A. Milillo and K. C. Hsieh (and the SHEA fans!)

2nd SERENA-HEWG meeting - Mykonos, June Why energetic neutral atoms are a useful tool for planetary investigations? The neutral atoms do not interact with electromagnetic fields. Hence, if their energies are high enough to consider the gravitational effects negligible, they have the property to maintain their characteristics (energy distribution and direction) unchanged since the generation time. In this way, the information about the (remote) generation process can be obtained through energetic neutral atom detection. We know two main generation mechanisms of atoms at energies well above the gravitational effects: Charge-exchange and ion sputtering processes.

Plasma ions keV ENA ENA features in the solar system Charge-Exchange SurfaceIon-Sputtering 10s eV ENAions AtomosphericIon-Sputtering ions (fm.: Orsini et al., SERENA-HEWG meeting, Santa Fe, NM, USA) 2nd SERENA-HEWG meeting - Mykonos, June. 2009

Outline Surface ion sputtering process Other surface release processes (example of Mercury) Expected outcomes from Sputtered High- Energy Atoms (SHEA) observations in the Hermean environment, in the solar wind - asteroid interaction, in the Jupiter’s system. Conclusions 2nd SERENA-HEWG meeting - Mykonos, June. 2009

Surface Ion Sputtering process Ion sputtering products depend on: the composition and the chemical structure of the surface; the impinging plasma flux. Release of neutrals due to bombardment of a surface by energetic ions 2nd SERENA-HEWG meeting - Mykonos, June. 2009

Ion-sputtering process The emitted neutral flux  n is proportional to the yield, Y, (the number of sputtered atoms produced by one single impinging ion) much higher for higher energies and for heavier ions (Baragiola et al., 2003). c is the surface relative abundance of the atomic species considered,  I is the ion flux, and f s is the energy distribution function. 2nd SERENA-HEWG meeting - Mykonos, June Note that the yields obtained by laboratory simulations could be different (lower or higher) in the planetary environments since the aggregation status of the surface material could be different from the sample. New observations are really important in this frame!

Energy distribution function Directional neutrals E i incident particle energy E b binding energy  1 eV E e Energy of the released particle 2nd SERENA-HEWG meeting - Mykonos, June. 2009

Particle release processes (Killen et al., SSR,2008) Thermal Desorption (TD): mainly volatiles at very low energies (<.5 eV) Photon Stimulated Desorption (PSD): mainly volatiles at low energy (~ 1 eV) Micrometeoroid Impact Vaporization (MIV): all the surface components at high energy but, anyway, below few eVs Ion Sputtering (IS): all the surface components at higher energy up to 100s eV 2nd SERENA-HEWG meeting - Mykonos, June. 2009

Particle release processes at Mercury: Na case TD PSD ISMIV (Milillo et al. PSS in press,2008) 2nd SERENA-HEWG meeting - Mykonos, June. 2009

Detection of exospheric gas Observation of the global exospheric distribution (released particles) with characterization of composition can be obtained by mass spectrometers as well as UV spectrometers. Na, K and Ca densities of the Hermean exosphere can be obtained by ground-based observations. These exospheric observations cannot provide univoche information about the parent surface release process neither about the location of the release. Only speculations about asymmetries induced by solar wind-planetary magnetosphere interaction or preferential released species can be done to discriminate the ion sputtering process. Why is it important to discriminate the action of the ion sputtering? The ion-sputtering process is one of the main processes responsible for refractories release and escape. Its action could be much more important in the last phases of the Solar System formations (about 4.5 Gy ago) when the early Sun was emitting a 100-times more intense slar wind (Lammer et al. 2008) 2nd SERENA-HEWG meeting - Mykonos, June. 2009

SHEA The Sputtered High Energy Atoms (SHEA), that is, the high energy tail of the sputtered distribution (let’s say at energy E e >10 eV) are between few % and tens % of the total release depending mainly on the ion impacting energy. 2nd SERENA-HEWG meeting - Mykonos, June Directional neutrals Why do we wish to detect SHEA (neutrals at energies >10 eV) to investigate the ion sputtering process? Because below 10 eV the ion-sputtering product is negligible compared to other release process, and the particles do not maintain the initial direction, since the gravitational effects are not negligible. Energy distribution of the sputtered H 2 O particles emerging from the surface of Europa, in case of S + at 10 keV and 100 keV. Binding energy is assumed 0.45 eV.

(Environment Simulation Particles released at different energy ranges Energy range: <0.06 eV Energy range: eVEnergy range: eVEnergy range: eVEnergy range: eVEnergy range: eV 2nd SERENA-HEWG meeting - Mykonos, June SHEA detection provides a map of plasma precipitation regions and an imaging of particle emission from surface. We do not need to perform line-of-sight integration, hence the geometry of deconvolution is easier. The problem here is the number of unknown parameters: Y (surface mineralogy, E b,...), c, F ion (species, energy, impact angle,...).

