic history but also opens new avenues in the search for past life and future colonization.
Ancient River Valleys and Lake Basins
One of the most compelling lines of evidence for ancient water comes from the surface features captured in high resolution by orbiters like NASA’s Mars Reconnaissance Orbiter and ESA’s Mars Express. These include valley networks, which are tree-like branching channels found predominantly in Mars’ southern highlands. These formations strongly resemble river systems on Earth and suggest sustained surface runoff from rainfall or melting snow. Their complexity indicates long-term erosion by flowing water during the Noachian period, over 3.7 billion years ago, when Mars was warmer and wetter.
Another striking example is the presence of outflow channels—massive, carved landscapes that hint at catastrophic floods. These are often several hundred kilometers long and tens of kilometers wide, pointing to the sudden release of water, possibly from melting subterranean ice or ruptured aquifers. Channels like Kasei Valles and Ares Vallis are believed to have been formed in this way. Additionally, deltaic deposits provide some of the most visual and persuasive evidence of lakes. These fan-shaped sedimentary structures form where rivers empty into standing water. Jezero Crater, where NASA’s Perseverance rover now operates, displays such a delta, suggesting a long-lived lake once existed there.
Craters with both inlet and outlet valleys also indicate that water filled their interiors, creating lakes that persisted over geologically significant periods. One notable example is Gale Crater, the landing site of the Curiosity rover. There, sedimentary rock layers, mudstones, and cross-bedding patterns suggest water slowly deposited these materials in a stable lake environment. These features collectively present a powerful case for the presence of lakes and rivers on ancient Mars, shaped by both persistent and sudden hydrological processes.
Mineral Clues: Traces of Water in the Rocks
Supporting this visual evidence is the planet’s mineralogy. Remote sensing instruments aboard orbiters and rovers have identified a wide range of minerals that only form in the presence of water. Chief among these are clay minerals, or phyllosilicates, which require neutral-to-alkaline water for extended periods to form. Detected across the southern highlands and other regions, these clays are relics of Mars’ earliest and most habitable era. Their presence suggests that Mars was not just briefly wet, but sustained water over millions of years.
In addition to clays, sulfates like gypsum and kieserite have been detected by rovers such as Opportunity and orbiters like Mars Express. These minerals typically form in acidic, evaporative conditions—often when shallow lakes or seas dry up. At Meridiani Planum, Opportunity discovered layered sulfate-rich rocks and hematite spheres, indicative of long-term interaction with liquid water. Similarly, high-silica deposits found by the Spirit rover suggest former hydrothermal environments like hot springs, which are considered ideal habitats for microbial life on Earth. Carbonate minerals, while less common on Mars, have also been detected and suggest water with mild pH levels in certain regions, further supporting the theory of a habitable Mars.
Notably, various salts like perchlorates have been found in Martian soils. These salts can dramatically lower the freezing point of water, allowing liquid brines to potentially exist even under present-day Martian conditions. Such brines are believed to be responsible for recurring slope lineae (RSL)—dark streaks that appear seasonally on crater rims and hillsides. While the exact nature of RSL is debated, their seasonal behavior and connection to salt deposits make them candidates for modern transient water activity.
Water Beneath the Surface
Even more fascinating is the discovery of water beneath the surface. Using radar instruments like MARSIS on Mars Express and SHARAD on the Mars Reconnaissance Orbiter, scientists have detected strong radar reflections beneath Mars’ south polar ice cap. In 2018, researchers announced the likely presence of a subglacial lake beneath 1.5 kilometers of ice, made possible by the antifreeze effect of briny water. Although controversial and still under investigation, this finding suggests that Mars may still host stable liquid water underground.
Subsurface ice deposits have been confirmed by multiple missions. Mars Odyssey, MRO, and the Phoenix lander have detected large expanses of ice just below the surface, especially in mid-latitude regions. These frozen water reserves could one day be vital for human missions. Ice cliffs revealed by erosion also suggest relatively accessible water sources, further bolstering the view that Mars is not entirely dry.
Confirmation from Surface Rovers
Data from ground missions have provided critical confirmation of orbital observations. NASA’s Curiosity rover, exploring Gale Crater since 2012, has uncovered detailed sedimentary structures such as mudstones and finely layered rocks, which only form in calm lake environments. It also detected ancient streambeds and chemical markers for water-altered minerals like clays and sulfates. These signs point to a lake that may have lasted millions of years, with potentially habitable conditions.
NASA’s Perseverance rover, which landed in Jezero Crater in 2021, has continued this legacy. Jezero was chosen because of its prominent river delta and the likelihood it once held a large lake. Perseverance’s imagery of rounded pebbles, sedimentary layering, and the surrounding delta formations all reinforce the idea of sustained water flow. The rover is also collecting rock and soil samples that will eventually be returned to Earth, offering an unprecedented opportunity to analyze Martian geology for signs of ancient life.
Earlier rovers Spirit and Opportunity also made valuable contributions. Spirit found evidence of past hot springs and volcanic interaction with water in Gusev Crater, while Opportunity discovered widespread hematite spheres (“blueberries”) and sulfate layers, pointing to ancient water-rich environments. Both rovers vastly exceeded their life expectancy and played key roles in confirming that water once shaped Mars’ surface on a massive scale.
New Discoveries and Future Missions
More recent findings continue to push the boundaries of our understanding. The Mars Sample Return (MSR) mission planned by NASA and ESA is designed to retrieve samples collected by Perseverance and return them to Earth for detailed analysis. These could reveal biosignatures or chemical evidence of ancient life. China’s Tianwen-1 mission, which includes the Zhurong rover, is also contributing new data from the Utopia Planitia region, helping to build a more complete picture of Mars’ climate and hydrological history.
Advanced climate models are shedding light on how Mars could have supported liquid water billions of years ago. These simulations suggest that volcanic activity, greenhouse gas release, or impacts from large asteroids might have warmed the atmosphere temporarily, allowing rivers and lakes to form. Meanwhile, upcoming missions from countries like India and the UAE, as well as private agencies, aim to use radar and seismic tools to search for more hidden water beneath the surface.
Altogether, the evidence for rivers and lakes on Mars is overwhelming. From ancient valley networks and massive flood channels to mineral clues and subsurface ice, it is clear that Mars was once a much wetter and possibly habitable world. These findings reshape our understanding of planetary evolution and hold promise for future exploration. If water existed on Mars in significant amounts—and if it still exists underground—then the next question is inevitable: did life ever arise there? The answer may lie in the rocks and samples now being gathered, waiting for a future return to Earth.
As science progresses and technology improves, we move ever closer to unraveling the mystery of Mars’ watery past. Each new discovery not only enriches our knowledge of the Red Planet but also brings us a step closer to finding out whether we are truly alone in the universe.
