Mars has long captivated scientists with the question of whether it once hosted vast bodies of water, perhaps even a sprawling ocean. Scientific investigations over decades have transformed our understanding of this seemingly arid world. Evidence increasingly suggests that Mars was once a warmer, wetter environment, a stark contrast to its current cold and dusty state. This past presence of water hints at a potentially habitable history for the Red Planet.
Signs of Past Water
Decades of exploration reveal compelling geological and mineralogical evidence pointing to widespread liquid water on ancient Mars. Orbiter images show features resembling ancient riverbeds, deltas, and lakebeds, suggesting extensive surface water flow. The Mars Global Surveyor spacecraft found gullies formed by flowing water, along with debris and mud deposits. The Mars Orbiter Laser Altimeter (MOLA) revealed that a watershed for an ocean on Mars would cover three-quarters of the planet.
Rovers provide direct, on-the-ground confirmation of water’s past influence. Key findings include:
The Opportunity rover discovered hematite spherules, informally known as “blueberries,” mineral grains that typically form in the presence of water.
It also found sedimentary rocks with subtle ripples, similar to those found at the bottom of shallow, moving bodies of water on Earth.
The Spirit and Opportunity rovers found acid-sulfate salts, indicating that ancient briny seas evaporated.
The OMEGA instrument on Mars Express detected hydrated minerals like phyllosilicates and sulfates, which contain water in their crystalline structure and form through water-rock interactions.
A comprehensive map from data by ESA’s Mars Express and NASA’s Mars Reconnaissance Orbiter identified hundreds of thousands of aqueous mineral deposits, particularly in older regions of the planet’s surface.
The Hypothetical Martian Ocean
Building on the evidence for widespread water, scientists hypothesize a large, ancient ocean on Mars. This theorized primordial ocean, sometimes called Paleo-Ocean or Oceanus Borealis, existed in the northern lowlands, specifically the Vastitas Borealis basin. This region lies 4 to 5 kilometers below the planet’s average elevation, making it a natural basin for water accumulation.
This hypothetical ocean covered nearly a third of the Martian surface. Estimates suggest it could have held a volume of water equivalent to a global ocean 430 meters deep. This massive body of water existed during the Noachian period, roughly 4.1 to 3.8 billion years ago. Evidence supporting this ocean includes the unique distribution of crater types below 2400 meters elevation in the Vastitas Borealis, and features resembling ancient shorelines, such as the Arabia and Deuteronilus shorelines, stretching for thousands of kilometers. Recent in-situ exploration by China’s Zhurong rover also revealed surficial marine sedimentary rocks and subsurface sedimentary structures, supporting the existence of an ancient northern ocean.
Mars’ Water Disappearance
Mars’ transformation from a potentially wet world to its current arid state is primarily attributed to the loss of its protective magnetic field. Unlike Earth, Mars’ core cooled, causing its internal dynamo to cease operating between 3.7 and 4.2 billion years ago. This loss left the planet vulnerable to the solar wind, a continuous stream of charged particles from the Sun.
Without a global magnetic field, the solar wind began stripping away Mars’ atmosphere. The Mars Atmosphere and Volatile Evolution (MAVEN) mission has directly observed this process, identifying that the solar wind accelerates ions from Mars’ upper atmosphere into space. MAVEN data indicate that Mars is currently losing about 100 grams of gas per second, a rate that would have been significantly higher billions of years ago when the Sun was more active. This atmospheric stripping led to a dramatic drop in atmospheric pressure and temperature, causing much of the remaining surface water to either freeze or sublimate into the thin atmosphere.
Water on Mars Today
Although Mars is now a cold, dry desert, water still exists, primarily in frozen forms. The most visible reservoirs are the polar ice caps, containing both water ice and frozen carbon dioxide. If all the water ice in these caps melted, it could cover the entire planet to a depth of 30 meters (100 feet).
Beyond the polar regions, subsurface ice deposits, similar to permafrost on Earth, are present. A large underground body of water ice in Utopia Planitia holds a volume equivalent to Lake Superior. Ongoing research also examines recurring slope lineae (RSL), dark streaks that appear seasonally on Martian slopes. While initially thought to be caused by transient flows of briny water, their exact composition is debated, with some studies suggesting they relate to flowing sand and dust. In 2018, radar data from the Mars Express satellite suggested a subglacial lake about 1.5 kilometers beneath the southern polar ice cap, a finding supported by independent laser-altimeter measurements of the ice cap’s surface shape.