Eckart Böhm

Measurements of Energetic Particle Radiation in Transit to Mars on the Mars Science Laboratory

Going to Mars The Mars Science Laboratory spacecraft containing the Curiosity rover, was launched from Earth in November 2011 and arrived at Gale crater on Mars in August 2012. Zeitlin et al. (p. 1080) report measurements of the energetic particle radiation environment inside the spacecraft during its cruise to Mars, confirming the hazard likely to be posed by this radiation to astronauts on a future potential trip to Mars. Williams et al. (p. 1068, see the Perspective by Jerolmack) report the detection of sedimentary conglomerates (pebbles mixed with sand and turned to rock) at Gale crater. The rounding of the rocks suggests abrasion of the pebbles as they were transported by flowing water several kilometers or more from their source. The radiation dose on a round-trip to Mars could represent a large fraction of the accepted lifetime limit for astronauts. The Mars Science Laboratory spacecraft, containing the Curiosity rover, was launched to Mars on 26 November 2011, and for most of the 253-day, 560-million-kilometer cruise to Mars, the Radiation Assessment Detector made detailed measurements of the energetic particle radiation environment inside the spacecraft. These data provide insights into the radiation hazards that would be associated with a human mission to Mars. We report measurements of the radiation dose, dose equivalent, and linear energy transfer spectra. The dose equivalent for even the shortest round-trip with current propulsion systems and comparable shielding is found to be 0.66 ± 0.12 sievert.

Mars’ Surface Radiation Environment Measured with the Mars Science Laboratory’s Curiosity Rover

The Radiation Assessment Detector (RAD) on the Mars Science Laboratory’s Curiosity rover began making detailed measurements of the cosmic ray and energetic particle radiation environment on the surface of Mars on 7 August 2012. We report and discuss measurements of the absorbed dose and dose equivalent from galactic cosmic rays and solar energetic particles on the martian surface for ~300 days of observations during the current solar maximum. These measurements provide insight into the radiation hazards associated with a human mission to the surface of Mars and provide an anchor point with which to model the subsurface radiation environment, with implications for microbial survival times of any possible extant or past life, as well as for the preservation of potential organic biosignatures of the ancient martian environment.

Charged particle spectra obtained with the Mars Science Laboratory Radiation Assessment Detector (MSL/RAD) on the surface of Mars

The Radiation Assessment Detector (RAD)—situated inside the Mars Science Laboratorys Curiosity rover—is the first ever instrument to measure the energetic particle radiation environment on the surface of Mars. To fully understand the influence of this surface radiation field in terms of potential hazard to life, a detailed knowledge of its composition is necessary. Charged particles are a major component of this environment, both galactic cosmic rays propagating to the Martian surface and secondary particles created by interactions of these cosmic rays with the atoms of the Martian atmosphere and soil. Here we present particle fluxes for a wide range of ion species, providing detailed energy spectra in the low-energy range (up to several hundred MeV/nucleon particle energy), and integral fluxes for higher energies. In addition to being crucial for the understanding of the hazards of this radiation to possible future manned missions to Mars, the data reported here provide valuable input for evaluating and validating particle transport models currently used to estimate the radiation environment on Mars and elsewhere in space. It is now possible for the first time to compare model results for expected surface particle fluxes with actual ground-based measurements.

Measurements of the neutron spectrum on the Martian surface with MSL/RAD

The Radiation Assessment Detector (RAD), onboard the Mars Science Laboratory (MSL) rover Curiosity, measures the energetic charged and neutral particles and the radiation dose rate on the surface of Mars. An important factor for determining the biological impact of the Martian surface radiation is the specific contribution of neutrons, with their deeper penetration depth and ensuing high biological effectiveness. This is very difficult to measure quantitatively, resulting in considerable uncertainties in the total radiation dose. In contrast to charged particles, neutral particles (neutrons and gamma rays) are generally only measured indirectly. Measured spectra are a complex convolution of the incident particle spectrum with the detector response function and must be unfolded. We apply an inversion method (based on a maximum likelihood estimation) to calculate the neutron and gamma spectra from the RAD neutral particle measurements. Here we show the first spectra on the surface of Mars and compare them to theoretical predictions. The measured neutron spectrum (ranging from 8 to 740 MeV) translates into a radiation dose rate of 14±4μGy/d and a dose equivalent rate of 61±15μSv/d. This corresponds to 7% of the measured total surface dose rate and 10% of the biologically relevant surface dose equivalent rate on Mars. Measuring the Martian neutron and gamma spectra is an essential step for determining the mutagenic influences to past or present life at or beneath the Martian surface as well as the radiation hazard for future human exploration, including the shielding design of a potential habitat.

