2. The Goldel Age of Solar System Exploration Stefano Orsini Settembre 2012 OUTLINE General scientific aspects: Mercurys Exosphere.General scientific aspects: Mercurys Exosphere. Surface emission: Ion Sputtering and BackscatteringSurface emission: Ion Sputtering and Backscattering ELENA Instrument description (part of the BepiColombo/SERENA suite)ELENA Instrument description (part of the BepiColombo/SERENA suite)

3. Plasma ions keV ENA Plasma-airless bodies interaction Charge-Exchange SurfaceIon-Sputtering 10s eV ENAions SurfaceIon-Backscattering 100s eV ENA The Goldel Age of Solar System Exploration Stefano Orsini Settembre 2012

4. 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 The Goldel Age of Solar System Exploration Stefano Orsini Settembre 2012

5. Solar wind impact on a surface An ion impacting (1) on the surface produces different effects: (2) back scattering, (3) neutralization and back scattering, (4) surface ion sputtering (5) electron emission (6) photon emission (7) adsorption (8) chemical sputtering: displacement and change of the surface mineralogy The Goldel Age of Solar System Exploration Stefano Orsini Settembre 2012

6. Reflection Backscattering Ion sputtering Chemical sputtering. Solar wind impact on a surface The Goldel Age of Solar System Exploration Stefano Orsini Settembre 2012

7. Surface Ion Sputtering process Ion sputtering products depend on: the composition and the chemical structure of the surface; the impinging plasma flux. Surface release of neutrals due to energetic ion impacts The Goldel Age of Solar System Exploration Stefano Orsini Settembre 2012

8. Back-scattering from the Moon The BS energy spectrum goes up to almost 1 keV (Wieser et al., 2009) The backscattering reflection yield is between 10-20% (Mc Comas et al., 2009) The Goldel Age of Solar System Exploration Stefano Orsini Settembre 2012

9. Energy distribution of sputtered particles H escape O escape Ca escape Fe escape CaOHFe The Goldel Age of Solar System Exploration Stefano Orsini Settembre 2012

10. Surface release processes Different release processes can have different efficiencies as a function of latitude and longitude/LT at Mercury due to surface compositions and mineralogy together with external conditions, as solar irradiance or plasma precipitation. (from Killen and Ip, 1999) The Goldel Age of Solar System Exploration Stefano Orsini Settembre 2012

11. Generally, the upper part of the atmosphere where the column density is so low (less than 10 14 cm -2 ) that the collision frequency between particles becomes negligible. Exobase is the boundary between collisional and not collisional regimes (Earth case: about 500 km altitude). In the case of absence of an atmosphere (Mercury case), we refer to surface-bounded exospheres. It is the result of a complex dynamic equilibrium with the surrounding systems, mainly: surface, magnetosphere and outer space. In this case the exobase is considered the surface itself. Exosphere The exosphere is the boundary between the planet and the open space. So its investigation is a way to know the mechanisms and interactions acting today as a proxy of what happened in the ancient times. The Goldel Age of Solar System Exploration Stefano Orsini Settembre 2012

12. Sputtered High Energy Atoms 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 products mix to other release process, and the particles do not maintain the initial direction (due to gravitational effects) so that the emission location on the surfaces cannot be easily recognized (Milillo et al., 2011). The Goldel Age of Solar System Exploration Stefano Orsini Settembre 2012

13. (Environment Simulation Tool@IFSI) The Goldel Age of Solar System Exploration Particles released at different energy ranges Energy range: <0.06 eV Energy range: 0.06-0.3 eVEnergy range: 0.3-1.5 eVEnergy range: 1.5-10 eVEnergy range: 10-40 eVEnergy range: 40-200 eV SHEA detection provides a map of plasma precipitation regions and an imaging of particle emission from surface. Stefano Orsini Settembre 2012

14. Instrument package on board BepiColombo/MPO SERENA Search for Exospheric Refilling and Emitted Natural Abundances Units: ELENA :Emitted Low-Energy Neutral Atoms STROFIO : Start from a ROtating FIeld spectrOmeter MIPA : Miniature Ion Precipitation Analyser PICAM : Planetary Ion CAMera The Goldel Age of Solar System Exploration Stefano Orsini Settembre 2012

