γ-ray spectroscopy in Mars orbit during solar proton events

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    the rover will search for evidence of life on Mars and will map a sub-section of the Martian surface, extracting compositional informa-

    Particles (mainly protons and electrons) are ejected fromthe Sun and contribute to the solar wind. Their energies are

    larly, galactic cosmic radiation (GCR) that is composed pri-marily of protons (87%) with energies ranging between 0.1and 10 GeV/nucleon may also interact with planetary atmo-

    sphere. Neutron interactions with matter can include inelas-tic scattering or neutron capture. Boynton et al. (2004) have

    energy (Kaplan, 1963). c-ray energies can vary between200 keV and 10 MeV (Boynton et al., 2004), and the energyof the emitted c-ray is characteristic of the element it has beenemitted from. The high-energy protons can undergo similarscattering and capture reactions with planetary surfaces andatmospheres to produce c-rays. c-ray emission may also

    * Corresponding author. Tel.: +44 116 252 3519.E-mail address: mss16@star.le.ac.uk (M.S. Skidmore).

    Available online at www.sciencedirect.com

    Advances in Space Research 44typically in the range of eV to keV (Hundhausen, 1995), andthey have speeds of approximately 400 km s1 when theyreach the Earth (Barnes, 1992). When the solar particlesdirectly interact with a planetary atmosphere or surface, acomplex series of interactions occur (including spallation,ionisation and absorption; these and other reactions aredetailed in Krane (1988)), producing secondary particlessuch as neutrons and high-energy c-rays (see Fig. 1). Simi-

    estimated that 10 neutrons are produced per primary par-ticle. The type of interaction is dependent on the neutronenergy, as discussed inMasarik andReedy (1996) and Boyn-ton et al. (2004). The interaction of the neutron with thenucleus (via capture, inelastic scatter or non-elastic scatter)can leave the nucleus in an excited state; the de-excitationprocess often results in the emission of a particle or c-rayand in some cases the re-release of a neutron at a lowertion. Currently our understanding of the bulk composition (and mineralogy) of Mars relies on orbital data from instruments on-boardsatellites such as 2001 Mars Odyssey, Mars Reconnaissance Orbiter and Mars Express, in addition to in-situ instrumentation on roverssuch as Spirit and Opportunity. c-ray spectroscopy can be used to determine the composition of Mars, but it has yet to be successfullycarried out in-situ on Mars. This study describes some of the results obtained from the c-ray spectrometer on 2001 Mars Odyssey duringsolar proton events and discusses whether the increased emissions are useful in c-ray spectroscopy. The study shows that althoughincreased c-ray emissions were expected from the Martian surface during a solar proton event, they were not detected from orbit prob-ably due to insucient signal-to-background. However, this does not preclude the possibility of measuring changes in c-ray ux corre-sponding to changes in solar activity on the surface of the planet. 2009 COSPAR. Published by Elsevier Ltd. All rights reserved.

    Keywords: Solar proton events; c-ray spectroscopy; Mars; Composition; 2001 Mars Odyssey

    1. Introduction

    1.1. Planetary c-ray emission

    spheres and surfaces, thus contributing to the secondary par-ticle uence (Reedy and Arnold, 1972). The secondaryparticles created consist primarily of neutrons and c-rayswhich are attenuated by the planetary surface and atmo-c-ray spectroscopy in Mars o

    M.S. Skidmore

    Space Research Centre, Department of Physics and Astronomy,

    Received 10 March 2009; received in revi

    Abstract

    Understanding the evolution of Mars requires determining thepean Space Agencys ExoMars rover mission, which is expected to0273-1177/$36.00 2009 COSPAR. Published by Elsevier Ltd. All rights resedoi:10.1016/j.asr.2009.06.004it during solar proton events

    R.M. Ambrosi

    iversity of Leicester, University Road, Leicester LE1 7RH, UK

    form 4 June 2009; accepted 4 June 2009

    position of the surface and atmosphere of the planet. The Euro-nch in 2016, is part of the Aurora programme. The instruments on

    www.elsevier.com/locate/asr

    (2009) 10191029rved.

  • s inoccur as a result of the decay of radioactive nuclei, such asU,Th and K. A suitable c-ray spectrometer can be used todetect the c-rays on the surface of a planet or in orbit.

