UTILIZING MEASUREMENTS FROM THE RADIATION ASSESSMENT DETECTOR (RAD) ON THE SURFACE OF MARS TO IMPROVE OUR UNDERSTANDING OF THE MARTIAN RADIATION ENVIRONMENT

Abstract

Exposure to space radiation remains one of the major risks for human space exploration. On Mars this exposure mainly stems from Galactic Cosmic Radiation (GCR) and spontaneous Solar Energetic Particles (SEPs) emitted from the Sun during Coronal Mass Ejections (CMEs) or solar flares. As the Martian atmosphere is comparably thin, GCRs and SEPs can penetrate deep into the atmosphere and even reach the surface. Note here that, even while the atmosphere is thin, protons need about 150 – 170 MeV to be able to reach the surface. To characterize the resulting Martian surface radiation environment and how it changes over time with the solar cycle, the Mars Science Laboratory (MSL) Radiation Assessment Detector (RAD) has been measuring this radiation in Gale Crater on Mars aboard NASA’s Curiosity rover since August 2012. In this presentation we focus on two specific sets of measurements and describe how they can be used to improve our understanding of the Martian surface radiation environment, and thus better understand the potential health hazards for future human explorers. (1) We present RAD measurements of SEP events and other associated effects (such as Forbush decreases) as observed on the surface of Mars. Thereby, we focus on the timing and intensity of these events and describe how comparison to measurements from other spacecraft and instruments can further our understanding of SEP propagation throughout the inner heliosphere. Here, RAD provides a unique data set as it is able to measure in detail the high-energy (> 150 – 170 MeV) component of these SEP events. (2) We present RAD measurements of the observed radiation shielding effect provided by local terrain surrounding the Curiosity rover. If the rover is situated close to massive terrain, such as canyon walls or cliff sides, part of the incoming radiation from the upper hemisphere can be blocked from reaching the rover, resulting in a decrease of the measured radiation dose. To qualify and quantify this shielding effect we present an overview of all instances that RAD was able to measure. Based on orbital altitude maps and rover localization data, we can calculate the amount of hemisphere that is blocked by a terrain feature and correspondingly the average zenith angle of obstruction for each instance. These data sets of obstruction angles and corresponding decrease in radiation dose provide valuable ground-truth proof to validate radiation transport models. These models can then be used to determine the effectiveness of planned radiation or storm shelters to be utilized by future Mars explorers. Here, it is important to note that it will most likely be more efficient to utilize existing terrain on Mars, such as crater walls or lava tubes, to build operation bases and emergency radiation shelters.