A spacecraft slowly descends to the surface of Mars. Once arid and lifeless, the Red Planet now features a lush, green landscape. Passengers watch as a city comes into view. People walk along busy streets, venture into the local park and breathe the Martian air.

Many science fiction writers have envisioned futures like this for Mars. In these stories, humans use so-called terraforming technology to make other planets more Earthlike.  

Anyone living on a terraformed Mars should “be able to walk around outside without a spacesuit,” says ​​Paul Byrne. At Washington University in St. Louis, Mo., Byrne studies how planets form. To be hospitable, Mars would need an atmosphere thick enough to retain heat. And there would need to be enough oxygen for life.  

Bringing the heat 

In Earth’s atmosphere, greenhouse gases such as carbon dioxide, methane and water vapor trap the sun’s heat. This helps keep our planet warm enough for water to exist as a liquid — which all life needs.

Most of Mars’ atmosphere is made up of carbon dioxide, or CO₂. But Mars doesn’t have enough CO₂ to trap heat. In fact, the Red Planet’s atmosphere ​is ​100 times thinner than our own. Being less dense than Earth and half its size, Mars has weaker gravity than our home planet, says Byrne. That makes it “harder for it to hang on to an atmosphere.”

Without a heat-absorbing atmosphere, Mars sees temperatures as low as –153° Celsius (–225° Fahrenheit). And Mars’ extremely low air pressure lowers the boiling point of water. Any liquid water quickly evaporates or freezes, depending on the temperature.

Jumpstarting the greenhouse effect would be key to increasing Mars’ temperature and air pressure, says Byrne. But future Martians would first need a way to produce enough CO₂ to fill an entire atmosphere. 

Shaking things up 

In the past, some researchers have suggested extracting CO₂ from Mars itself. Engineers would create the gas from carbon and oxygen found in Martian minerals. Or they might release CO₂ trapped in Mars’ polar ice caps or below the surface.  

But observations of Mars suggest this likely wouldn’t work. “There probably isn’t enough [CO₂] to make an atmosphere even close to what we would need,” says Byrne. 

In 2018, researchers publishing in Nature Astronomy used spacecraft observations to estimate how much CO₂ is locked away in Martian minerals and ice caps. The team found that these sources don’t contain enough CO₂ to create a significant greenhouse effect. The entire planet would only produce enough CO₂ to thicken the atmosphere to about 7 percent of Earth’s.

Other scientists suggest triggering volcanic eruptions to pump heat-trapping CO₂ into Mars’ atmosphere. Volcanoes can help planets replenish atmospheres lost to space, says Byrne. Researchers suspect this is how Venus maintains its CO₂-rich atmosphere.

The Red Planet once had active volcanoes, which may have spewed carbon dioxide a few billion years ago. “Most of the activity ended a very long time ago,” says Byrne.

Future civilizations might try to redirect asteroids to create these eruptions. Humankind has already inched toward that feat, says ​​Sara Seager. This astrophysicist studies planets around other stars at the Massachusetts Institute of Technology in Cambridge.  

In 2022, NASA’s DART spacecraft successfully bumped the asteroid Dimorphos closer to the larger rock it orbits. “With asteroid orbits, you just have to nudge it a little bit,” says Seager. But such impacts probably wouldn’t be too useful for triggering volcanic eruptions on Mars.  

​​You’d likely need to whip a lot of space rocks at Mars to release enough CO₂ for an atmosphere, says Byrne. And the sheer speed of the incoming asteroids would lead to “catastrophically damaging impacts.”  

Just breathe 

Let’s say future engineers do figure out some way to warm and thicken up Mars’ atmosphere with a heavy helping of CO₂. Once there’s enough CO₂ in the air, Mars colonists would want to start tweaking it to resemble ours. “We [would] need to have enough free oxygen that we can breathe,” says Byrne. Too much, though, could be toxic. (Free oxygen is a form of this element that is not chemically bonded to any others.)

Free oxygen makes up about 21 percent of the air we breathe. The rest is mostly nitrogen with a smattering of other gases. Engineers would want to replicate this blend, says Byrne.

Oxygen-producing microbes could help, says Seager. Research suggests cyanobacteria kicked off a rise in free oxygen in our atmosphere a little over 2 billion years ago. Scientists could tweak the genes of these microbes to help them better withstand Mars’ freezing temps and intense levels of radiation. ​​​ 

Through photosynthesis, these tiny workers could then take in CO₂ and pump breathable oxygen into Mars’ atmosphere. And using microbes might offer a safer method of oxygen production than relying on more complex machines. With machines, says Seager, “if one little thing goes wrong, we’re all dead.” 

No place like home 

Creating a stable atmosphere is only one of many hurdles engineers would face in terraforming Mars. Without a magnetic field to shield it, Mars gets blasted with deadly amounts of radiation from outer space​​​ and the sun​. The dusty, pulverized rock covering the planet’s surface contains toxic salts called perchlorates. And since Mars has only about a third of Earth’s gravity, anyone living there would risk their muscles and bones weakening, says Byrne.

Humans can’t reshape Mars with today’s tech. Space agencies around the world are still working toward getting the first astronauts to the Red Planet. NASA aims to achieve this as early as the ​​2030s.

That makes it tough to estimate when humans might be able to terraform a planet like Mars, says Byrne. Terraforming tech could take anywhere from a few hundred to several thousands of years to perfect, he says. “Certainly nothing remotely in our time.”

Such tech would have to be reliable enough to protect human lives. Even on Earth, humans can’t survive extremes such as the cold of Antarctica or the intense pressures of our oceans without life support. Small malfunctions to terraforming tech could be catastrophic. “We’re just so fragile,” says Seager. “That’s why the whole terraforming question is so challenging.”