Radiation on Planetary Surfaces M. S. Clowdsley 1, G. DeAngelis 2, J. W. Wilson 1, F. F. Badavi 3, and R. C. Singleterry 1 1 NASA Langley Research Center, Hampton, VA 2 Old Dominion University, Norfolk, VA 3 Christopher Newport University, Newport News, VA Solar and Space Physics and the Vision for Space Exploration Meeting Wintergreen, Virginia October 16-20, 2005

Outline Requirements for Evaluating Risk Due to Radiation on Planetary Surfaces –Description of the free space radiation environment near the planet (types of particles and their energy spectra) –Model of planetary magnetic field (if one exists) –Models of planetary surface material and atmosphere (if planet has an atmosphere) –Radiation transport code or codes –Guideline defining how much of each type of radiation is too much Examples Calculations –The Moon –Mars –Callisto Conclusions

Free Space Radiation Environment Galactic Cosmic Rays (GCR) –Made up of heavy ions as well as alpha particles and protons –Modeled using the Badhwar-O’Neill formulation –Modulated by the solar wind Vary with the solar cycle Dependant on distance from the sun Solar Particle Events (SPE) –Made up of a large number of particles, mostly protons –Correspond to large coronal mass ejections Large SPE rare Last only a few hours Could result in fatality

Free Space GCR Environments at 1 AU 1977 Solar Minimum (solid) 1990 Solar Maximum (dashed)

Solar Sunspot Numbers and Deep River Neutron Monitor Count Rates (Measured and Predicted) Deep River Neutron Monitor Solar Sunspot Number

Free Space Solar Particle Event Proton Spectra at 1 AU

Planetary Surface Material and Atmosphere (Simonsen et al.) Mars Induced Fields GCR ion Diffuse neutrons High energy particles

Radiation Transport Codes Monte Carlo Codes: MCNPX, HETC, FLUKA, TIGRE –Accurately model the transport of neutrons, protons, and other light ions (and electrons in the case of TIGRE) –GCR ions being added –Require large amounts of computer time Deterministic Codes: HZETRN, GRNTRN, Electron Transport Code (Nealy et. al.) –Accurately model the transport of neutrons, protons, light ions, and GCR (and electrons in the case of the electron transport code) –Provide rapid transport calculations

Lunar Surface GCR Environments 1977 Solar Minimum (solid) 1990 Solar Maximum (dashed)

Lunar Surface “Worst Case SPE” Environment

Dose Equivalent on Lunar Surface Due to GCR

Mars Surface GCR Environments 1977 Solar Minimum (solid) 1990 Solar Maximum (dashed)

Mars Surface Neutrons

Mars Surface “Worst Case SPE” Environment

Dose Equivalent on Mars Surface Due to GCR

Mars Surface Mapping Charged Ions – 1977 Solar Minimum from Space Ionizing Radiation Environment and Shielding Tools (SIREST) web site

Mars Surface Mapping Neutrons – 1977 Solar Minimum from Space Ionizing Radiation Environment and Shielding Tools (SIREST) web site

Mars Surface Mapping Low Energy Neutrons – 1977 Solar Minimum from Space Ionizing Radiation Environment and Shielding Tools (SIREST) web site

Mars Surface Environment

Model for Mars Atmosphere Atmospheric chemical and isotopic composition modeled using results from in-situ Viking 1 & 2 Landers measurements for both major and minor components: CO 2 % N 2 % Ar % O 2 % CO % 00.08

Model for Mars Surface The surface altitude, or better the atmospheric depth for incoming particles, determined using a model for the Martian topography based on the data provided by the Mars Orbiter Laser Altimeter (MOLA) instrument on board the Mars Global Surveyor (MGS) spacecraft. The Mars surface chemical composition model based on an averaging process over the measurements obtained from orbiting spacecraft, namely the Mars 5 with gamma-ray spectroscopy, and from landers at the various landing sites, namely Viking Lander 1, Viling Lander 2, Phobos 2 and Mars Pathfinder missions.

Model for Mars Surface The adopted Mars surface chemical composition: SiO 2 % 44.2 Fe 2 O 3 % 16.8 Al 2 O 3 % 08.8 CaO % 06.6 MgO % 06.2 SO 3 % 05.5 Na 2 O % 02.5 TiO 2 % 01.0

Model for Mars Surface The composition, different with respect to the regolith (e.g. CO 2 ice, H 2 O ice), of seasonal and perennial polar caps has been taken into account by modeling the deposition of the possible volatile inventory over the residual caps, along with its geographical variations all throughout the Martian year, for both the Mars North Pole and South Pole, from results from imaging data of orbiter spacecraft and from groundbased observations No 3D time dependent models for the Martians polar caps was previously available for radiation studies

Callisto Surface GCR Environments 1977 Solar Minimum (solid) 1990 Solar Maximum (dashed)

Dose Equivalent Rate on Callisto Due to GCR for Jan. 1, 2047

Sample ISS Calculations Ray-trace MeshDirectional Dose Directional Dose distributions Dose Maps

Conclusions Surface radiation calculations have been performed for the Earth’s moon, Mars, and Callisto These calculations show that radiation shielding will be an important consideration in planning of long term missions to these surfaces These calculations also demonstrate the large variation in exposure rates due to solar cycle The advantages of using shielding materials containing hydrogen were demonstrated The ability of the HZETRN code to calculate the radiation environment on the surface of any planet or moon has been demonstrated

Table 1 – Dose Equivalent Limits (Sv) BFOEyeSkin CareerTable Year Day Exposure Limits for LEO Operations (NCRP 98) Table 2 – Career Dose Equivalent to BFO Limits (Sv) Age at Exposure Male Female Based on 3% excess career fatal cancer risk Note: limits not yet defined for missions beyond LEO Limits defined in terms of dose equivalent (H) H =  Q(L) D L dL where D L is the dose (energy absorbed per unit mass) from particles with linear energy transfer between L and L+dL and Q(L) is a quality factor. ALARA – In addition to the above limits, radiation exposure must be kept “as low as reasonably achievable.”

Table 2 – Career Effective Dose Limits (Sv) Age at Exposure Male Female Table 1 – Gray Equivalent Limits (Gy-Eq) BFOEyeSkin CareerTable Year Day New radiation protection quantities Gray equivalent to BFO, eyes, and skin used to evaluate risk due to deterministic effects Gy-Eq =  i RBE i D i Whole body effective dose used to evaluate health risk due to stochastic effects E=  w T H T Based on 3% excess career fatal cancer risk Note: limits not yet defined for missions beyond LEO Proposed Exposure Limits for LEO Operations (NCRP 132) ALARA – In addition to the above limits, radiation exposure must be kept “as low as reasonably achievable.”

Possible Exposure Limits for Lunar Missions (NASA-STD-3000 Vol. VIII - Feb. 1, 2005 Draft) Table 1 – Gray Equivalent Limits (Gy-Eq) BFOEyeSkin CareerREID Year Day REID: “Occupational radiation exposure is limited to not exceed 3% probability of radiation exposure induced death (REID). NASA will assure that this risk limit is not exceeded at a 95% confidence level using a statistical assessment of the uncertainties in the risk projection calculations…” ALARA – In addition to the above limits, radiation exposure must be kept “as low as reasonably achievable.”