15. The Goldel Age of Solar System Exploration Stefano Orsini Settembre 2012 ELENA is a Time-of-Flight (TOF) system, based on oscillating shutter (operated at frequencies up to a 100 kHz) and mechanical gratings: the incoming neutral particles directly impinge upon the entrance with a definite timing (START) and arrive to a STOP detector after a flight path. In this way the low-energy neutral particles are directly detected, without using elements of interaction. ParameterActual Energy range<0.02- 5 keV (mass dependent) Velocity resolution v/v Down to 10% Viewing angle4.5 o x76 o Angular resolution4.5 o x4.5 o (actual) 4.5°x2.4° (nominal pixel) Mass resolution M/ M H and heavy species Optimal temporal resolution40 s Geometric factor G2. 10 –5 cm 2 sr Integral Geometric factor6 10 –4 cm 2 sr The ELENA Instrument

16. The Goldel Age of Solar System Exploration Stefano Orsini Settembre 2012 ELENA START Section: shutter system The shuttering element of the ELENA detector consists in a couple of nano- patterned self standing silicon nitride membranes, one facing the other and separated by a distance between 1 and 5 μm, in order to have the correct number of time of flight channels. One membrane is fixed while the second one is moved respect to the other by means of a piezoelectric actuator, at a frequency up to 50 kHz. ELENA membranes are of 10x10 mm 2 area, 1µm thick Si 3 N 4 with slits of the order of 200nm and 1,4 mm pitch.

17. SERENA PM#19, Milan, 22-23 May, 2012 ELENA shutter Static and dynamic High-frequency test with ion beam Dynamic case: V=23V: freq=42kHz Static case: 0-100V (0-6 m) piezo elongation : 3 apertures at step 1,4 m. Static test: verifies the 1.4mm pitch of the membranes; it confirms the open/closed condition inside the 24 Volt elongation available. Dynamic test: up to 52kHz (without damage) in the requested voltage range (DV24V). The offset has to be optimized to see the double aperture in the oscillation.

18. SERENA PM#19, Milan, 22-23 May, 2012 Static test 1keV He + beam Dynamic test (10kHz) 1keV H beam Dynamic test (52 kHz) 2keV He + beam

19. The Goldel Age of Solar System Exploration Stefano Orsini Settembre 2012 MCP Efficiency Efficiency measurement for the standard MCP with Hydrogen, Helium and Oxygen at energies from 10 to 1000 eV. We can observe a good agreement with the only existing data on this argument in the range of 30- 1000 eV (Stephen et al, 2000; Peko et al., 2000)

20. The Goldel Age of Solar System Exploration Stefano Orsini Settembre 2012 Time-of-flight resolution for a membrane hole of 200 nm, 0.7 um of amplitude, and 1.5 um of membrane separation (those parameters are best expectations from technological development) Expected ToF resolution is 1/8.

21. The Goldel Age of Solar System Exploration Stefano Orsini Settembre 2012 Left panel: analytical prediction of countrates for Ion Sputtering signal. Right panel: Montecarlo simulation of countrates.

22. The Goldel Age of Solar System Exploration Stefano Orsini Settembre 2012 Left panel: analytical prediction of countrates for Ion Back-scattering signal. Right panel: Montecarlo simulation of countrates.

23. The Goldel Age of Solar System Exploration Stefano Orsini Settembre 2012 Noise – Background due to the neutral generation of ion-sputtering and back- scattering inside the instrument.

24. Simulation of SHEA detection at Mercury by BepiColombo/MPO/SERENA-ELENA (Orsini et al, 2008) Thanks to the BepiColombo/SERENA package we will have simultaneous measurements of precipitating plasma (MIPA), circulating plasma (PICAM), and neutral gas composition (STROFIO) and SHEA observation (ELENA), for a complete investigation of the IS process. The Goldel Age of Solar System Exploration Stefano Orsini Settembre 2012

25. The Goldel Age of Solar System Exploration Stefano Orsini Settembre 2012 IN SUMMARY….. The exosphere of Mercury is the region where the interaction between planet and Solar System environment occurs. The SERENA particle suite is designed for observing this region along the BepiColombo MPO Orbit. In particular, ELENA will be able to measure the planets surface emission via Ion Sputtering and Backscattering, thus providing an image of the ion precipitation areas, together with an estimate of the soil emitted elements.