Mars attacks. Its wisp of an atmosphere means that if you were standing on the surface, it’d be a race to see if suffocation or the sub-zero temperature killed you first. But that’s all tarring the red planet with too broad a brush, perhaps. It’s not all a rusty, frozen hellscape. At the mid-latitudes, just inches down into the ground, you’d find ice—frozen gases like carbon dioxide, even frozen water.If only Mars were warmer, wetter, more oxygen-y, the would-be Martians whine. If only people wouldn’t have to carry bubbles of home with them to colonize Mars and make manifest humanity’s destiny among the stars, or something. As most humans busily make Earth less and less habitable, a few humans propose making Mars more Earth-like, via a process called terraforming.Carl Sagan pitched the idea back in 1971, and even then he knew the main problem would be that gossamer atmosphere. It lets in too much ultraviolet radiation and lets out too much infrared—that’s heat—to turn Mars’ ice into hospitable-to-life water. Either the planet already lost all its insulating CO2, or it’s bottled up underground somehow. On Earth, the greenhouse effect is about to go runaway; on Mars, it ran away. And yet: “Sunlight penetrating a few centimeters through the surface into the snowpack can cause great increases in temperature, leading to sublimation,” says Robin Wordsworth, a planetary scientist at Harvard. The frozen CO2 turns to gas and geysers out of the ground. It’s called a solid-state greenhouse effect—light penetrates the surface, passes through the translucent ice, and then hits darker regolith, which warms up. And so Wordsworth, who studies the climate, evolution, and potential habitability of other worlds, wondered: Could you do that artificially? Could an insulating material create a solid-state greenhouse effect warm enough to make Mars habitable? “If you wanted to take an atmosphere and compress it down to a few centimeters, what would you need?” Wordsworth asks. “The key is how transparent the material is, how light propagates through it, and how thermally insulating it is.”In a new paper in Nature Astronomy, Wordsworth proposes a candidate: silica aerogel. You remember this stuff—it’s the “solid smoke” that the Stardust probe used to collect dirt in space, a mostly-air nanocrystalline matrix of silicon oxide that has an extremely low thermal conductivity. Which is to say, it’s an insulator good enough for spacecraft.
It’s also translucent. Silica aerogel’s clever molecular structure lets photons of visible and infrared light get through with enough efficiency to palpably raise the temperature behind it. But ultraviolet, at the wavelengths that would give a human a sunburn and at the wavelengths that would break a human’s DNA, gets bounced off the outside like it’s a private party and no one named “ultraviolet” is on the list.Wordsworth hasn’t tested the idea on Mars yet, of course. His team did it in a lab, mounting aerogel particles and tiles inside a polystyrene box and then shining a light attenuated to approximate the Martian spectrum and flux. The result? A temperature increase of 50 degrees K.1 “The fact that it’s smoky means that it’s scattering the light, but the bulk of the light is getting through,” Wordsworth says. “Perfectly transparent aerogel, you’d get to the point where you could get hundreds and hundreds of K of warming. The limit is on the materials science, not on the basic theory.”Then they fed their measurements of temperature change into a computer model that included Martian regolith— “with the Martian seasonal cycle, atmospheric pressure, and the rest of it to extrapolate our results,” Wordsworth says. And the numbers say that under aerogel layers, Martian soil would quickly get warm enough to allow for liquid water. Given the presence of other critical nutrients already there, Wordsworth says, a constrained area under the aerogel, maybe lightly pressurized, could even support life. “The major question is going to be, do you want to grow biomass or crops,” he says. “If it’s the latter, then something more like a greenhouse would make more sense. If you’re doing something like just growing algae on the surface, it could literally just be a surface layer.”Maybe most importantly, the warming is fast—way faster than the hundreds, perhaps thousands of years it could take to either liberate CO2 on Mars (if it’s there at all) or to synthesize and emit super-greenhouse gases to the job. On the flip side, says Chris McKay, a NASA astrogeophysicist and Mars expert, the effect would necessarily be local. That’s not terraforming. “Perhaps they could make a cover the size of Midwestern county,” McKay says. “The only way we know how to make a greenhouse over an entire planet is with gases in the atmosphere. As we know from Earth, this is an effective way to warm a planet.”Last year researchers released an analysis of Mars data that suggested the planet had lost most of its CO2, that there wasn’t enough left to induce natural greenhouse warming. One of the authors of that paper says that Wordsworth’s idea, even if only practicable over small scales, is worth a shot. “I imagine there would be questions about how it would work in practice,” says Bruce Jakosky, a planetary scientist at the University of Colorado. “Would dust settling out from the atmosphere eliminate the effect? Would the aerogel be strong enough to withstand the real-world environment on Mars? But these are questions that can be addressed.”
Wordsworth says that small scale is actually an advantage when it comes to terraforming. “You can pick regions where you’re confident there’s no extant life to be worried about,” he says. Still, you’d have to do it where there’s water ice, which leaves room for the possibility of extremophiles . “There’s definitely ethical considerations that need to be thought about, but they’re much more controllable than if you were trying to do something that was global.”The really cool part, though? This is testable on Earth first, in Mars analogs like the Atacama Desert, or the McMurdo Dry Valleys of Antarctica. That’s the next step, trying out a sheet of aerogel on Dry Valley gravel to see if the ground warms up and the ice melts. And if it does, maybe its next stop: Mars.
1Updated to fix the math.
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