Digging deep on Mars could protect future astronauts from radiation

Digging deep into Mars's surface could offer protection against harmful radiation and potentially provide building materials for future astronauts, a new study suggests.

Before humankind ventures beyond Earth's atmosphere and journeys to Mars, scientists must assess the many threats to life on the Red Planet's surface. That includes the amount of cosmic radiation that rains down on Mars —in particular, galactic cosmic ray (GCR) particles. 

According to the U.S. National Oceanic and Atmospheric Administration, GCRs are highly energetic particles that "consist of essentially every element" in existence. They originate from outside our solar system — likely emitted from explosive cosmic events, such as supernovae — and largely bounce off the magnetic fields that surround Earth, called the magnetosphere. 

Extensive exposure to GCR particles could lead humans to experience many health issues, such as the development of cancers, cataracts and central nervous system damage. And Mars lacks a similar protective global magnetic field allowing GCR particles to freely pass into its atmosphere and reach the planet's surface. 

In the absence of a magnetosphere, Mars' atmosphere is the only line of defense against GCRs. And that defense is quite thin: on average, the Red Planet's air is just 1% as dense as that of Earth at sea level.

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When GCRs enter Mars's atmosphere — which is composed mainly of carbon dioxide and nitrogen — they lose a lot of energy through ionization, which can prevent them from reaching the surface. This can largely depend on the thickness of the atmosphere and subsequently, the amount of atmospheric pressure applied to the surface, the study authors found. 

Like Earth, the topography of Mars varies widely. From the peak of Olympus Mons (which is 16 miles, or about 26 kilometers tall) to the deepest Martian crater Hellas Planitia (around 4.4 miles, or 7.1 km, deep), the thickness of Mars' atmosphere can vary in different areas, changing the amount of radiation that reaches the surface. Mars' atmospheric thickness can be more than 10 times different from one place to another, the researchers wrote. 

The researchers also discovered that the interaction between GCRs and the atmosphere also creates another harmful type of radiation called secondary neutron particles. They found that more atmospheric shielding resulted in enhanced contributions of the secondary neutrons to the surface. 

The tesearchers used state-of-the-art computer modeling called the Atmospheric Radiation Interaction Simulator (AtRIS) and radiation data collected by NASA's Curiosity rover — which landed insiden Mars's Gale Crater in 2012 — to simulate GCR exposure on the planet's surface and measure how deeply it penetrates into the surface dirt and rock (known as the regolith).

The result of their analysis revealed that the effective radiation dose peaked at around 12 inches (30 centimeters)  into the regolith. Beyond that, researchers proposed that for safe habitation on Mars — defined to be annual radiation exposure of no more than 100 millisieverts), a regolith shield of between 3.3 to 5.5 feet (1 to 1.6 meters) would be required. "At a deep crater where the surface pressure is higher, the needed extra regolith shielding is slightly smaller," the study authors wrote. 

Understanding how Martian material is affected by GCRs and the role Mars' atmosphere plays in altering radiation exposure is a step forward in developing a potential base on Mars. 

"It has long been argued that astronauts could make use of natural geological structures, such as cave skylights or lava tubes as radiation shelters on Mars," the study authors wrote. "Our study may serve for mitigating radiation risks when designing future Martian habitats using natural surface material as shielding protection."

The new study was published in February in the Journal of Geophysical Research: Planets.

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