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Lucy Kissick, a first year DPhil in Earth Sciences

When the team behind NASA’s Mariner 9 mission first glimpsed the surface of Mars forty-five years ago, they were shocked to discover an entirely different planet to their predecessors’ observations. Mariners 4, 6, and 7 all by chance observed the same crater-scarred, moonlike highlands during their brief flybys, but the images of Mariner 9 revealed colossal volcanoes, dry river beds, flood-scoured planes, and great systems of apparently water-carved canyons.

Not quite H.G. Wells’ super-intelligent lifeforms or Giovanni Schiaparelli’s flowing canals, but to the budding field of planetary geology, Mars was alive again.

One of the most intriguing discoveries from this time is the eponymous Valles Marineris: a valley complex on the edge of a volcanic bulge that makes Earth’s Grand Canyon want to downgrade its name. At times over ten kilometres deep and as long as the mainland United States, Marineris opens into the largest known overland floodplains in the Solar System, leading some early researchers to suppose it once contained great lakes that burst their banks. Most of these megafloods, however, seem sourced from strange cracked pits with no Earthly analogue, appropriately termed chaos terrain. Incredibly, it seems the chaos formed when pressured groundwater explosively burst to the surface as geysers hundreds of metres high, which then flowed downstream.

It’s a beautiful image to contemplate: alien rivers turned blood-red by iron oxide sands, gushing down rust-coloured canyons under a rose-pink sky. But what could have caused such catastrophic outpourings?

It began with gradual cooling following the loss of the martian atmosphere some three and a half billion years ago. As the climate cooled, the depth at which water freezes extended far below the surface, trapping deeply buried liquid water between ice and the planet’s interior warmth. Any event that placed additional pressure on the system – a meteor strike perhaps, or a volcanic eruption – would have forced this water upwards through points of weakness, rupturing the surface and forming the high-pressure fountains.

But if this was a process happening all across Mars, why are the megafloods – and associated chaos terrain – almost exclusively near Marineris?

Perhaps pressure came from the build-up of the great volcanoes next door; but then we would expect floods all around their bases. Perhaps the weight of an ocean downstream was the trigger; but then we would expect floods all around the proposed shores of this (rather questionable) ocean. What we observe instead is flooding solely downgradient and radial of Marineris. In fact, we observe the most explosive events (chaos-forming) closest to the canyons and the most effusive (gentle upwelling) thousands of kilometres away as water-altered rocks. Put together, Marineris is the focal point of pressure for the megafloods. Something was happening in these canyons to place incredible pressure on the groundwater system. But what?

We already know groundwater reaches the surface through points of weakness. These are often craters, areas of softer sediments, or tectonic faults. As the longest and deepest fracture on not only Mars but the entire Solar System, Marineris is about as great a point of weakness as one could ask for. And as one might expect, we do indeed find evidence of water leaking from the ground: chaos terrain right in its centre, spring-sourced channels, a suite of water-altered minerals. But on a whole other scale to short-lived outburst floods, we also see features pointing to massive lakes or glaciers possibly once covering the entire system.

Was the first impression of the Mariner 9 scientists right? Could Marineris have held a lake that burst down already well-established floodplains? Lakes big and small have been proposed here for years, but have never gained traction for one fatal reason: the water’s source is always assumed to be rain or snowfall. With a canyon volume around three times that of the Greenland ice sheet, no climate models can account for such a fill.

Now consider such a lake but sourced instead by groundwater. This is the same groundwater that erupted as the megafloods; that is trapped across Mars beneath frozen rock; that carved channels connected to the opening of the canyons. Imagine the weight of this lake, the pressure it placed on the ground, and the pressure on the groundwater system. Imagine floods bursting radially downhill from Marineris as the over-pressured water erupted in surely the grandest waterworks the Solar System has ever seen.

If the weight of a lake in Valles Marineris three and a half billion years ago was indeed responsible for incepting the largest overland floods in the Solar System, then two long-standing mysteries – whether there were lakes, and what caused the floods – will have been solved with one hypothesis. One way to find out for sure would be to visit the canyons for ourselves as explorers do in National Geographic’s recent Mars docudrama, or to send a rover to drill deep into this proposed lake bed. Geophysical data from NASA’s InSight mission in 2018 could offer further information on crustal movement, but for now such a conclusion accounts for all recorded regional mineralogic, morphologic, and structural observations.

Since Mariner 9’s first glimpse at the floods forty-five years ago have their origins remained unresolved; perhaps this preliminary hypothesis can offer new perspective on so long-standing an issue.