Expected outcomes of SHEA observations: Mercury case 2nd SERENA-HEWG meeting - Mykonos, June. 2009

The Solar wind and IMF at Mercury The Parker spiral forms an angle of about 20° with the solar wind radial direction, less than half of the value at the Earth’s orbit (~45°); this implies a change of the relative ratio of the Interplanetary Magnetic Field (IMF) components with respect to the near-Earth conditions, and an increase of the weight of the IMF B x component. The average solar wind density is about ten times higher than that at the Earth, but this value varies considerably due to the high eccentricity of the orbit of the planet (from 32 cm -3 to 73 cm -3 ). B The IMF intensity changes by a similar factor. Bv v 2nd SERENA-HEWG meeting - Mykonos, June. 2009

Mercury has an internal magnetic field The Mariner 10 observations, confirmed by the first MESSENGER flyby, reveal the existence of an internal dipolar-like magnetic field (Ness et al, 1975). (Milillo et al, 2005) The estimated dipole moment ranges between 284 and 358 nT R M 3, nothward oriented. Hermean magnetosphere is only 5% of that of the Earth. In a zero order analysis, the spatial dimensions must be scaled by a factor of ~7 or 8 2nd SERENA-HEWG meeting - Mykonos, June. 2009

Solar wind entry in the Hermean magnetosphere Density of the solar wind protons in from 10 5 to m -3. The yellow lines represent the magnetic field (Kallio et al., 2008) The IMF orientation and intensity and the weak internal magnetic field of Mercury likely produce a direct entry through the cusps inside the Hermean dayside magnetosphere. Solar wind plasma circulates inside the magnetosphere and eventually may hit the surface. 2nd SERENA-HEWG meeting - Mykonos, June. 2009

SW precipitation The SW impacting region is in a first approximation the footprint of open field lines. Open field line maps on the Hermean surface depend on solar wind conditions. IMF B z <0 nT causes reconnection and symmetric SW entry. IMF B y variations cause longitudinal shifts. Strong P dyn causes a poleward extension of the mapped area. Higher energies and higher fluxes precipitate at lower latitudes (Massetti et al, 2003) B IMF =(0,0,-10) nT B IMF =(0,5,-10) nT P dyn =16 nPa B IMF =(0,5,-10) nT P dyn =60 nPa Flux(cm -2 s sr keV) -1 2nd SERENA-HEWG meeting - Mykonos, June. 2009

Color and albedo controlled by maturity and composition PC 2 interpreted to represent compositional variation Caloris Basin smooth plains Low albedo material Smooth plains Low albedo MNF (minimum noise fraction, similar to PC) as R-G-B Low albedo material “streak” From Robinson et al. LPSC nd SERENA-HEWG meeting - Mykonos, June Fm: Sprague et al., SERENA-HEWG meeting, Santa Fe, NM, May 2008

As well as for the cameras, a FOV perpendicular to the s/c motion is the most suitable for this kind of surface imaging. 2nd SERENA-HEWG meeting - Mykonos, June. 2009

Simulation of SHEA detection at Mercury by BepiColombo/MPO/SERENA-ELENA (Orsini et al, 2008) 2nd SERENA-HEWG meeting - Mykonos, June Thanks to the BepiColombo mission we will have simultaneous measurements of precipitating plasma (MIPA), surface composition (MIXS) and mineralogy (MERTIS) and neutral composition (STROFIO and PHEBUS) and SHEA observation (ELENA), for a complete investigation of the IS process.

Near Earth Object (NEO) have orbits with perihelion distances q <1.3 appear heterogeneous in shapes, sizes, spin rates and compositions are subjected to continuous interaction with the interplanetary medium: solar wind interaction with surface (ion sputtering), solar and cosmic ray bombardment, micrometeoroids gardening. NEO (433) Eros (John Hopkins University) The NEO superficial composition is modified by this space weathering 2nd SERENA-HEWG meeting - Mykonos, 8-11 June nd SERENA-HEWG meeting - Mykonos, June. 2009

Ion NEO Ion fluxes: Solar wind at 1 AU, 1-keV H +, flux 10 8 (cm -2 s -1 sr keV) The binding energy E b holding the regolith (strongly dependent from NEO type) ranges between 0.5 and 3-4 eV (Lammer et al. 2003). We consider 2 eV. The average yield Y, in this case is about 0.05 (Lammer et al. 2003). Bulk element abundances for CI type chondrites (Plainaki et al, 2008, adapted from Brown et al., 2000) 2nd SERENA-HEWG meeting - Mykonos, June. 2009

Sputtered fluxes of individual species The most important contribution to the total sputtered particle flux comes from sputtered H. Mg is also significantly lost for ion-sputtering action (Plainaki et al. 2008). m -2 s -1 2nd SERENA-HEWG meeting - Mykonos, June. 2009

Total sputtered flux from a CI type NEO m -2 s -1 Particles fluxes (up to particles m -2 s -1 ) appear in a region up to 1 km above the NEO surface (in the solar wind direction). The erosion rate f·ω is 0.3 Å/year, where ω= m 3 is the atomic volume in a solid substance (Plainaki et al., 2008). This result is similar to the estimation performed for the lunar surface, 0.2 Å/year, in case of solar wind sputtering (Starukhina, 2003). 2nd SERENA-HEWG meeting - Mykonos, June The SHEA (>10 eV) flux results in about 10 9 particles m -2 s -1.

