Which parts of Mars are most immune to cosmic radiation?

Artist’s impression of a Martian colony with cross-sectional view. Credit: NASA Ames Research Center

Over the next decade, NASA and China plan to send the first crewed missions to Mars. This will consist of both agencies sending spacecraft in 2033, 2035, 2037 and every 26 months thereafter to coincide with Mars opposition (i.e. when Earth and Mars are closest in their orbits). The long-term goal of these programs is to establish a base on Mars that will serve as a hub for future missions, although the Chinese have said they intend to make their base permanent.

The prospect of sending astronauts on the six- to nine-month journey to Mars presents several challenges, not to mention the dangers they will face while conducting science operations on the surface. In a recent study, an international team of scientists surveyed the Martian environment – from the peaks of Mount Olympus to its subterranean recesses – to find where the radiation is weakest. Their findings could inform future missions to Mars and the creation of Martian habitats.

The team was led by Jian Zhang, assistant professor at the School of Earth and Space Sciences (ESS) at the University of Science and Technology of China. He was joined by colleagues from the ESS and CAS Center for Excellence in Comparative Planetology in China, the Institute for Experimental and Applied Physics (IEAP) in Kiel, Germany, and the Institute for Biomedical Problems in the Russian Academy of Sciences (RAS) and the Skobeltsyn Institute of Nuclear Physics (SINP) in Moscow. The article describing their findings recently appeared in the Journal of Geophysical Research: Planets.

When it comes to missions to Mars and other places beyond Low Earth Orbit (LEO), radiation is always a constant concern. Compared to Earth, Mars has a thin atmosphere (less than 1% of atmospheric pressure), and there is no protective magnetosphere to shield the surface from solar and cosmic radiation. As a result, scientists theorize that harmful particles, especially galactic cosmic rays (GCRs), could spread and interact directly with the atmosphere and even reach the subsurface of Mars.

However, the level of radiation exposure depends on the thickness of the atmosphere, which varies with altitude. In low-lying areas like Mars’ famous canyon system (Valles Marineris) and its largest crater (Hellas Planitia), atmospheric pressure is estimated to be over 1.2 and 1.24 kPa, respectively. This is about double the average 0.636 kPa and up to 10 times atmospheric pressure at high altitude locations like Olympus Mons (the tallest mountain in the solar system).

Dr. Jingnan Guo, an esteemed IEAP Professor of Christian-Albrechts University and a Fellow of the Chinese Academy of Sciences (CAS), was Professor Jian Zhang’s Ph.D. supervisor and co-author of the article. As she explained to Universe Today via email:

“Different elevation means different atmospheric thickness. Places at high altitudes generally have a thinner atmosphere at the top. High-energy particle radiation must pass through the atmosphere to reach the surface of Mars. If the atmospheric thickness changes, surface radiation can also change. So elevation could influence the surface radiation of Mars.”

To this end, the team examined the influence of atmospheric depths on Martian radiation levels. This included absorbed dose measured in rads; the dose equivalent, measured in rems and sieverts (Sv); and the effective dose rates to the body induced by GCRs. The aim was to model the radiative environment using a state-of-the-art simulator based on the GEOmetry And Tracking (GEANT4) software developed by CERN.

Known as the Atmospheric Radiation Interaction Simulator (AtRIS), this software uses Monte Carlo probability algorithms to simulate particle interactions with the Martian atmosphere and terrain. As Dr. Guo illustrated:

“We use a Monte Carlo approach called “GEANT4” to model the transport and interaction of energetic particles with the Martian atmosphere and regolith. The environment of Mars is configured taking into account the atmospheric composition and structure of Mars and the properties of the regolith.

“Ingress particle spectra above the Martian atmosphere are also obtained from calibrated data models that describe the ubiquitous particle radiation environment in interplanetary space that includes charged particles of different species which are mostly protons (~87%), helium ions (12%) along with small traces of heavier ions such as carbon, oxygen and iron.”

They found that higher surface pressures can effectively reduce the amount of heavy ion radiation (GCR), but additional shielding is still needed. Unfortunately, the presence of this shielding can lead to “cosmic ray showers”, where the impact of GCRs against the shielding creates secondary particles that can flood the interior of a habitat with varying levels of neutron radiation (aka neutron flux). These can contribute significantly to the effective dose of radiation that astronauts will absorb.

They determined that the neutron flux and effective dose peaked about 30 cm (1 foot) below the surface. Fortunately, these discoveries offer solutions when it comes to the use of Martian regolith for shielding. Said Dr. Guo:

“For a given threshold of biologically weighted annual effective dose of radiation, say 100 mSv (a quantity often considered the threshold below which the risk of radiation-induced cancer is negligible), the required regolith depth varies between about 1 m and 1.6 m In this range, in a deep crater where the surface pressure is higher, the additional regolith shielding needed is slightly smaller.While at the top of Mount Olympus, the additional regolith shielding needed is more raised.

Based on their findings, the best sites for future habitats on Mars would be in low-lying areas and at depths of 1m and 1.6m (3.28 to 5.25ft) below the surface. Therefore, the northern lowlands, which make up most of the northern hemisphere (aka Vastitas Borealis), and Valles Marineris would be very suitable locations. In addition to having thicker atmospheric pressure, these regions also have abundant water ice just below the surface.

If all goes according to plan, astronauts will set foot on the Martian surface in just over a decade. This will consist of transits lasting six to nine months (barring the development of more advanced propulsion technology) and surface operations of up to 18 months. In short, astronauts will face the threat of high radiation for up to three years. As such, detailed mitigation strategies should be developed well in advance.

NASA and other space agencies have invested considerable time, energy, and resources to develop habitat designs that take advantage of 3D printing, in-situ resource utilization (ISRU), and even shielding electromagnetic to ensure the health and safety of astronauts. However, there remain unanswered questions about how effective these strategies will be in practice, especially considering the amount of time crews will spend on the Martian surface.

“Our study can be used to mitigate radiation risks when designing future Martian habitats using natural surface materials as shielding,” Dr Guo said. “Research like this will therefore be of considerable value when mission planners begin to consider designs for future Martian habitats that rely on natural surface materials to provide radiation protection.”

Mars astronauts will create fuel by taking a shower

More information:
Jian Zhang et al, From the summit of Martian Olympus to deep craters and below: the radiation environment of Mars under different atmospheric and regolith depths, Journal of Geophysical Research: Planets (2022). DOI: 10.1029/2021JE007157

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Quote: Which parts of Mars are most sheltered from cosmic radiation? (2022, May 3) retrieved May 4, 2022 from https://phys.org/news/2022-05-mars-safest-cosmic.html

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