    The Martian atmosphere, although thin, signicantlyattenuates the proton ux below 100 MeV, so few protonsare able to interact with the surface (A Monte Carlo modelbeing developed at theUniversity of Leicester (UoL) that sim-ulates theMartian atmosphere and surface has indicated thatnearly all of the sub- 100 MeV protons are attenuated in theatmosphere, several kilometers above the surface). However,the neutrons and secondary particles created in the atmo-sphere via these proton reactions are able to penetrate intothe surface. As a result normal solar proton emission fromthe Sun does not create a signicant amount of c-ray emissionfrom the surface ofMars comparedwith theGCR interactionwith Mars. Unlike the solar protons, the GCR protons areable to directly interactwith the sub-surface because their typ-ical energies are in excess of 100 MeV instead of keV. Typicalproton uences ofGCRand solar origin detected by the spaceenvironment monitors (SEM) on the geostationary opera-tional environmental satellites (GOES) at 1 AU are of theorder of 10 protons cm2 sr1 s1 for 110 MeV protons,5 protons cm2 sr1 s1 for 10100 MeV protons and1 proton cm2 sr1 s1 for >100 MeV protons (NGDC,

    Fig. 1. Diagram showing the possible interactions of protons with aplanetary surface.1020 M.S. Skidmore, R.M. Ambrosi / Advance2008). Since the total GCR ux can be assumed to be 12 cm2 sr1 s1, with energies ranging from 0.1 to 10 GeV/nucleon (Reedy and Arnold, 1972), the protons >100 MeVare assumed to be ofGCRorigin.During a solar proton event(SPE), protons canbe acceleratedout from the Sunat energies>100 MeV; and the overall proton ux is enhanced. Theseprotons will be able to directly interact with the Martianand lunar surfaces and sub-surfaces, and could lead toincreased c-ray emission from the planet. This study discusseswhether this increased c-ray emission could be exploited inorder to obtain a deeper understanding of the interactionmechanisms and composition of a planetary surface.

    1.2. c-ray spectroscopy

    The composition of the surface and sub-surface of a pla-net can be extracted from the c-ray spectra collected, giventhat the c-rays emitted by these mechanisms are elementspecic. In addition, the intensity of a c-ray line is propor-tional to the concentration of the associated element in thesurface, sub-surface or atmosphere of a planet.

    c-ray spectroscopy (GRS) can be carried out from orbitif the planetary body of interest has a thin enough atmo-sphere. It has been carried out in the orbit of Mars (Mars5, Phobos 2 and 2001 Mars Odyssey, hereafter called MO)but has yet to be successfully applied in-situ (Surkov,1997). Detection of c-rays from orbit is dependent on thepresence and density of an atmosphere. The atmosphericattenuation varies with atmospheric thickness, and theatmosphere itself will contribute to the c-ray spectra(deconvolving an orbital c-ray spectrum into its atmo-spheric and surface contributions has yet to be carriedout). On the Martian surface or sub-surface the c-ray uxcan be signicantly higher than that in orbit about Mars asa result of the attenuation by the atmosphere (the averagecount rate from the surface of Mars detected by the c-rayspectrometer on MO is 190 s1 (Evans et al., 2007)).

    A research team led byR.Ambrosi at theUoL is currentlydeveloping instrumentation for geophysical applications.The instrumentation includes a geophysical package for afuture in-situ planetary science missions that can simulta-neously carry out GRS, c-ray densitometry and radiometricdating of a planetary surface. The geophysical package couldbe installed on a lander or (ideally) a sub-surface probe thatwould carry out measurements at depths of up to 5 m. Theadvantages, goals and challenges of a sub-surface GRS mis-sion are detailed in Skidmore et al. (2009). Current designsfor the c-ray portion of the detector feature a lanthanumbro-mide (LaBr3(Ce)) scintillator detector, which is compact,radiation tolerant, operates at a range of temperatures, islow in mass and has a low power requirement compared tothe cooling power requirements of a high-purity germanium(HPGe) detector (Skidmore et al., 2009).

    Currently the geophysical package will rely on naturalc-ray emissions caused by the proton interaction with the sur-face. The addition of a neutron generator could increase thesurface c-ray emission further. An increase in the c-ray uxwould reduce the accumulation time required to get a spec-trum with sucient counts in it. This was discussed in detailby Ambrosi et al. (2005). Although neutron generators arebecoming more compact (e.g. a 252Cf source could weigh grams (Ambrosi et al., 2005)), they require shielding to pro-tect the rest of the lander/probe, which increases the mass aswell as posing a radiation hazard. The additional proton uxinteracting with a planetary atmosphere, surface and sub-sur-face during a SPE results in an increase in the neutron uxwithin the surface, and an increase in the c-ray emission fromthe surface. This may positively aect the counting statistics.The mechanism that creates a SPE is discussed in Section 1.3.

    1.3. Solar proton events

    Space Research 44 (2009) 10191029A SPE refers to an enhancement in solar proton emis-sions from the Sun, but generally speaking these events

  • ground and will positively impact the counting statistics.Improved counting statistics would be of great benet toa lander-based c-ray spectrometer. Often lander-basedinstruments have to share limited resources with otherinstruments thus reducing data acquisition times.