Diurnal variations of energetic particle radiation at the surface of Mars as observed by the Mars Science Laboratory Radiation Assessment Detector

The Radiation Assessment Detector onboard the Mars Science Laboratory rover Curiosity is detecting the energetic particle radiation at the surface of Mars. Data collected over the first 350 Martian days of the nominal surface mission show a pronounced diurnal cycle in both the total dose rate and the neutral particle count rate. The diurnal variations detected by the Radiation Assessment Detector were neither anticipated nor previously considered in the literature. These cyclic variations in dose rate and count rate are shown to be the result of changes in atmospheric column mass driven by the atmospheric thermal tide that is characterized through pressure measurements obtained by the Rover Environmental Monitoring Station, also onboard the rover. In addition to bulk changes in the radiation environment, changes in atmospheric shielding forced by the thermal tide are shown to disproportionately affect heavy ions compared to H and He nuclei.

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 s urface of Mars. With these first-ever measurements on the Martian surface, RAD observe d 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 mag netic field, which correlates with long-term solar activity and the rotation of th e 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 quantitati vely demonstrate how the long-term influences ([b] and [c]) are related to the measure d 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.

Variations of dose rate observed by MSL/RAD in transit to Mars

Aims: To predict the cruise radiation environment related to future human missions to Mars, the correlation between solar modulation potential and the dose rate measured by the Radiation Assessment Detector (RAD) has been analyzed and empirical models have been employed to quantify this correlation. Methods: The instrument RAD, onboard Mars Science Laboratorys (MSL) rover Curiosity, measures a broad spectrum of energetic particles along with the radiation dose rate during the 253-day cruise phase as well as on the surface of Mars. With these first ever measurements inside a spacecraft from Earth to Mars, RAD observed the impulsive enhancement of dose rate during solar particle events as well as a gradual evolution of the galactic cosmic ray (GCR) induced radiation dose rate due to the modulation of the primary GCR flux by the solar magnetic field, which correlates with long-term solar activities and heliospheric rotation. Results: We analyzed the dependence of the dose rate measured by RAD on solar modulation potentials and estimated the dose rate and dose equivalent under different solar modulation conditions. These estimations help us to have approximate predictions of the cruise radiation environment, such as the accumulated dose equivalent associated with future human missions to Mars. Conclusions: The predicted dose equivalent rate during solar maximum conditions could be as low as one-fourth of the current RAD cruise measurement. However, future measurements during solar maximum and minimum periods are essential to validate our estimations.

Measurements of the neutron spectrum in transit to Mars on the Mars Science Laboratory

The Mars Science Laboratory spacecraft, containing the Curiosity rover, was launched to Mars on 26 November 2011. Although designed for measuring the radiation on the surface of Mars, the Radiation Assessment Detector (RAD) measured the radiation environment inside the spacecraft during most of the 253-day, 560-million-kilometer cruise to Mars. An important factor for determining the biological impact of the radiation environment inside the spacecraft is the specific contribution of neutrons with their high biological effectiveness. We apply an inversion method (based on a maximum-likelihood estimation) to calculate the neutron and gamma spectra from the RAD neutral particle measurements. The measured neutron spectrum (12-436 MeV) translates into a radiation dose rate of 3.8±1.2 μGy/day and a dose equivalent of 19±5 μSv/day. Extrapolating the measured spectrum (0.1-1000 MeV), we find that the total neutron-induced dose rate is 6±2 μGy/day and the dose equivalent rate is 30±10 μSv/day. For a 360 day round-trip from Earth to Mars with comparable shielding, this translates into a neutron induced dose equivalent of about 11±4 mSv.

Solar energetic particle spectra from the SOHO-EPHIN sensor by application of regularization methods

Context. The Electron Proton Helium Instrument (EPHIN) on ESA’s Solar and Heliospheric Observatory (SOHO) measures solar energetic electrons, protons, and alpha particles with a stack of six solid-state detectors forming a telescope. The energy deposit in these detectors must be inverted to derive the original energy of the incident particles, thus leading to the original energy spectrum of solar energetic particles. Normal inversion techniques, such as least-squares methods, rely on fitting a known functional behavior of the spectral dependence (normally a power law) to the measured data with some account taken for the instrument response. Such procedures can fail to retrieve accurate particle spectra, e.g., when count rates are low and unphysical negative counts result from the fitting procedure. Aims. We show how regularization methods can be applied to energetic particle measurements to unambiguously derive the original particle spectrum without any assumptions about its functional behavior, while also satisfying constraints such as non-negative counts. Methods. Such inversion techniques still require knowledge of the instrument response function, however, it is an improvement upon normal least-squares or maximum-likelihood fitting procedures because it does not require any a-priori knowledge of the underlying particle spectra. Given the instrument response function in matrix form (here derived using Monte Carlo techniques), the original Fredholm integral equations reduce to a discrete system of linear algebraic equations that can be solved by ordinary regularization methods such as singular value decomposition (SVD) or the Tikhonov method. This procedure alone may lead to unphysical negative results, requiring the further constraint of non-negative count rates. This technique avoids full deconvolution because it involves the solution of ill-conditioned or singular linear systems. Results. We analyze data from SOHO/EPHIN by full deconvolution of the measured data with the instrument response function. We apply the SVD and Thikonov methods with and without constraints to measured data from SOHO/EPHIN. Conclusions. The derived results agree well with those of other methods that rely on a-priori knowledge of the spectral shape of the particle distribution function, demonstrating the power of the regularization method for more general cases.

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.