Regions of Mars could be made habitable with a material -- silica aerogel -- that mimics Earth's atmospheric greenhouse effect, according to a study by NASA and Harvard researchers. People have long dreamed of re-shaping the Martian climate to make it livable for humans. Carl Sagan was the first outside of the realm of science fiction to propose terraforming. In a 1971 paper, Sagan suggested that vapourising the northern polar ice caps would yield atmosphere over the planet, higher global temperatures through the greenhouse effect, and a greatly increased likelihood of liquid water.
Now, researchers from the Harvard University and NASA's Jet Propulsion Lab in the US, have shown that a two to three-centimeter-thick shield of silica aerogel could transmit enough visible light for photosynthesis, and block hazardous ultraviolet radiation. Doing so will also raise temperatures underneath permanently above the melting point of water, all without the need for any internal heat source, according to the study published in the journal Nature Astronomy.
"This regional approach to making Mars habitable is much more achievable than global atmospheric modification," said Robin Wordsworth, Assistant Professor at the Harvard John A Paulson School of Engineering and Applied Sciences (SEAS). "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," said Wordsworth.
"Mars is the most habitable planet in our Solar System besides Earth," said Laura Kerber, Research Scientist at NASA's Jet Propulsion Laboratory. "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," Kerber said.
The researchers, including those from the University of Edinburgh in the UK, were inspired by a phenomenon that already occurs on Mars. Unlike Earth's polar ice caps, which are made of frozen water, polar ice caps on Mars are a combination of water ice and frozen CO2.
Like its gaseous form, frozen CO2 allows sunlight to penetrate while trapping heat. In the summer, this solid-state greenhouse effect creates pockets of warming under the ice.
"We started thinking about this solid-state greenhouse effect and how it could be invoked for creating habitable environments on Mars in the future," said Wordsworth. "We started thinking about what kind of materials could minimise thermal conductivity but still transmit as much light as possible," he said.
The researchers landed on silica aerogel, one of the most insulating materials ever created. Silica aerogels are 97 per cent porous, meaning light moves through the material but the interconnecting nanolayers of silicon dioxide infrared radiation and greatly slow the conduction of heat.
These aerogels are used in several engineering applications today, including NASA's Mars Exploration Rovers. "Silica aerogel is a promising material because its effect is passive. It wouldn't require large amounts of energy or maintenance of moving parts to keep an area warm over long periods of time," said Kerber.
Using modelling and experiments that mimicked the Martian surface, the researchers demonstrated that a thin layer of this material increased average temperatures of mid-latitudes on Mars to Earth-like temperatures. "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.
This material could be used to build habitation domes or even self-contained biospheres on Mars. Next, the team aims to test the material in Mars-like climates on Earth, such as the dry valleys of Antarctica or Chile.