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4. Radiation


2005: IVA-5
2010: 2B
Priority: Medium

2010 Version

2B. Investigation. Characterize in detail the ionizing radiation environment at the martian surface, distinguishing contributions from the energetic charged particles that penetrate the atmosphere, secondary neutrons produced in the atmosphere, and secondary charged particles and neutrons produced in the regolith.

Risks to astronauts from radiation in space have been characterized for decades. Outside the shielding affects of the Earth’s magnetic field and atmosphere, the ever-present flux of Galactic Cosmic Rays (GCRs) poses a long term cancer risk. The particle energies in GCRs are such that shielding as a mitigation is in most situations impractical. Superimposed on the continual GCR background are Solar Energetic Particles (SEPs), generated episodically by a component of solar activity known as Coronal Mass Ejections (CMEs). SEPs are composed primarily of protons, generally lower in energy than GCRs, and possess much higher number fluxes. An individual SEP event could be fatal if a crewmember is caught unprotected. Given the energy distribution and fluxes of typical SEP events, the use of shielding to mitigate their impact is possible. However these shielded areas may be limited in size due to mass constraints. Hence avoiding SEP exposures would in large part rely on an understanding of space weather, with predictive and monitoring capabilities for CMEs and the SEPs that often accompany them, such that a crew could be warned ahead of time and appropriate actions taken.

The central issue for radiation exposure at Mars involves the validation of radiation transport codes and other tools designed to simulate and predict the biologically-relevant exposure at the surface, taking into account all of the major variables. On Earth, the relatively thick atmosphere combined with a sizeable, global magnetic field effectively shields humanity from the direct exposure to SEP events, while GCR fluxes are substantially reduced. The atmosphere of Mars is both geometrically thinner and of lower density than on Earth, and there is no global, intrinsic magnetic field; as a result, radiation exposure on the surface would be higher. As energetic particles dissipate energy into the Martian atmosphere and regolith, they would also produce a host of secondary particles. These include neutrons, which can be highly biologically effective and therefore contribute a significant share of the dose equivalent. Radiation dose would not only vary with solar activity and GCR levels, but also with topography and regolith composition. While GCR energies are such that the majority of these particles will pass through the atmosphere, many SEP events would likely deposit most of their energy into the atmosphere, with significant production of biologically relevant secondaries. Of these, the efficiency for the production of secondary neutrons is currently uncertain. Thus GCRs and SEPs are fairly distinct in terms of the physics of their interaction with the atmosphere. During future missions, SEP intensities would most likely be forecast and detected from the vantage point of space or the Earth. To assess the impact of these events at the surface of Mars, models and tools must account for the details of SEP energy deposition into the atmosphere. Hence successful development of these tools would require simultaneous, accurate measurements of the radiation field both above the atmosphere and on the surface, such that the inputs and resulting outputs of the model system are fully constrained.

MSL will carry the Radiation Assessment Detector (RAD), designed to assess radiation hazards from both neutrons and energetic charged particles on the surface of Mars. MSL will provide ground-truth measurements of the radiation environment on the surface of Mars, for both GCR and the SEP events which it will observe over the course of the MSL mission (nominally 2 years). These measurements will be useful in providing necessary boundary conditions to constrain radiation exposure models primarily for GCRs, whose input flux, energy spectra, and variations are approximately uniform over much of the solar system, but never measured on the Martian surface. MSL will also characterize the contribution to the surface radiation environment by SEP events which it samples; however, due to the highly variable spectral, spatial, and temporal properties of SEPs, the properties of the radiation input at the top of the atmosphere will be far less understood. Thus measurements on MSL will likely satisfy measurement goals a. and b. below for GCRs only. The impact of SEPs will not be fully characterized on MSL, either due to solar variability (few or no significant CMEs during the mission), but more importantly because of a lack of an orbital reference to compare measured inputs to and outputs from the Martian atmosphere (measurement goal c.).

The prioritization of the radiation precursor measurements was guided by the following factors: (1) under some circumstances, episodic, but highly intense SEP events could result in the loss or incapacitation of the crew, (2) there are known mitigations to this risk, but come at the expense of increased mass and system and/or operational complexity, and (3) there is a total lack of “ground truth” measurements on Mars. Thus radiation is ranked high in the impact of additional data on risk reduction, since it could potentially lead to the loss of the crew. While additional measurements would likely yield more optimized mission designs, it would nonetheless currently be possible to ensure a reasonable radiation protection strategy, with the risk that the system is overdesigned (for example, increased mass required by shielding, or operational complexities associated with space weather forecasting). Hence the impact of additional data on the mission design is ranked as significant.

Measurements:
a. Identify charged particles from hydrogen to iron by species and energy from 10 to 100 MeV/nuc, and by species above 100 MeV/nuc.
b. Measurement of neutrons with directionality. Energy range from <10 keV to >100 MeV.
c. Simultaneous with surface measurements, a detector should be placed in orbit to measure energy spectra in Solar Energetic Particle events.

Source:
MEPAG Goal IV Science Analysis Group (2010). “IV. Goal: Prepare for Human Exploration.”
Proposed replacement text for MEPAG (2008), Mars Scientific Goals, Objectives, Investigations, and Priorities. Submitted 2 August 2010.

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