The Martian surface radiation environment at solar minimum measured with MSL/RAD

Abstract

The radiation environment at the surface of Mars is mainly dominated by incoming galactic cosmic rays (GCRs) that propagate through the atmosphere, with sporadic strong contributions from solar energetic particles (SEPs). The main driver for changes in the radiation field, on time scales of years, is the solar modulation of the GCR flux. During times of higher solar activity, GCRs are more strongly attenuated, resulting in highest GCR fluxes during solar minimum and lowest fluxes at solar maximum. We report dosimetric measurements conducted with the Radiation Assessment Detector (RAD) from November 2019 to October 2020 during the recent deep solar minimum. RAD has been operating on board NASA's Curiosity rover on Mars since August 2012. We bring these measurements into context with RAD measurements from 2012 to 2013 around the (weak) maximum of Solar Cycle 24. The results show the impact of the changing solar modulation from 2012 to 2020 on the Martian surface radiation environment and have implications for future human exploration missions of Mars. We find that while the overall radiation dose rate has increased significantly by 50% between the two time frames, the biologically highly relevant dose equivalent rate shows a modest increase of 13%, yielding interesting input for the timing of such Mars missions within the solar cycle. We also report the first results of the analysis of the flux of medium-energy protons with 100–300 MeV on the Martian surface, yielding an important additional, in-situ measured data point for validating radiation transport models.