The Zenith‐Angle Dependence of the Downward Radiation Dose Rate on the Martian Surface: Modeling Versus MSL/RAD Measurement

The Radiation Assessment Detector (RAD) on board the Mars Science Laboratory's Curiosity rover has been monitoring the surface radiation environment on Mars for just over 10 years. It has been found by Wimmer‐Schweingruber et al. (2015, https://doi.org/10.1002/2015gl066664) that within the narrow view cone of RAD, the directionality of the radiation field is close to but not completely isotropic. In order to better understand the directionality of the surface radiation over a wide range of zenith angles (θ), we perform a three‐dimensional Geant4 Monte Carlo simulation to derive the θ‐dependence of the surface dose rate. The results show that galactic cosmic ray protons, coming in at θ ∼ 74° make the greatest contribution to the surface dose. For helium ions, this angle is at around 46°. This is a consequence of the increasing column depth at larger zenith angles and the complex interplay of the destruction of primary and the creation of secondary particles as the primary cosmic ray interacts with the Martian atmosphere. We also compared the simulated results with the RAD measurements and found a reasonable agreement. Our results are important for future human exploration of Mars, for instance, to estimate the effectiveness of radiation shielding of a given geometry or for optimizing the radiation shielding design of a Martian habitat.

Plain Language Summary: Space agencies and private companies are working to place humans on the surface of Mars. Astronauts would be exposed to a different and considerably harsher radiation environment on Mars than humans are on Earth. Space radiation is largely determined by galactic cosmic rays, which have sufficient energy to reach the Martian surface. Thus, a better understanding of the radiation on the surface of Mars is needed. The shielding provided by the atmosphere increases with the zenith angle, and it also causes an increase in the creation of secondary particles. To better understand this, we perform a Geant4 Monte Carlo simulation to derive the dependence of the surface dose rate on the zenith angle θ. The results show that the radiation dose on the surface of Mars depends on the incoming angle of the primary radiation. Moreover, the radiation dose rate is significantly modulated by solar activity, and the Mars surface dose rate differs by about 50% between solar maximum and minimum periods. We validate our simulation by comparing the dose measured by the Mars Science Laboratory Radiation Assessment Detector and find good agreement.

Key Points: We model the downward radiation dose on the surface of Mars and find that it only depends weakly on the zenith angle. The surface dose rate depends on solar modulation, and weaker modulation results in higher dose rate for each.The local topographical features influence the Martian surface radiation.