ENA in the Jupiter environment SHEA from Europa and Ganymede C-E ENA from plasma – tori interaction C-E ENA from auroral regions 2nd SERENA-HEWG meeting - Mykonos, June. 2009

(Paranicas et al., GRL, 2002). Krupp et al 2001 showed that the radiation belts extends inside the Callisto’s while the energetic particle fluxes decrease abruptly out of these distances. Plasma precipitation on the inner moons’ surfaces produces SHEA. Giovian radiation belt Ganymede’s orbit Callisto’s orbit Europa’s orbit H+H+ O+O+ S+S+

Europa Surface composed mainly by water ice and other components as Na, K, etc... In fact, Na and K atmosphere has been observed (Brown and Hill, 1996; Brown, 2001) Traces of non-icy material (a hydrate compound of H 2 O 2 (0.13%), SO 2 and CO 2 ) (Tiscareno and Geissler, 2003; Johnson et al., 1983). 2nd SERENA-HEWG meeting - Mykonos, June. 2009

The surface composition of Europa  Highest albedo among all the Galilean satellites (Kuskov and Kronrod, 2005).  Probably geologically young surface (crater preservation yields an age estimate of ~10–100 Ma) (Moore et al., 1998).  Possible existence of liquid water beneath the outer icy crust that ranges from a few kilometers to at least a few tens of kilometers (McKinnon, 1999; Ruiz and Tejero, 2003) Disrupted ice crust in the Conamara region of Jupiter's moon Europa (NASA courtesy) 2nd SERENA-HEWG meeting - Mykonos, June. 2009

Ion Europa Ion fluxes based on values given by Cooper et al (2001) and Strazzulla et al. (2003) at energy about keV ParticleFlux (ions cm -2 s -1 ) H+H+ 1.5 ∙10 7 C+C+ 1.8 ∙10 6 O+O+ 1.5 ∙10 6 S+S+ 9∙10 6 The binding energy E b holding the ice molecules on the surface of Europa, is characterized by the sublimation energy of water 0.45 eV per molecule (Johnson, 1998). ParticleSputtering Yield (molecules/ion) H+H+ 5 C+C+ 10 O+O+ 50 S+S+ 30 The yield Y, are based on the results by Shi et al. (1995), Johnson (1990) and Ip et al. (1997) as well as on the Galileo observations, 2nd SERENA-HEWG meeting - Mykonos, June. 2009

Europa SHEA flux produced by S + impinging particles (Plainaki et al., Cospar meeting, 2008) particles/s/m 2 The estimated SHEA (>10 eV) signal at 200 km over Europa’s surface is f=10 7 (cm 2 s) -1. Flux (particles m -2 s -1 ) H+H+ O+O+ C+C+ S+S+ (Plainaki et al., EGU, 2009) 2nd SERENA-HEWG meeting - Mykonos, June. 2009

Similar energetic ion fluxes observed by Galileo spacecraft at Europa (Paranicas et al., GRL, 2002) are expected at Ganymede that is composed by a similar iced surface but it has the peculiarity of an internal magnetic field that could shield the plasma or define preferential entries and plasma precipitation regions Comparative observations 2nd SERENA-HEWG meeting - Mykonos, June From the NASA/ESA JOINT SUMMARY REPORT: EUROPA JUPITER SYSTEM MISSION On the contrary, the ion fluxes at Callisto are two order of magnitude lower. Similar observations in different environment like the two moons Europa and Ganymede would offer the chance to investigate the different satellites evolution in the Jupiter system.

Conclusions: Why is ion-sputtering investigation important? Interaction plasma-surface or plasma-atmosphere. Characterization of the surface properties. Investigation of the particles escape mechanisms. Plasma entry and circulation in planetary environments Characterization of the effect of space weathering on the surfaces. Eventually, investigation of the evolution of Solar system. 2nd SERENA-HEWG meeting - Mykonos, June. 2009

Conclusions: Why is SHEA imaging important as well as of surrounding gas analysis? The detection of particles above 10 eVs is a method to identify that the ion-sputtering is active and to define the region of its action. A joint observation of surronding gas and SHEA will permit to know where, when and how the ion sputtering release takes place and will permit a clearer estimation of the escaping material. 2nd SERENA-HEWG meeting - Mykonos, June. 2009

ENVIRONMENTS INVESTIGATED VIA SHEA 2nd SERENA-HEWG meeting - Mykonos, June ENCELADUS NEO

Thank you for your attention! 2nd SERENA-HEWG meeting - Mykonos, June. 2009