    Studying the eects of a SPE on a planetary surface is ofgreat interest, not only for the purposes of planetary com-position studies, but also in understanding the radiationhazard astronauts could face on the surfaces of other plan-ets. The Earths atmosphere is sucient to protect us fromthe energetic particles released in a SPE, but the Martianatmosphere is not, and there would be no protection atall on the lunar surface. Models of the Martian surfaceradiation environment have been developed in order tostudy the dose rate that astronauts would be exposed toduring quiet solar conditions and during SPEs. Morthekaiet al. (2007) modelled the dose rate with depth in the Mar-tian regolith using GEANT-4 and compared it to the esti-mated dose rate at the surface determined by MARIE, aMartian radiation environment monitor on MO. They

    es in Space Research 44 (2009) 10191029 1021include heavy ions, which form a similar proportion of thetotal number of emitted particles as the heavy ions in theGCR particle distribution (Morthekai et al., 2007). Solarprotons dominate the spectrum and are accelerated to highenergies (>1 MeV) by shocks associated with coronal massejections (CMEs), magnetospheric and bow shocks, andco-rotating interaction regions (Reames, 1999a). Themechanism that generates the most intense events is theCME, which is the result of magnetic energy being releasedfrom reconnecting coronal elds. This process can causethe acceleration of protons to energies in excess of>100 MeV, even reaching energies of several GeV. Theseenergies are reached via collision-less shock waves thatare driven out from the solar corona. CMEs can occur(on average) 2.5 times a day at solar maximum; however,only about 12% of these actually form shocks (Reames,1999a). Proton acceleration occurs when protons are scat-tered back and forth across the shock many times by mag-netic turbulence upstream and downstream from the shock.Reames (1999a) provides a more detailed explanation ofthe acceleration mechanisms.

    The protons accelerated by the shock can stream out-wards from it, and tend to follow the magnetic eld linesthat make up the interplanetary magnetic eld (IMF)(Cleghorn et al., 2004). Protons of 10 MeV can have speedsup to 1 AU/h (Reames, 1999b); higher energy protons tra-vel faster, therefore the onset of the CME can be detectedvery quickly via radiation monitors in orbit about theEarth. The shock wave itself also travels outwards fromthe Sun at speeds of 7502500 km s1 (Reames, 1999b).Protons within the shocks generate resonant waves thattrap other protons in the shocks paths; these areaccelerated in the same way as the protons within theshocks. These shocks can spread over a wide angular range,reaching 180 width at 1 AU (Cane, 1988). The peak ux ofenergetic protons is usually observed when the shock wavepasses (Reames, 1999a), even for protons at energies of>500 MeV. If a planet is well connected to the CME viaone of the IMF lines, then an observer at 1 AU could seea sharp rise in particle in particle ux (associated withthe SPE) approximately 12 h after the onset of the event(Futaana et al., 2008). If the planet is not well connectedto the CME then the particle ux will increase more grad-ually, or not at all. The location of the CME emission pointon the Sun, the width of the shock wave and the speed ofthe shock all determine whether the SPE could aect theplanet or not.

    The high-energy solar protons associated with a SPE areable to penetrate further into a planetary atmosphere orsurface before an interaction occurs. In the case of Mars,if the particles were able to reach the denser surface mate-rial of the planet, the probability of a nuclear reactionoccurring would increase, therefore more secondary parti-cles would be produced than in any interaction with theMartian atmosphere. This should result in an increase in

    M.S. Skidmore, R.M. Ambrosi / Advancc-ray emission from the planetary surface. The increasein c-ray emission would result in increased signal-to-back-determined that GCR and solar event protons are onlyslightly attenuated in the Martian atmosphere; most aredeposited in the regolith. From balloon experiments car-ried out in the upper atmosphere on Earth, we know thatthe secondary particle production (including neutronsand c-rays) as a result of the nuclear interaction of protonswith planetary material reaches a maximum at approxi-mately 50 g cm2 (Morthekai et al., 2007). Given that theMartian atmospheric thickness is approximately 16 g cm2

    (Morthekai et al., 2007), this maximum would occur withinthe Martian regolith. This is in agreement with the MonteCarlo model being developed at the UoL. Fig. 2 shows pre-liminary results from the UoL Monte Carlo model, whichshows the ux of neutrons and c-rays as a function of

    Fig. 2. Relative amounts (%) of secondary particles in the MCNPXsimulated Martian surface. The plot indicates where the number ofsecondary particles reaches a maximum for protons of dierent energies.The energy stated in the brackets in the legend next to the particle typeindicates the energy of protons that were interacting with the simulated

    surface. For protons of >500 MeV,...