Posted by Water leaves the same signatures everywhere. It cuts branching valleys into high ground, it dumps sediment where a current slows into a standing body, it sorts pebbles by size and rounds their edges, and it leaves minerals behind that only form in its presence. All of those signatures show up across the Mars landscape, which is why the argument about ancient Martian water has shifted over the past thirty years from whether there was any to how much, for how long, and whether it stayed liquid on the surface or spent most of its history frozen.
The branching valleys nobody expected
Mariner 9 spotted channels in 1971 and the interpretation has been contested ever since, but the valley networks in the southern highlands are the least ambiguous evidence there is. They branch upstream like terrestrial drainage basins, they run downhill consistently, and they concentrate in terrain dated to the late Noachian, around 3.7 billion years ago. Something wet, flowing and reasonably persistent carved them.
The outflow channels are a different animal entirely. Kasei Valles, Ares Vallis and their relatives are enormous — tens of kilometres wide, scoured with teardrop-shaped islands — and they appear to have been cut in weeks or months rather than millennia, by catastrophic releases of groundwater under pressure. A flood like that does not require a warm climate. It requires a confined aquifer and a crack.
Those two landform families tell different stories about the same planet, and reconciling them is still an active argument in the literature.
Standing water: the deltas
A delta is the clearest thing a geologist can ask for. Sediment builds outward into a lake or sea and stops where the water surface was, which fixes both the existence and the level of a body of standing water.
Mars has dozens of them. Eberswalde crater, imaged by Mars Global Surveyor in the early 2000s, holds a fan with meander scars so tidy it could be a textbook figure. Jezero crater holds another, and that is why Perseverance was sent there on 18 February 2021. The rover’s first year of imaging showed the delta front layered exactly as a river-fed deposit should be, with a topping of boulders up to a metre across that only violent floods could have carried out that far — evidence of a climate that ran episodically rather than gently.
Gale crater, three thousand kilometres away, holds the other well-studied lake. Curiosity has driven up through hundreds of metres of section since 2012, reading a sequence of lake muds, river sands and later drying-out deposits. The estimated durations are the interesting part: not a flash flood, but a lake system that plausibly persisted for millions of years.
Surveys of orbital imagery have catalogued hundreds of former crater lakes, and a useful subset are open basins: craters with an inlet valley on one side and an outlet breach on the other, meaning water filled them to the rim and spilled over. That detail matters more than it sounds. A closed basin can be filled by a single wet spell; a chain of basins that overflowed into one another needs a hydrological system with a steady supply, whether from rain, melting snowpack or groundwater pushing up from below.
An ocean in the north, maybe
Then there is the big claim. In the late 1980s, Timothy Parker and Victor Baker traced what they read as ancient shorelines around the northern lowlands and proposed a hemispheric ocean, sometimes called Oceanus Borealis.
The idea took a hard hit when laser altimetry from Mars Global Surveyor showed that the proposed shorelines wander thousands of metres in elevation, which no real shoreline does. It came partly back in 2007, when Taylor Perron and colleagues showed that a shift in the planet’s spin axis — true polar wander — would deform an originally level shoreline in roughly the observed pattern. Not proof. A plausible rescue.
The independent line of evidence comes from isotopes. Water containing deuterium escapes to space more slowly than ordinary water, so a planet that has lost most of its ocean ends up enriched in the heavy stuff. Measurements published by Geronimo Villanueva’s team in 2015, using ground-based infrared telescopes, found Martian water roughly six times more deuterium-rich than Earth’s, implying an early inventory large enough to have covered a substantial fraction of the planet to a depth of tens of metres or more. Where that water ended up — space, ice, or bound into minerals — is split between all three in most models.
Anyone who tells you the northern ocean is settled either way is ahead of the evidence.
What the rovers actually touched
Orbital geology is inference. The ground truth came from wheels.
Opportunity landed at Meridiani Planum in January 2004 and almost immediately found rock with fine cross-lamination formed by water flowing in ripples, studded with hematite concretions the team nicknamed blueberries. The sulfate chemistry pointed to shallow, acidic, salty water — habitable in the loosest sense, hostile by most standards.
Curiosity’s mudstones at Yellowknife Bay in 2013 were the opposite: fine sediment from calm, roughly neutral water, carrying the elements biology needs. Perseverance has been sampling the Jezero delta and its margin, and its SHERLOC instrument detected organic molecules in sedimentary rock there in 2022 — organics being a necessary ingredient rather than evidence of life, since meteorites deliver them and geochemistry makes them without any help.
Those samples are sealed in metal tubes waiting on a return mission whose cost and schedule have been rewritten repeatedly. Nothing on Mars gets confirmed as biological until a laboratory on Earth has the rock.
The water that is still there
The modern Mars landscape is not dry so much as frozen. Both polar caps are largely water ice under a seasonal carbon dioxide frost, radar has traced buried glaciers through the mid-latitudes, and in 2018 a survey found cliffs where thick, relatively clean ice sits exposed in cross-section a few metres below the dust.
Liquid is harder. In 2018 the MARSIS radar aboard Mars Express returned a bright reflection beneath the southern polar cap that its team interpreted as a body of brine. Follow-up work has argued that layered clays, or certain frozen materials, could produce the same radar signature without any liquid at all, and the question has not closed.
Salts help either way. Perchlorates, which are common in the soil, drop water’s freezing point sharply, so any liquid surviving underground today would be cold, dense brine rather than anything a fish would recognize. The dark streaks on crater slopes that were once presented as seasonal flows now look, on closer study, more like dry sand avalanching.
Why the wet chapters matter
Habitability is a claim about environments, and lakes and river deltas are the best environments Mars is known to have offered: liquid water, dissolved minerals, energy gradients, and fine sediment that buries and preserves whatever falls into it. On Earth, that combination is exactly where the oldest traces of microbial life are found.
Whether Mars ever took the step from chemistry to biology is unknown, and it may stay unknown for a while. What has changed is that the question now has addresses attached — specific craters, specific layers, specific sample tubes sitting on a cold plain waiting for a ride. SETIworld keeps following the missions, the sample return arguments and the contested detections that will eventually decide it.
