Analysis of Mercury’s X-ray fluorescence M. Laurenza, M. Storini and A. Gardini IFSI-INAF, Via del Fosso del Cavaliere, 100, Rome 00133, Italy Joint SERENA – HEWG Conference Santa Fe, New Mexico, USA, 12 – 14 May 2008 IFSI-ROMA

Outline Introduction Model of the planetary environment & simulation basics Computation and evaluation of SEP induced photoemission Conclusions

Illumination by solar X-rays produces X-rays at the surface of Mercury, by exciting inner shell electrons in atoms of the surface material. These atoms return almost instantaneously to their ground state, emitting secondary fluorescence X-rays of the characteristic frequencies for chemical elements present on the surface. X-rays are also elastically scattered from the surface (background). Introduction The X-ray glow of Mercury follows the changes of input by the Sun during the solar cycle. Range of typical solar spectra at mid solar cycle (Clark and Trombka, 1997).

In addition, the Sun emits transient fluxes of solar energetic particles (SEPs) from suprathermal to relativistic energies, that are capable to induce X-ray emission as well. SEP effects are: - Production of secondary particles (by interaction with soil) - Possible contribution to changes of Mercury’s exosphere (e.g. Potter et al., 1999, Leblanc et al., 2003). Gamma-rays are also generated at far greater depths (centimeters to meters) than the extremely superficial fluorescent or scattered X-rays (tens of microns to millimeters).

BepiColombo (MPO) will be exposed to - Direct X-rays and particles from the Sun in the Hermean magnetospehere (SIXS) - X-rays from the Hermean surface, including components caused by (X-ray and particle induced) fluorescence, and scattering (MIXS) This study is devoted to investigate the contribution of SEPs (protons and electrons) to the production of secondary photons, with special attention to the X-ray energy band. We report an attempt to evaluate the X-ray fluorescence following several types of SEP events.

Simulation basics We model the SEP interaction with Mercury’s environment by using the GEANT4 Planetocosmics code. Inputs GEOMETRY: spherical. SOIL: composition is derived from Goettel, PARTICLE SOURCE: we studied several SEP spectra; monodirectional beam along the Sun – Mercury direction. MAGNETIC FIELD MODEL: we selected a dipolar field ( dipole moment 300nT R M 3, northward oriented ). Results Calculation of the motion of SEPs in the magnetosphere (Lorentz equation of motion) and the SEP flux reaching the surface. Computation of the flux of photons emitted from the surface.

Geometry The geometrical construction of the Herman environment is effected with the definitions of two spheres: - the inner one, the “core” consists of vacuum. - around this sphere, a mantle is draped, the “soil”. The soil thickness, d soil can vary, but the planet's radius should not. Therefore, the radius of the core has to vary with the soil thickness, such that always r core +d soil = r M. Outside the planet, two detection layers are defined at 400 and 1500 km.

Soil The soil is represented by a homogeneous layer with a thickness of 1 m and a density of 1.3 g/cm 3. The adopted chemical composition of the soil is presented in the Table. This composition is based on the preferred model by Goettel (1988). These values represent a composition between an extreme refractory-rich and an extreme volatile-rich soil model. MaterialAbundance [%] SiO 2 45 MgO35 Al 2 O 3 7 CaO7 FeO5 Na 2 O0.7 TiO 2 0.3

Simulation of SEP source Energy spectra of SEP events vary significantly from event to event. SEP spectra have characteristic power law spectra ~ E - . We investigate the effect of varying  (from 0 to 3), both for solar energetic protons and electrons (that represent the bulk of SEPs). Energy spectra used in the simulations for protons (left) and electrons (right) with several slopes.

SEP fluxes near Mercury SEP event whose spectrum is similar to one used in the simulations: -  = 2 -~ 6500 Part cm 2 s -1 sr -1 at peak flux time over the whole energy range. Solar proton fluxes recorded by Helios 1 at 0.3 AU in three energy channels, during the 28 April 1978 SEP event. The electron energy spectrum with  = 3 resembles the event reported by Simnett, 1974 properly scaled at Mercury’s orbit (Leblanc et al., 2003).

Photon emission from the simulated proton events High energy protons ( MeV) impacting the planet surface, produce photons at energies from about 1 keV to 10 MeV. In particular, some K lines of different elements are present for  = 0 (flat proton distribution). They progressively disappear with increasing , i.e. when the proton flux at higher energy is lower. Si CaFe

Photoemission vs proton energy X-ray emission due to incident protons is relatively small when compared to the total photon flux (from a ratio of for  = 0 to 0 for  = 3). The total photon flux decreases linearly with decreasing proton mean energy, as expected, while the X-ray emission scales as an inverse law (regression coefficient R 2 =0.98). No X-ray emission is detected below 5.96 MeV.

Photon emission from the simulated electron events High energy electrons ( MeV) give a higher contribution to the X-ray fluorescence than protons (about a factor 10 2 ).

X-ray emission induced by incident electrons is comparable to the total photon flux (less than a factor 3 for all spectral indices). Both the total and the X-ray photon flux decreases linearly with the electron mean energy (regression coefficient R 2 =0.98). The mean electron energy corresponding to a null X-ray emission is MeV. Photoemission vs electron energy

 Ratio (solar flare) Ratio (quiet Sun) X-ray fluorescence evaluation We evaluate whether X-ray fluorescence resulting from the impact of SEPs can be detected (in particular by the MIXS instrument aboard BepiColombo), relatively to that induced by solar flux (e.g. Clark and Trombka, 1997). Burbine et al., 2005 computed the photon total flux in the energy range 1 – 10 keV, by assuming a flat distribution for a typical solar flare (1.91  10 8 photons cm -2 s -1 ) and for the quiet Sun (2.51  10 7 photons cm -2 s -1 ). The ratios of photons produced by high energy electrons in the same energy range respect to those derived from a solar flare and the quiet Sun are reported for different spectral indices in table 2. As far as protons are concerned, a flat distribution in the considered range 4 – 44 MeV produces a ratio of respect to the quiet Sun.

Conclusions Solar energetic particles produce a significant X-ray emission from Mercury surface. It can be expressed by a linear or inverse law vs the spectrum mean energy of high energy electrons and protons, respectively. The minimum proton mean energy needed to produce X-rays is 5.96 MeV for a typical SEP event at Mercury (total proton flux of about 6500 Part cm -2 s -1 sr -1 in the range 4-44 MeV). The mean electron energy corresponding to a null X-ray emission is MeV (given a total electron flux of Part cm -2 s -1 sr -1 in the range 4-44 MeV). The bulk Mercury’s fluorescence (1-10 keV range) is produced by solar energetic electrons, up to 21 % of photons due to the solar radiation. This value is a lower estimate and could increase by assuming a more realistic representation of the solar spectrum.

Acknowledegments Work performed under ASI contract N. I/090/06/0 for SIXS/MPO Science. M. L. thanks the cosmic ray group at the University of Bern for a visiting stage. References Burbine, T. H. et al., Lunar and Planetary Science XXXVI, 1416.pdf, Clark, P. E. and J. I. Trombka, Planetary and Space Science, 45, 57, Goettel, K. A., Present bounds on the bulk composition of mercury: Implications for planetary formation processes. In F. Vilas, C. R. Chapman, and M. S. Matthews (Eds.), Mercury, The University of Arizona Press, Tucson, USA. Leblanc, F. et al., Planetary and Space Science, 51, 339, Potter, A. E. et al., Planetary and Space Science, 47, 1441, Simnett, G. M., Space Science Reviews, 16, 257, 1974.