Detecting Upward Directed Charged Particle Fluxes in the Mars Science Laboratory Radiation Assessment Detector

Abstract

The Mars Science Laboratory rover Curiosity, operating on the surface of Mars, is exposed to radiation fluxes from above and below. Galactic Cosmic Rays travel through the Martian atmosphere, producing a modified spectrum consisting of both primary and secondary particles at ground level. These particles produce an upward directed secondary particle spectrum as they interact with the Martian soil. Here we develop a method to distinguish the upward and downward directed particle fluxes in the Radiation Assessment Detector (RAD) instrument, verify it using data taken during the cruise to Mars, and apply it to data taken on the Martian surface. We use a combination of Geant4 and Planetocosmics modeling to find discrimination criteria for the flux directions. After developing models of the cruise phase and surface shielding conditions, we compare model-predicted values for the ratio of upward to downward flux with those found in RAD observation data. Given the quality of available information on Mars Science Laboratory spacecraft and rover composition, we find generally reasonable agreement between our models and RAD observation data. This demonstrates the feasibility of the method developed and tested here. We additionally note that the method can also be used to extend the measurement range and capabilities of the RAD instrument to higher energies. Plain Language SummaryThe MSL rover Curiosity is exposed to energetic particles from above and below on the Martian surface. Particles enter the Martian atmosphere from above and travel through it until they reach the ground. Particles lose energy and can produce secondary particles while passing through the atmosphere, resulting in an energy distribution on ground level that is different from that on the top of the atmosphere. The resulting particles produce an upward directed particle distribution in the soil. We develop a method to distinguish the upward and downward particle fluxes in the RAD instrument, verify it using data taken during the cruise to Mars, and apply it to data taken on the Martian surface. We use a combination of models to find criteria for discriminating the flux directions. After developing models of the cruise phase and surface shielding conditions, we compare simulated values for the ratio of upward to downward flux with those found in observation data. We find generally reasonable agreement between our models and RAD observation data. This demonstrates the feasibility of the method developed and tested here. The method can also be used to extend the measurement range and capabilities of the RAD instrument to higher energies.

A new model describing Forbush Decreases at Mars: combining the heliospheric modulation and the atmospheric influence

“…In the current model setup, we use the atmosphere profile from MCD above the ground at the location of Gale Crater, which is the landing site of the Curiosity rover. More detailed descriptions of features of the MCD implemented in the model used here can be found in Guo JN et al (2018a, 2019a and Appel et al (2018). Particularly relevant to the current study is that the GEANT4/PLAN-ETOCOSMICS model has been shown to be able to predict reliably the surface charged particle spectra and dose rates of the GCR-induced surface radiation (Matthiä et al, 2016) as actually measured by RAD.…”

Directionality of the Martian Surface Radiation and Derivation of the Upward Albedo Radiation

“…Appel et al (2018) have studied the charged protons detected in RAD's vertical telescope cone (with a half angle of 18°) and found a 10% ratio of the upward to downward flux of 100-200 MeV protons. This ratio is different from what we obtain here mainly for two reasons: (1) dose is contributed by all particles over a wide energy range and cannot be represented solely by protons in this energy range and (2) downward particles within the semi-vertical cone used in Appel et al (2018) traverse through a much thinner atmosphere than those coming from more horizontal directions so that the downward flux/dose ratio in this cone is different from the averaged downward value.…”

Energetic ions in the Venusian system: Insights from the first Solar Orbiter flyby

“…While any difference between the fraction of sky obscured by Venus and the fractional decrease in GCR count rate is small enough to potentially be a result of instrumental uncertainty, it is possible that some of the discrepancy could be due to primary GCRs interacting with the atmosphere and surface of Venus, leading to the production of secondary particles (i.e., albedo particles) which are then detected by HET. Such albedo particles have been measured at Mars with the RAD instrument on Mars Science Laboratory (Appel et al 2018) and at the Moon with the CRaTER (Schwadron et al 2016) andLND (Wimmer-Schweingruber et al 2020) instruments. Measured fluxes are on the order of 10% of the downward flux in the case of Mars in the energy range that can be measured by the RAD instrument (Appel et al 2018).…”

“…Such albedo particles have been measured at Mars with the RAD instrument on Mars Science Laboratory (Appel et al 2018) and at the Moon with the CRaTER (Schwadron et al 2016) andLND (Wimmer-Schweingruber et al 2020) instruments. Measured fluxes are on the order of 10% of the downward flux in the case of Mars in the energy range that can be measured by the RAD instrument (Appel et al 2018). If these albedo particles are being produced at Venus, it would seem that their flux at~2 R V is small.…”