There are very good reasons why Mars is such a desolate, barren landscape. With no thick atmosphere nor a magnetic field, the Red Planet's surface is bombarded daily by radiation up to 900 times higher than seen on Earth. However, some places are sheltered. New research has found that cave entrances are shielded from the harmful radiation that normally hits Mars. This may make them ideal as both sites for future settlements and robotic missions meant to scour for signs of alien life.
Despite amazing advances in space exploration in the last decade, if we're going to take the idea of settling Mars sometime during this century seriously, there are many challenges that need to be overcome. That's unless we're content with one-way suicide missions.
There's no shortage of environmental hazards out to kill any astronaut bold enough to dare set foot on Mars. For one, the planet only has 0.7% of Earth's sea-level pressure, meaning any human on Mars must wear a full pressure suit or stay barricaded inside a pressure-controlled chamber, otherwise oxygen wouldn't flow through the bloodstream and the body could swell and bleed out.
Then there's the issue of radiation. Mars is farther away from the Sun than Earth, receiving roughly 60% of the power per square meter seen on a similar site on Earth. But since Mars doesn't have a magnetic field to deflect energetic particles, coupled with the paper-thin atmosphere, its surface is exposed to much higher levels of radiation than Earth. Furthermore, besides regular exposure to cosmic rays and solar wind, it receives occasional, lethal radiation blasts due to strong solar flares.
Measurements performed by the Mars Odyssey probe suggest that ongoing radiation levels on Mars are at least 2.5 times higher than what astronauts experience on the International Space Station. That's about 22 millirads per day, which works out to 8000 millirads (8 rads) per year. For comparison, the people in the U.S. are exposed to roughly 0.62 rads/year on average.
Any attempt to colonize the Red Planet will require measures to ensure radiation exposure is kept to a minimum. Some of the proposed ideas thus far involve habitats built directly into the ground or even above-ground habitats using inflatable modules encased in ceramics.
But a better idea may be to take advantage of the natural shelters already in place. Mars is dotted with deep pits, caves, and lava tube structures across its surface. According to a new study performed by researchers led by Daniel Viúdez-Moreiras at Spain’s National Institute for Aerospace Technology, many of these caverns could offer ample protection to human settlers.
"Caves and their entrances have been proposed as habitable environments and regions that could have preserved evidence of life, mostly due to their natural shielding from the damaging ionizing and non-ionizing radiation present on the surface. However, no studies to date have quantitatively determined the shielding offered by these voids on Mars," the researchers wrote in the journal Icarus.
The researchers found that the levels of UV radiation inside Martian caverns were, in some cases, ~2% of those values found on the surface.
"Numerical simulations of cave entrances show a reduction even more than two orders of magnitude in UV radiation, both in the maximum instantaneous and cumulative doses, throughout the year and at any location of the planet," the researchers found.
What's more, the amount of active radiation is still higher than the minimum required for Earth-like photosynthesis. In other words, cave entrances could shelter both humans and their plant food source. However, it's unclear whether ionizing radiation -- the kind of electromagnetic radiation associated with cancer -- is blocked in the same way as UV radiation.
“Ionizing radiation doesn’t present exactly the same behavior as UV radiation,” Viúdez-Moreiras. told New Scientist. “However, it is expected that ionizing radiation will also be strongly attenuated in pit craters and cave skylights.”
In 2009, researchers led by Dr. Armando Azua-Bustos, a scientist at the Department of Planetology and Habitability Center of Astrobiology (CSIC-INTA) in Madrid, described the behavior of a particular Cyanidium eukaryote red algae growing in the Mars-like Atacama Desert. These microorganisms formed biofilms in seemingly inhospitable coastal caves where there is little light, but just enough it seems to support life. If Martian caves are anything like those across the barren Atacama Desert, the driest place on Earth, life could find a way to thrive there as well, Azua-Bustos and colleagues proposed.
High-resolution surface imaging data recorded over the past couple of decades by instruments like the Mars Reconnaissance Orbiter Context Camera system (CTX), together with Mars Odyssey's thermal emission imaging system (THEMIS), suggest that the Tharsis bulge may be the best region for cave candidates on Mars. More than 1,000 suitable caves have been identified in this region, which also contains three enormous shield volcanoes, Arsia Mons, Pavonis Mons, and Ascraeus Mons.
Tharsis city sounds like an awesome name for the first human settlement on Mars. Remember the name.
UPDATE (August 30, 2021): The article was updated to include the findings made by Azua-Bustos et al. in the Atacama Desert, which complement the radiation quantification in Martian caves.