The Martian Surface Radiation Environment- A Comparison of Models and MSL/RAD Measurements

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

Modeling the variations of Dose Rate measured by RAD during the first MSL Martian year: 2012-2014

The Radiation Assessment Detector (RAD), on board Mars Science Laboratory's (MSL) rover Curiosity, measures the {energy spectra} of both energetic charged and neutral particles along with the radiation dose rate at the surface of Mars. With these first-ever measurements on the Martian surface, RAD observed several effects influencing the galactic cosmic ray (GCR) induced surface radiation dose concurrently: [a] short-term diurnal variations of the Martian atmospheric pressure caused by daily thermal tides, [b] long-term seasonal pressure changes in the Martian atmosphere, and [c] the modulation of the primary GCR flux by the heliospheric magnetic field, which correlates with long-term solar activity and the rotation of the Sun. The RAD surface dose measurements, along with the surface pressure data and the solar modulation factor, are analysed and fitted to empirical models which quantitatively demonstrate} how the long-term influences ([b] and [c]) are related to the measured dose rates. {Correspondingly we can estimate dose rate and dose equivalents under different solar modulations and different atmospheric conditions, thus allowing empirical predictions of the Martian surface radiation environment

The Lunar Lander Neutron and Dosimetry (LND) Experiment on Chang'E 4

Chang'E 4 is the first mission to the far side of the Moon and consists of a lander, a rover, and a relay spacecraft. Lander and rover were launched at 18:23 UTC on December 7, 2018 and landed in the von K\'arm\'an crater at 02:26 UTC on January 3, 2019. Here we describe the Lunar Lander Neutron \& Dosimetry experiment (LND) which is part of the Chang'E 4 Lander scientific payload. Its chief scientific goal is to obtain first active dosimetric measurements on the surface of the Moon. LND also provides observations of fast neutrons which are a result of the interaction of high-energy particle radiation with the lunar regolith and of their thermalized counterpart, thermal neutrons, which are a sensitive indicator of subsurface water content.Comment: 38 pages, submitted to Space Science Review

RADIATION MEASUREMENTS AND DOSIMETRY FOR DEEP-SPACE EXPLORATION

Dosimetry and radiation monitoring are key factors for the planning of future human exploration on deep-space missions outside of Low-Earth Orbit (LEO), as radiation exposure poses one of the major health risks for astronauts in deep space. The amount of radiation dose astronauts would accumulate during typical mission scenarios and durations can potentially exceed currently allowable exposure limits within months. In space radiation monitoring the three main areas of concern are: 1) high-energy and deeplypenetrating Galactic Cosmic Rays (GCRs); 2) impulsive Solar Energetic Particle (SEP) eventswith high flux intensities up to a few hundred MeVs; and 3) secondary neutrons created by interactions of primary radiation with spacecraft material or planetary atmospheres and soils. Furthermore, in-situ radiation measurements provide invaluable input for radiation transport models that are used to calculate expected radiation exposures and shielding effectiveness, and subsequently potential health risks for future deep-space mission scenarios

THE SEPTEMBER 2017 SOLAR ENERGETIC PARTICLE EVENT OBSERVED BY MSL/RAD ON THE SURFACE OF MARS

The Radiation Assessment Detector (RAD) has been continuously monitoring the Martian surface radiation environment in Gale crater as part of NASA’s Mars Science Laboratory (MSL) mission since August 2012. On September 11 2017, RAD detected the strongest SEP event on the Martian surface to date. This event lead to the highest radiation levels since the beginning of the RAD operations in Gale crater. The SEPs associated with this solar storm increased the surface radiation dose by a factor of 3 over the course of a few hours, and the peak radiation dose was more than 50% higher than the previous maximum measured in October 2013.Here, we give an overview of the heliospheric conditions during the September 2017 solar storm, and present measurements of charged particle spectra during and before the event. RAD saw significant increases in the surface proton and helium fluxes, as well as in the neutral particles (neutrons and gamma-rays) created by interactions of the SEPs with the atmosphere and soil

THE 2015 - PRESENT RISE OF THE GCR AS OBSERVED BY RAD ON MARS

The radiation exposure due to galactic cosmic rays has been rising since early 2015 and is reaching some of the highest levels observed in the space age. This observation is not limited to the near-Earth space environment, but is also seen at Mars in measurements performed by the highly capable Radiation Assessment Detector (RAD) on NASA’s Curiosity rover. We will discuss the increased radiation exposure at Mars and elsewhere, put into the larger heliospheric context and discuss implications for human exploration

On determining the zenith angle dependence of the Martian radiation environment at Gale Crater altitudes

We report the zenith angle dependence of the radiation environment at Gale Crater on Mars. This is the first determination of this dependence on another planet than Earth and is important for future human exploration of Mars and understanding radiation effects in the Martian regolith