Elon Musk is not giving up on his dream of transforming Mars into humanity’s second home. The SpaceX CEO has been debating over Twitter this week about whether the red planet has the right resources to support an Earth-like atmosphere. A new study claims there isn’t enough carbon dioxide available to support the plan, but Musk points to evidence that humans could release sufficient gases from the Martian soil using the right technology.

The debate started on Monday, when a study published in the journal Nature Astronomy explored two approaches to terraforming Mars. The first is to create an atmosphere sufficient for supporting liquid water on Mars’ surface, meaning humans could freely walk around and breathe the air, while the second involves raising the planet’s atmospheric pressure so humans will only need to use a small breathing apparatus rather than the bulky spacesuits of today. The researchers, Bruce Jakosky of the University of Colorado Boulder and Christopher S. Edwards of Northern Arizona University, found that the planet doesn’t have enough carbon dioxide to support either plan, as Mars only holds enough carbon dioxide to create around 15 millibars of atmospheric pressure. Destroying Martian sedimentary rocks would only release around 12 millibars. For comparison, Earth’s atmosphere reaches 1,000 millibars at sea level.

Musk disputed the findings, which were reported in Discover Magazine.

Musk’s goal is to transform humanity into a “multi-planetary species.” While SpaceX plans to send humans to Mars as soon as 2024, the long-term goal is to set up a permanent Mars base that could eventually span out into a city. His June 2017 article in the journal New Space claimed Mars is a better candidate for colonization than the moon because it has an atmosphere, more natural resources, and a 24.5-hour day compared to the moon’s 28-day day. At the International Astronautical Congress in Adelaide, Australia, last September, Musk showed an artist’s impression of how this city may look.

A Mars city.

This is just the beginning, though. Musk wants to move away from small extraterrestrial bases and instead toward humans as a star-hopping species. In June 2017, he wrote that “there are really two fundamental paths … One path is we stay on Earth forever, and then there will be some eventual extinction event. I do not have an immediate doomsday prophecy, but eventually, history suggests, there will be some doomsday event. The alternative is to become a space-bearing civilization and a multi-planetary species, which I hope you would agree is the right way to go.”

Musk has spoken before about how he could transform Mars into a livable planet. He told TV host Stephen Colbert in September 2015 that humans could fire nuclear weapons at Mars to melt the frozen stores of carbon dioxide, inducing a rapid climate change to benefit humans. Another, suggested the following month, is to create “tiny pulsing suns over the poles [that are] above the planet, not on the planet.” These would “gasify frozen carbon dioxide, thicken the atmosphere and warm up the water and all of that would have a greenhouse effect, a cascading effect, to continue warming up the planet.”

The magazine responded by speaking to Jakosky and Edwards about their research. The pair admitted Musk is right, Mars does contain adsorbed carbon dioxide, but Jakosky could only conceive of two methods for releasing the gases. The first would be to release chlorofluorocarbons, or CFCs, to induce rapid climate change, while the other would involve using a mirror as big as the day side of Mars to warm it up quickly. Both of these ideas are centuries away, and even then it’s unclear whether Martian rock adsorbs gas as efficiently as Earth rock. While a favorable ecosystem and warm temperatures encourage gas adsorption on Earth, NASA’s Curiosity rover has found few signs of rocks with carbonates.

Musk responded by citing a 1993 paper, written by Robert M. Zubrin of Martin Marietta Astronautics and Christopher P. McKay of the NASA Ames Research Center, which claims that humans could indeed use today’s technology to transform the planet and develop a suitable atmosphere. People would need to use a breathing apparatus, but they could ditch the pressure suits and walk around freely. The paper describes this dramatic project, which would require multi-terawatt power sources, as “a leap in human power over nature as dramatic as that which accompanied the creation of post-Renaissance industrial civilization.”

“There is positive feedback- we warm Mars a few degrees C with CF4. this will cause CO2 to outgas from the soil,” Zubrin wrote in support of Musk on Twitter. “That will warm Mars more, releasing more CO2, resulting in a runaway greenhouse effect. Contrary to Jakowsky, the required CO2 is almost certainly there. If the soil contains one percent adsorbed CO2 by wt, it will contain enough CO2 in the top 200 m to give Mars a 300 mb (5 psi) atmosphere - same pressure as Skylab - or Mt Everest. No need for spacesuits on such a Mars.”

Zubrin also criticized Jakowsky for claiming in the ‘90s that there was no ice at mid-latitudes on Mars, a claim disputed by scientists as they discover further potential stores of water over the coming years. Zubrin claims that, as with his ice claims, Jakowsky has no data to assert that Mars does not have enough carbon dioxide.

As for when this could take place if Musk and Zubrin are correct? Zubrin estimates that it would take around 100 years for heat to penetrate Mars to 40 meters’ depth, and a further 500 years to reach 100 meters’ depth.