Raising crops on Mars is far easier in science fiction than it will be in real life: The Red Planet is an inhospitable world. Among other challenges, subzero temperatures mean water can persist on the surface only as ice, and the planet's atmosphere offers little protection to plants (or people) from the Sun's radiation.
Of course, NASA has plans to eventually put humans on Mars, using lessons it will learn from its Artemis lunar explorations. And those humans will need to eat. Being able to produce food on Mars would help reduce the quantity of supplies consuming valuable space and fuel on crewed missions to the Red Planet. But figuring out how - and where - to produce that food, while also being exceedingly careful not to contaminate Mars with Earth-borne bacteria, are some of the challenges scientists and engineers face.
In a new paper in Nature Astronomy, researchers propose that a material called aerogel might help humans one day build greenhouses and other habitats at Mars' mid-latitudes, where near-surface water ice has been identified. The study was funded by Harvard University's Faculty of Arts and Sciences.
Aerogel is a Styrofoam-like solid that is 99% air, making it extremely light. It's adept at preventing the transfer of heat as well, making it an excellent insulator; in fact, it's been used for that purpose on all of NASA's Mars rovers. Moreover, aerogel is translucent, allowing visible light to pass through while blocking ultraviolet light's harmful radiation. Most aerogel is made from silica, the same material found in glass.
In an experiment conducted by lead author Robin Wordsworth of Harvard, 2-3 centimeters of silica aerogel allowed light from a lamp tuned to simulate Martian sunlight to heat the surface beneath it by up to 150 degrees Fahrenheit (65 degrees Celsius) - enough to raise temperatures on the Martian surface and melt water ice.
"The study was meant as an initial test of aerogel's potential as a Martian building material," said second author Laura Kerber, a geologist at NASA's Jet Propulsion Laboratory in Pasadena, California.
Kerber participated in a 2015 NASA workshop to identify the best places on Mars to send astronauts. "The ideal place for a Martian outpost would have plentiful water and moderate temperatures," she said. "Mars is warmer around the equator, but most of the water ice is located at higher latitudes. Building with silica aerogel would allow us to artificially create warm environments where there is already water ice available."
Broadening the regions on Mars where humans could grow things also opens up new areas where they could conduct valuable scientific research, Kerber added.
'Dark Spots' on Mars
The aerogel experiment was inspired by the heating process that creates so-called dark spots that dot Mars' carbon dioxide ice caps during the spring. This kind of ice is better known on Earth as dry ice. Like aerogel, carbon dioxide ice is translucent, allowing sunlight to heat the surface below. As the soil warms, carbon dioxide gas accumulates between the ice and the warm surface, eventually causing the ice to rupture. That, in turn, creates a puff of gas that tosses soil beneath the ice onto its surface.
The experiment explored a similar process with aerogel. The paper details how both a solid piece of aerogel as well as chunks of crushed aerogel can be used to heat the surface below. The researchers used varying levels of illumination produced by Martian seasons. The results suggest aerogel could even provide a heating effect in the bitter Martian winter. In the mid-latitudes, winter nighttime temperatures can be as cold as minus 130 degrees Fahrenheit (minus 90 degrees Celsius).
The next step, Wordsworth said, is taking the experiment out of the lab and into Martian analogues like Chile's Atacama Desert or Antarctica's McMurdo Dry Valleys. Like Mars, these environments reach subzero temperatures and are exceptionally dry.
"Our prediction is that aerogel shielding should provide more efficient heating as it scales in size," Wordsworth said. "That would be important to see under field conditions."
Challenges to Be Overcome
While the experiment was an encouraging proof of concept, Wordsworth acknowledged there are still significant engineering challenges to overcome. Based on a climate model produced along with the experiment, it would take lots of aerogel and at least two Mars years (or four Earth years) of warming to produce a permanent region of liquid water underneath. Although aerogel is several times lighter than air, building structures with roofs made out of the material would require shipping large quantities of it to Mars or somehow manufacturing it there.
Silica aerogel is very fragile and porous; layering it within another translucent material, or combining it with flexible materials, could prevent fracturing. Doing so could increase air pressure under a structure made with an aerogel roof or shield as well, allowing liquid water to pool more easily on the surface instead of vaporizing in the thin Martian atmosphere.
But the study's authors noted that developing small habitability zones on Mars is more plausible than attempting to "terraform" the planet, as science-fiction writers have proposed doing in the past. A NASA study last year dashed the hopes of thickening the Martian atmosphere enough to create an Earth-like greenhouse effect.
"Anything that would help make long-term habitability possible is exciting to consider," Wordsworth said.
More information about NASA's Mars program is at:
News Media ContactAndrew Good
Jet Propulsion Laboratory, Pasadena, Calif.
NASA Headquarters, Washington