Dr. Pascal Lee exploring a cave in Iceland
Credit: Pascal Lee
We’ve all seen the images for many, many years: sprawling habitats on the surface of the Moon, domed settlements on Mars, and every variant imaginable. It’s a glorious vision that appeals on many levels. After all, these are sheltered variants of how we live on Earth today, and what could be more appealing than a view of the lunar Earthrise out your living room window, or the fading sunset across a ruddy Martian plain?
We still see such images, even in modern designs. But there’s a hitch—the solar system has become a much more hostile place since these visions first came into being in the early 20th century. Space is suffused with radiation from the sun and beyond our solar system, and it is not kind to humans. Measurements from the Moon made during the Apollo program and more recent readings from the Curiosity rover on Mars have made it abundantly clear that living on the surface of these worlds in an unshielded habitat is off the table. Even a reasonably thick metal hull just serves to magnify the effects of high-energy particles as they punch through the structure, their effect magnified into multiple offenders that are not kind to human anatomy.
What, then, will future settlers do to live safely in these hostile environments?
We have known for a long time that a dense, thick blanket of regolith—the soil of the lunar or Martian surface—will block most of the offending radiation. But this would need to be on the order of 10-to-30 feet (3 to 9 meters) thick to be effective, and on first glance this seems to be quite labor-intensive. But in the past two decades, an appealing alternative has been suggested.
Over the last 20 years we have spotted openings in the pockmarked surfaces of the Moon and Mars that are not mere craters, but are actually pits—portals to what appear to be caverns below. Orbital investigations of the Moon have revealed vast structures of subterranean caves formed by once flowing, incandescent lava that formed rivers across the surface. Over time, when the top-most part of these streams of molten rock cooled and crusted over, they left flowing magma below a solid surface. As the magma drained out, a tube was left below that solid crust, creating a lava tube. Some are quite roomy—enough to house a medium-sized lunar city in some cases. From the observations of generations of Mars orbiters, it appears likely that the same is true on that planet.
What better place to build a settlement? The interior of these structures would be protected by a massive amount of rock overhead, handily provided by nature, and it is even possible that some of these cavernous spaces could be sealed to maintain an internal atmosphere. Our dream of vast, sprawling settlements was simplified, and plans to utilize these geological features have been evolving since then.