Mars, the fourth planet from the Sun, has long captivated scientists and dreamers alike. Once considered a potential abode for life, it is now a barren, dusty world with a thin atmosphere. However, mounting evidence suggests that Mars was once a much wetter planet, with flowing rivers, vast lakes, and possibly even a northern ocean. Today, the remnants of these ancient waterways exist as “ghost rivers” and “hidden lakes”—dry valleys, sediment deposits, and subterranean reservoirs that hint at a lost aquatic past.
Early Observations and Hypotheses
For centuries, astronomers speculated about water on Mars. In the 19th century, telescopic observations revealed dark patches that changed with the seasons, leading some (like Percival Lowell) to propose the existence of canals built by an intelligent civilization. While these ideas were later debunked, they set the stage for serious scientific inquiry.
By the mid-20th century, spacecraft missions began to reveal Mars’ true nature. Mariner 4 (1965) showed a cratered, Moon-like surface, but later missions like Mariner 9 (1971) and the Viking orbiters (1976) found dried-up river valleys, outflow channels, and sedimentary layers—clear signs of past water activity.
Present Scientific Data on Mars’ Ghost Rivers and Hidden Lakes
Mars shows clear geological evidence of a watery past, with data from orbiters and rovers revealing ancient river valleys, dry lakebeds, and possible underground water reservoirs. Modern missions have collected spectral, radar, and geomorphological data that support the theory of a once warmer, wetter Mars. This section explores the key scientific findings that help us understand the planet’s hydrological history.
One of the strongest pieces of evidence for past water on Mars comes from spectral analysis of hydrated minerals. Instruments like the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on the Mars Reconnaissance Orbiter (MRO) have detected phyllosilicates (clays), which form in the presence of neutral-pH water over long periods. These clays, including smectite and kaolinite, have been found in regions like Mawrth Vallis and Jezero Crater, suggesting stable liquid water existed there billions of years ago. Additionally, sulfates such as gypsum and jarosite—discovered in Meridiani Planum by the Opportunity rover—indicate acidic, evaporative environments, possibly from drying lakes or hot springs. Other minerals, like carbonates in Leighton Crater and chloride salts in Terra Sirenum, further support the idea that Mars once had widespread liquid water that later evaporated or froze.
The planet’s surface also bears the scars of ancient rivers and catastrophic floods. Valley networks, such as those in Nirgal Vallis, display dendritic (branching) patterns similar to Earth’s river systems, hinting at rainfall or snowmelt in the Noachian period (3.7–4.1 billion years ago). Meanwhile, massive outflow channels like Kasei Valles and Ares Vallis suggest sudden, enormous floods, possibly caused by underground aquifers bursting or glaciers melting rapidly. Some of these features, like the inverted channels in Eberswalde Crater, have been preserved because erosion-resistant sediment filled the original riverbeds, leaving behind raised ridges.
Perhaps the most compelling evidence for standing water comes from paleolakes and deltas. Jezero Crater, now being explored by the Perseverance rover, contains a well-preserved delta structure, confirming that it once held a long-lived lake. The rover has also found carbonate-rich rocks, which on Earth often form in microbial-rich waters, raising the possibility of ancient Martian life. Similarly, the Curiosity rover in Gale Crater has studied lakebed mudstones containing organic molecules, such as thiophenes and benzene, which could be remnants of past biological activity—though non-biological origins are also possible.
While Mars’ surface is now dry, recent radar discoveries suggest that liquid water might still exist underground. The Mars Express orbiter’s MARSIS instrument detected a possible subglacial lake beneath the south polar ice cap in 2018. This lake, buried 1.5 kilometers deep, is estimated to be 20 kilometers wide and remains liquid due to dissolved salts that lower its freezing point. Follow-up studies have found additional bright radar reflections, hinting at more subsurface brine pockets. However, some scientists argue that these signals could also come from clay deposits or highly conductive ice, so further research is needed to confirm the presence of liquid water.
Isotopic studies of Mars’ atmosphere provide more clues about its water history. The deuterium-to-hydrogen (D/H) ratio in the Martian atmosphere is five times higher than Earth’s, indicating that Mars lost much of its water to space over billions of years. However, some ancient clays have a lower D/H ratio, suggesting that water loss was less severe in Mars’ early history. Additionally, the Curiosity rover has detected seasonal methane spikes in Gale Crater, which could be produced by subsurface microbial life or geological processes—though no definitive link to biology has been proven.
Future missions aim to uncover more about Mars’ water and potential for life. The ESA’s ExoMars rover, set to launch in 2028, will drill two meters below the surface to search for organic molecules. NASA’s Mars Sample Return mission (planned for the 2030s) will bring back rocks from Jezero Crater for detailed analysis on Earth. There are also proposals for an orbital ice-mapping mission to locate accessible water sources for future human explorers.
In conclusion, multiple lines of evidence—spectral, geomorphological, radar, and isotopic—confirm that Mars was once a much wetter world with rivers, lakes, and possibly even a northern ocean. While its surface is now arid, the discovery of subsurface brines keeps open the possibility of liquid water—and maybe even life—existing today. The next decade of exploration, including deep drilling and sample return missions, will be crucial in answering these enduring questions.
