Posted by In 1973, Carl Sagan published a short paper in the journal Icarus with a disarmingly casual idea. If you could darken the polar caps of Mars, perhaps by spreading dark material or plants over them, they would absorb more sunlight, release carbon dioxide, thicken the air and warm the planet further in a runaway feedback. He was careful to call it speculation. Fifty years later, the terraforming of Mars is still mostly speculation, but it’s a much better-informed kind, and the numbers have not been kind to the simplest versions of the dream.
The appeal is obvious. Mars has a day of 24 hours and 40 minutes, seasons, polar ice and a surface carved by ancient rivers and lakes. It once had liquid water at the surface, possibly for long stretches. Why not give it back?
What would actually need to change
Start with the air. Average surface pressure on Mars is around 6 millibars, roughly 0.6 percent of Earth’s at sea level. At that pressure, unprotected human blood would boil at body temperature; the threshold where that happens on Earth, called the Armstrong limit, is about ten times higher than Martian surface pressure. Liquid water can’t last long on most of the surface either. It freezes or evaporates, sometimes both in the same afternoon.
Temperatures average around minus 60 degrees Celsius. There’s no global magnetic field and only a thin blanket of gas, so ultraviolet light and charged particles hit the ground almost unfiltered. And the atmosphere that does exist is about 95 percent carbon dioxide, with only a trace of oxygen.
So the terraforming of Mars is really three separate projects stacked on top of each other: warm the planet, thicken the atmosphere enough for liquid water and unpressurized (but still masked) humans, and then, much later and much harder, make the air breathable. People often talk about them as one thing. They aren’t.
The carbon dioxide that isn’t there
The classic plan, Sagan’s included, relied on Mars having enough frozen and mineral-bound carbon dioxide to rebuild a thick atmosphere once someone tipped the climate. Release it and the planet warms itself.
In 2018, Bruce Jakosky and Christopher Edwards ran the inventory using data from orbiters including MAVEN and Mars Reconnaissance Orbiter. Their conclusion, published in Nature Astronomy, was deflating. Vaporizing all the carbon dioxide in the polar caps would only roughly double today’s pressure. Adding what is plausibly accessible in dusty soils and shallow rocks would still leave Mars far short of what’s needed to keep water liquid, let alone to let a person walk around in a shirt and oxygen mask. Most of the planet’s ancient atmosphere, they argued, has been lost to space or locked deep in rocks where it can’t easily be retrieved.
That loss is still happening. MAVEN, which arrived in 2014, measured the solar wind stripping gas from the upper atmosphere at something like 100 grams per second in calm conditions, with spikes during solar storms. It’s slow on human timescales. Over hundreds of millions of years, it helped turn a wetter world into a desert.
Elon Musk’s 2015 suggestion of detonating nuclear weapons over the poles to release carbon dioxide runs into the same wall. You can’t release what isn’t there.
Newer ideas: warming without a thick atmosphere
If the gas doesn’t exist, maybe you don’t need it. Researchers have started looking for other ways to trap heat.
In 2019, Robin Wordsworth, Laura Kerber and Charles Cockell proposed a local fix: cover patches of ground with a layer of silica aerogel two or three centimeters thick. Their models suggested it could let sunlight through, block harmful ultraviolet, and raise temperatures beneath it by around 50 degrees, enough to melt ice in some mid-latitude regions. It’s not terraforming in the planetary sense, more like a giant passive greenhouse roof.
A bolder 2024 study led by Samaneh Ansari and Edwin Kite suggested releasing engineered nanoparticles, tiny rods made from iron or aluminum available in Martian dust, into the atmosphere. In their simulations, these particles trap heat thousands of times more effectively per kilogram than the artificial greenhouse gases proposed before, and could warm the planet by tens of degrees within decades. The authors were clear about the unknowns: how long the particles stay aloft, how they interact with clouds and dust, and what the warming would do to the existing ice. Warmer is not the same as habitable. It also does nothing, on its own, to raise the pressure.
Keeping an atmosphere once you’ve made one
Suppose someone manages to warm Mars and thicken its air. The new atmosphere would leak, just as the old one did, though over geological rather than human timescales. In 2017, NASA’s Jim Green and colleagues presented a concept of placing a powerful magnetic shield at the Mars-Sun L1 point, to deflect the solar wind and protect the planet like an artificial magnetosphere. Their models suggested atmospheric loss would drop sharply. It was a conceptual study for a workshop, not a mission plan, and the engineering would be beyond anything ever flown.
Oxygen: the slowest step
Breathable air is the part that tends to quietly disappear from documentaries. Earth’s oxygen accumulated over billions of years of photosynthesis. Planetary scientist Chris McKay and colleagues estimated decades ago that warming Mars could, in principle, take a century or so, while turning a thickened carbon dioxide atmosphere into one rich in oxygen using plants and microbes might take on the order of a hundred thousand years. Machines could speed that up, but the energy needed to split that much carbon dioxide is staggering.
That’s the gap between a Mars where hardy, engineered microbes might survive in the open and a Mars where people take off their helmets. The first is a stretch. The second is so far off that most serious researchers treat it as a thought experiment.
Domes, tents and the less romantic alternative
There’s a middle path, sometimes called paraterraforming: instead of reworking an entire planet, you create Earth-like conditions inside enclosed volumes. Pressurized domes, buried habitats, aerogel-roofed valleys. It isn’t as cinematic as a blue Mars with lakes and forests, but it scales from a single greenhouse up to a town, and every step can be tested before the next.
For any foreseeable settlement, this is the realistic approach. People would adapt to Mars far more than Mars adapts to them.
Should we even try?
There’s one more complication, and it may matter more than the physics. If Mars still harbours native microbes, perhaps deep underground or in salty brines, transforming its climate could wipe out the only independent example of life we’d ever have found. Even if Mars is sterile, its rocks preserve a record of the early solar system that a global thaw could scramble. Planetary protection rules exist to stop us contaminating Mars with a few spacecraft. A deliberate terraforming project would be contamination on purpose, at planetary scale.
Some philosophers argue a lifeless Mars has no interests to protect. Others, including Sagan himself, felt that if there is life on Mars, Mars belongs to the Martians, even if they’re microbes. Nobody has to resolve that today. Nobody can resolve the engineering either.
For now, the terraforming of Mars is a useful way of asking hard questions about climate, atmospheres and what makes a world habitable at all. If those questions grab you, SETIworld has plenty more on Martian water, atmospheric loss and the search for life there. Come explore, and bring your own verdict on Sagan’s dark polar caps.
