The Martian surface radiation environment – a comparison of models and MSL/RAD measurements

Abstract

Context: The Radiation Assessment Detector (RAD) on the Mars Science Laboratory (MSL) has been measuring the radiation environment on the surface of Mars since August 6th 2012. MSL-RAD is the first instrument to provide detailed information about charged and neutral particle spectra and dose rates on the Martian surface, and one of the primary objectives of the RAD investigation is to help improve and validate current radiation transport models. Aims: Applying different numerical transport models with boundary conditions derived from the MSL-RAD environment the goal of this work was to both provide predictions for the particle spectra and the radiation exposure on the Martian surface complementing the RAD sensitive range and, at the same time, validate the results with the experimental data, where applicable. Such validated models can be used to predict dose rates for future manned missions as well as for performing shield optimization studies. Methods: Several particle transport models (GEANT4, PHITS, HZETRN/OLTARIS) were used to predict the particle flux and the corresponding radiation environment caused by galactic cosmic radiation on Mars. From the calculated particle spectra the dose rates on the surface are estimated. Results: Calculations of particle spectra and dose rates induced by galactic cosmic radiation on the Martian surface are presented. Although good agreement is found in many cases for the different transport codes, GEANT4, PHITS, and HZETRN/OLTARIS, some models still show large, sometimes order of magnitude discrepancies in certain particle spectra. We have found that RAD data is helping to make better choices of input parameters and physical models. Elements of these validated models can be applied to more detailed studies on how the radiation environment is influenced by solar modulation, Martian atmosphere and soil, and changes due to the Martian seasonal pressure cycle. By extending the range of the calculated particle spectra with respect to the experimental data additional information about the radiation environment is gained, and the contribution of different particle species to the dose is estimated.

Dependence of the Martian radiation environment on atmospheric depth: Modeling and measurement

“…That is, incident GCR ions are modeled with an isotropic distribution and transported in straight lines; secondary ions are transported along their initial velocity vectors; secondary neutrons can be transported bidirectionally parallel or antiparallel to their initial velocity vectors. Previous verification studies on the lunar surface [ Slaba et al , ] and validation studies on the Mars surface [ Matthiä et al , ] have shown this transport model to be reasonably accurate compared to Monte Carlo simulations. Detail of the nuclear physics models used in HZETRN2015 can be found in Wilson et al [, , ].…”

“…Results within a specified solid angle field of view may also be obtained by simply integrating over the ray results falling inside the cone of interest. This approach has been shown to provide reasonably accurate spectral results for various particles compared to Monte Carlo simulation and MSL/RAD measurements [ Matthiä et al , ], at least in the context of known uncertainties associated with nuclear physics models used in radiation transport codes [ Norbury and Miller , ].…”

“…In fact, a recent attempt of comparing RAD‐measured particle spectra and different model predictions of GCR spectra on the surface of Mars has shown quite some discrepancies between modeled and measured results [ Matthiä et al , ]. This could be partly due to similar reasons listed here, and the local shielding environment around the RAD detector has modified the original surface spectra, making the direct comparison very difficult.…”

Ready functions for calculating the Martian radiation environment

“…Indeed different physics lists could have been utilized here. However, Matthiä et al (2016) have compared different physics models applied to the Martian environment and the predictions of surface GCR dose rate are agreeing with each other within ∼ 10 percent of uncertainty which is generally not larger than the uncertainty of the input particle spectra.…”

“…the composition and depth of the atmosphere and the soil, can be used in the simulations. Modeling the radiation environment on the surface of Mars using PLANETOCOSMICS has been carried out in various studies (e.g., Dartnell et al, 2007;Gronoff et al, 2015;Matthiä et al, 2016;Ehresmann et al, 2011) and has been validated ) when compared to energetic charged and neutral particle spectra on the surface of Mars measured by MSL/RAD. GEANT4 offers a wide variety of models for handling physical processes of particle interactions at different energy ranges (Geant4 Collaboration, 2017).…”