How to protect future Martian astronauts from radiation?

Digging deep into the Martian surface could not only protect against harmful radiation but could also provide building materials for future astronauts living on Mars, a new study suggests. Before humans break through Earth's atmosphere and embark on a journey to Mars, scientists must assess the many threats to life on the red planet's surface. This includes the massive amount of cosmic radiation that is pouring down on Mars specifically charged high-energy particles of galactic cosmic rays (GCRs).

According to the National Oceanic and Atmospheric Administration (NOAA), galactic cosmic ray particles "contain essentially all the elements that exist." These particles originated outside the solar system and were likely released by violent cosmic events such as supernova explosions. Upon encountering the Earth, most particles are reflected back by the magnetic field around the Earth, known as the magnetosphere.

High exposure to galactic cosmic ray particles may cause many health problems in humans, such as cancer, cataracts, and central nervous system damage. Because Mars lacks an Earth-like global protective magnetic field, galactic cosmic ray particles can freely enter the Martian atmosphere and reach the Martian surface.

In the absence of a magnetosphere, Mars' atmosphere is the only line of defense against galactic cosmic rays. However, this line of defense is very weak: the air density of the red planet is, on average, only 1% of Earth's sea level. When galactic cosmic rays enter the Martian atmosphere, which is mostly composed of carbon dioxide and nitrogen, they lose a lot of energy due to ionization, which could prevent them from reaching the Martian surface. The study authors found that this process is largely dependent on the thickness of the atmosphere, and the subsequent atmospheric pressure applied to the surface.

Like Earth, the topography of Mars varies widely. From the summit of Mount Olympus (about 26 kilometers high) to the Greek Plain (about 7.1 kilometers deep), the deepest impact basin on Mars, the thickness of the atmosphere varies greatly in different regions of Mars, and the amount of radiation reaching the surface of Mars also varies greatly. The thickness of the atmosphere in different regions of Mars can vary by a factor of more than 10, the researchers wrote.

The researchers also found that interactions between galactic cosmic rays and the atmosphere also produce another type of harmful radiation called secondary neutron radiation. The greater the barrier effect of the atmosphere, the more secondary neutron radiation reaches the Martian surface.

The researchers used an advanced computer model, the Atmospheric Radiation Interaction Simulator (AtRIS), and radiation data collected by NASA's Curiosity rover to simulate the exposure of galactic cosmic rays to the Martian surface, And measure how deep these particles penetrate into surface dust and rock (regolith). The Curiosity rover landed in Gale Crater on Mars in 2012. Its mission is to detect the presence of Martian climate, geology, and water, and to detect whether the environment in Gale crater was once able to support life.

The analysis showed that the effective radiation dose of galactic cosmic rays peaked at the 30 cm thickness of the Martian regolith. The researchers also proposed that safe habitation on Mars (defined as annual radiation exposure of no more than 100 millisieverts) would require a regolith "shield" of 1 to 1.6 meters. "In a deep crater with a higher surface pressure, the additional regolith shield required can be slightly thinner," the study authors wrote.

Understanding how Martian material is affected by galactic cosmic rays, and the role the Martian atmosphere plays in altering radiation exposure is an important step in developing a future settlement base on Mars. "It has long been thought that astronauts could use natural geological structures such as cavernous skylights or lava tubes as radiation shelters on Mars," the study's authors wrote. Shielding protection with natural surface materials may help reduce radiation risks when exposed to light." The new study was published in the February 2022 issue of the Journal of Geophysical Research: Planets.

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