Today, Mars is a dry, dead, desolate world. The distinctive rust-coloured surface stretches out across the landscape. Overhead, the sky is dyed butterscotch by dust suspended in the thin carbon dioxide atmosphere that barely clings to the planet.
Things were once very different on the Red Planet though. Rivers, lakes and perhaps even oceans of liquid water used to cover the surface of Mars. Wrapped around this wet world was a thick atmosphere, a cocooning blanket keeping the planet insulated.
So, where did it all go?
What happened to Mars’s missing water and carbon atmosphere has long been a question for those studying Mars. But the answer may have been finally found this past year. A pair of studies suggest both retreated deep beneath Mars's surface.
Most intriguingly, this means both the water and the carbon are still on Mars. A future mission to the surface may be able to access them.
Mars’s wet and windy past
Centuries before the first space mission to Mars, people speculated about what lay on the surface. Writers imagined a landscape covered with dense jungles of red-leaved foliage and vast alien civilisations.
NASA’s Mariner 4 put those ideas to bed in 1964 when it flew past the planet for the first time. Its images revealed Mars to be a cold, dead world.
Over time, new missions sent back higher resolution images of the planet. These revealed deep canyons and fan-shaped deltas, like those carved by rivers here on Earth.
In 1997, the Pathfinder mission touched down in a field strewn with boulders apparently carried there by an ancient flood.
Finally, in the early 2000s, the Spirit and Opportunity rovers discovered sedimentary rocks and other minerals which can only be created in the presence of liquid water. This finally confirmed that yes – water once flowed across the surface of Mars.
Meanwhile, orbital spacecraft examined the planet’s tenuous atmosphere, revealing an overabundance of heavy isotopes. As lighter isotopes are more likely to escape into space, this strongly indicates the planet has lost much of its atmosphere.
Even more intriguingly, when they measured the relative abundance of carbon to krypton – an unreactive gas that’s remained largely unchanged over Mars’s 4.6-billion-year history – they found Mars has around a tenth the carbon of Earth or Venus.
Presumably, all three rocky planets started out with roughly the same primordial atmosphere. If so, this would mean Mars’s atmosphere has lost at least 90 per cent of its carbon.
This all indicated that once Mars was covered in water and possessed a carbon rich atmosphere. Then, around 3.6 billion years ago, both vanished.
Mars’s water and atmosphere make their escape
Initially, the leading theory was Mars’s missing atmosphere and water were simply lost to space. On Earth, tectonic activity and our magnetic field keep the atmosphere in check.
Tectonic activity helps regulate our atmosphere, as shifting plates and volcanic activity bring new rocks to the surface which can interact and absorb water and the atmosphere, or drag old rocks back into the mantel. The magnetic field, meanwhile, helps guide the Sun’s solar wind around our planet.
Mars has no plate tectonics and only a very weak magnetic field. There’s no volcanism replenishing the gases, and the solar wind is free to strip away the planet’s atmosphere and any water vapour along with it.
Less atmosphere means a lower surface pressure, which means water evaporates more readily, allowing more and more water to escape.
When researchers ran simulations to examine how the planet’s climate evolved over time, it quickly became apparent that atmospheric loss couldn’t be the only thing at play.
It could only account for a fraction of Mars’s missing water, and didn’t explain why the carbon levels specifically had fallen so dramatically.
So where did the rest of Mars’s water and atmosphere go?
Mars’s missing water revealed
In August 2024, a team of geophysicists made a remarkable announcement – they’d found Mars’s missing water, hidden deep beneath the ground for over three billion years.
The team used data from NASA’s InSight lander, which spent four years from 2018 to 2022 detecting the tiny vibrations of Martian seismic activity, or ‘marsquakes’.
As seismic waves travel through a planet, they are subtly changed by the density and structure of the rock they pass through.
The team were able to pick out and analyse these changes. Putting them together, they built a picture of what the planet looks like under the surface.
What they found was enough water to flood Mars’s surface to a depth between 1 and 2 km( about 1 mile). The water was around 11.5 to 20km (7 to 13 miles) underground, trapped between tiny cracks and pores in the rock.
“Establishing that there is a big reservoir of liquid water provides some window into what the climate was like or could be like,” says Michael Manga from University of California, Berkeley who took part in the study.
“And water is necessary for life as we know it. I don’t see why [the underground reservoir] is not a habitable environment.
It’s certainly true on Earth – deep, deep mines host life, the bottom of the ocean hosts life. We haven’t found any evidence for life on Mars, but at least we have identified a place that should, in principle, be able to sustain life.”
Finding the lost Martian atmosphere
Only a month later, another team of planetary scientists from MIT revealed they may have tracked down Mars’s missing atmosphere, locating it, once again, hidden underground.
Initally, the team weren’t even investigating Mars, but our own planet. They were looking at a type of clay mineral called smectites.
The grains of smectites are covered with tiny folds resembling the side of an accordion, which trap and retain carbon dioxide and methane molecules for billions of years (unfortunately the process occurs on much longer timescales than would be helpful against the current climate crisis).
By chance, one team member, Oliver Jagoutz, happened to look at a map of Mars. He realised the surface was covered in smectite clay. Could they have captured Mars’s missing atmosphere?
“We know this process happens, and it is well-documented on Earth. And these rocks and clays exist on Mars,” says Jagoutz. “So, we wanted to try and connect the dots.”
Tectonic activity creates smectites on Earth. That can’t be the case on Mars, so the team looked into alternative ways the Martian smectites could form.
The answer seems to be water dripping through Mars’s crust and reacting with the rock.
The Martian crust is mostly igneous rock – solidified lava – and is rich in olivine, the mineral that comprises peridot gemstones.
Olivine is rich in iron, which reacts with the oxygen in water molecules. This creates iron oxide (i.e. rust, which gives Mars its red colour) but also releases hydrogen. This in turn bonds with carbon dioxide dissolved in the water, creating methane.
Over time, the process would transform the olivine into smectite, which then traps the methane.
The study found that if Mars were covered in a 1,100m (0.7 mile) deep layer of smectites, it would be able to store 1.7 bar of carbon dioxide – around 80 per cent of Mars’s missing atmosphere.
“We find that estimates of global clay volumes on Mars are consistent with a significant fraction of Mars’ initial carbon dioxide being sequestered as organic compounds within the clay-rich crust,” says Joshua Murray, who led the study.
“In some ways, Mars’ missing atmosphere could be hiding in plain sight.”
Could future Martian explorers access Mars’s missing atmosphere and water?
It’s no secret there are people who want to mount a human mission to Mars. Both NASA and the Chinese space agency have ambitions to set a human on the Martian surface within the next few decades. Both of these deposits could be a useful resource on such a mission.
The water trapped underground has great scientific potential. Studying it would allow geologists insight into how that water shaped Mars, while astrobiologists could investigate it as a potential haven for Martian life.
It could also be a vital resource for humans to drink, cooling and even being used to make rocket fuel.
Alas, this potential will almost certainly remain unrealised.
The water is as much as 20 km (13 miles) below the surface, jammed into tiny cracks in the rock.
While it may be possible to find a small amount of water closer to the surface for scientific study, extracting it in bulk from that depth would be challenging on Earth, let alone another planet.
The methane trapped in the smectites, however, is much closer to the surface. It could be possible that future Martian explorers could one day mine the clay of Mars, and use the methane trapped there as fuel.
It might not be too long, then, before at least some of Mars’s missing atmosphere and water are returned to the surface.
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