Photon Luminescence of the Moon

Introduction: Luminescence is typically described as light emitted by objects at low temperatures, induced by chemical reactions, electrical energy, atomic interactions, or acoustical and mechanical stress. An example is photoluminescence created when photons (electromagnetic radiation) strike a substance and are absorbed, resulting in the emission of a resonant fluorescent or phosphorescent albedo. In planetary science, there exists X-ray fluorescence induced by sunlight absorbed by a regolith – a property used to measure some of the chemical composition of the Moon’s surface during the Apollo program. However, there exists an equally important phenomenon in planetary science which will be designated here as photon luminescence. It is not conventional photoluminescence because the incoming radiation that strikes the planetary surface is not photons but rather cosmic rays (CRs). Nevertheless, the result is the same: the generation of a photon albedo. In particular, Galactic CRs (GCRs) and solar energetic particles (SEPs) both induce a photon albedo that radiates from the surface of the Moon. Other particle albedos are generated as well, most of which are hazardous (e.g. neutrons). The photon luminescence or albedo of the lunar surface induced by GCRs and SEPs will be derived here, demonstrating that the Moon literally glows in the dark (when there is no sunlight or Earthshine). This extends earlier work on the same subject [1-4]. A sideby-side comparison of these two albedos and related mitigation measures will also be discussed. Method – the Monte Carlo: Monte Carlos have the distinct advantage that certain physics can be turned on and off. In this respect, they are a “mathematical experiment” which can isolate specific physical phenomena that actual experiment cannot. This feature has been exploited here. The radiation transport code chosen for the study is FLUKA, which has already been described [1-5]. It has been benchmarked against experiment at the world’s largest accelerators and is currently used to support the Large Hadron Collider (LHC) at CERN. Lunar Surface Model: The chemical composition of soils found at various landing sites during the Apollo and Luna programs [7] has been taken to be the model of the lunar surface, averaging over all such sites to define a generic regolith for the present analysis. This is the same model as used in other studies [1-4]. The lunar regolith is assumed to have a mean density of 2.85 g cm and a negligible magnetic field [1-3]. GCR-Induced Photon Albedo of the Moon: For the case of the GCR-induced photon luminescence, the differential GCR flux was taken from Simpson [7], obeying a power-law spectrum dN ~ E dE with γ = 2.75. This was modulated for solar activity (<10 GeV/nucleon) using the model of O’Neill [8]. The modulation is accomplished by adopting appropriate modulation potentials Φ (MV) in the solar magnetic field for the given epoch. The result for GCRs is given in Figure 1 [1], necessary for generation of and comparison with the SEPinduced case that follows in Figure 2. The luminescent albedo produced by the lunar regolith model is given as a fluence (the time-integral of flux) with the abscissa in GeV’s as well as wavelength in meters [λ=1.23984x10 6 m/E(eV)]. First protons (H), then α–particles (He), and finally everything else (Z>2) have been analyzed. The term “pr” on the ordinate axis represents primary GCR component (ionized H, He, etc.). That is to say, Figure 1 represents the fluence per primary particle.