Earth is the only currently habitable planet in our solar system, but that's never stopped scientists from proposing ways to make Mars a little more human-friendly.
In a 1971 paper, astronomer Carl Sagan suggested a way of terraforming Mars, or making it more habitable for humans. His proposal included vaporizing the northern Martian polar ice caps to add to Mars' thin atmosphere and increase temperatures through the greenhouse gas effect, which would also allow for liquid water on the planet.
Researchers tested this proposal and found that the total yield would fall short of making the planet habitable. So they narrowed the focus from planet-wide to regional.
A new study published Monday in the journal Nature Astronomy proposes that the material silica aerogel could be used to mimic the greenhouse effect in our own atmosphere to make parts of Mars habitable for humans.
"Mars is the most habitable planet in our Solar System besides Earth," said Laura Kerber, research scientist at NASA's Jet Propulsion Laboratory, in a statement. "But it remains a hostile world for many kinds of life. A system for creating small islands of habitability would allow us to transform Mars in a controlled and scalable way."
So how would that work? The researchers used models and experiments to show that a silica aerogel shield, only 2-3 centimeters thick, could block radiation, raise temperatures and allow for photosynthesis.
Scientists believe a thin layer of this material could be used to build domes or biospheres in which temperatures would feel more like the ones we experience on Earth. Going forward, this material will be tested in environments on Earth that could be comparable to Mars, like Antarctica or Chile.
"This regional approach to making Mars habitable is much more achievable than global atmospheric modification," said Robin Wordsworth, study author and assistant professor of environmental science and engineering at the Harvard John A. Paulson School of Engineering and Applied Sciences.
"Unlike the previous ideas to make Mars habitable, this is something that can be developed and tested systematically with materials and technology we already have."
A similar process that already happens on the red planet inspired the researchers. The Martian polar ice caps are comprised of water ice and carbon dioxide. During summers on Mars, the carbon dioxide allows sunlight to penetrate the ice, which traps heat beneath it and creates little warm pockets.
"We started thinking about this solid-state greenhouse effect and how it could be invoked for creating habitable environments on Mars in the future," Wordsworth said. "We started thinking about what kind of materials could minimize thermal conductivity but still transmit as much light as possible."
Silica aerogel is known for being incredibly insulating and it protected the electronic instruments on the Mars Spirit and Opportunity rovers from extreme temperature shifts. Its porous nature allows allows light to pass through it while layers within the material slow heat conduction.
"Silica aerogel is a promising material because its effect is passive," said Kerber. "It wouldn't require large amounts of energy or maintenance of moving parts to keep an area warm over long periods of time."
"Spread across a large enough area, you wouldn't need any other technology or physics, you would just need a layer of this stuff on the surface and underneath you would have permanent liquid water," said Wordsworth.
But the proposal also raises engineering and ethical questions for any future exploration and habitation of Mars.
"If you're going to enable life on the Martian surface, are you sure that there's not life there already? If there is, how do we navigate that," Wordsworth said. "The moment we decide to commit to having humans on Mars, these questions are inevitable."
Material could make parts of Mars habitable for humans, study says