serena

search for exospheric refilling and emitted natural abundances

science goals

SERENA (http://serena.iaps.inaf.it/) is an experiment composed of four sensors of complementary neutral and ionised particle detectors on board the BepiColombo/Mercury Planetary Orbiter (MPO). It investigates Mercury's complex particle environment that is a tightly-coupled system where the magnetosphere, exosphere, and surface experience temporal and spatial variations linked to each other. The particle environment is composed of thermal and directional neutral atoms (exosphere) originating from surface through release and charge-exchange processes, and of ionized particles caused by entry of the solar wind particles into the magnetosphere, photo-ionisation of neutrals as well by charge exchange and surface release processes. SERENA provides information on the whole surface-exosphere-magnetosphere system and the involved processes, plus the interactions between energetic particles, the solar wind, micrometeorites and the interplanetary medium.


 

More specifically, STROFIO measures in-situ the composition and density of low-energy neutral particles within the exosphere; ELENA covers the higher energy spectrum of neutral particles released from the surface; MIPA monitors the solar wind particle dynamics inside the magnetosphere, whereas PICAM is optimized to measure the planetary ions. Each unit operates individually to achieve specific scientific objectives of its own, however, operating simultaneously allows to cover the complex scientific objectives as illustrated in the above figure, and allows for additional objectives.

science objectives from the mission Science Traceability Matrix - TMX:

  • Chemical and elemental Composition of the Exosphere
  • Exospheric distribution, variability and circulation
  • Particles and Photons Interaction with the Surface
  • Solar Activity effect on exosphere
  • Meteoroid space weathering and exosphere
  • Dynamics and kinetic processes in the magnetosphere
  • Solar wind – Magnetosphere interactions and structure
  • Volatiles and their cycles

​measurement principle (per sub-unit)

ELENA (Emitted Low-Energy Netrual Atoms) is a neutral particle camera that investigates neutral gases escaping from the surface of Mercury, their dynamics and the processes responsible for such a population. ELENA has a field of view of 4.5ºx76º, directed towards the surface of Mercury (nadir) with angular resolution of 4.5º x 4.5º. The second dimension of the map will be provided by the movement of the spacecraft. ELENA measures energetic neutrals between 20 eV – 5 keV. The ELENA measurements will allow mapping neutral particle released from the surface after ion back scattering and ion sputtering, as well as charge-exchange ENA generated from exosphere interaction with the ion close to the surface.


 

ELENA technological highlights

The basic concept of ELENA is based on direct particle detection removing the background signal and leaving the Energetic Neutral Atoms (ENA) untouched, so that the directionality is preserved. The composite radiation made by neutral atoms, ions, and light impinges onto the ELENA sensor entrance. A powered wired grid system at the entrance minimizes the ion and electron background radiation. After that a nanometric grid filter the UV radiation. An internal ion deflector is a stack of particle cross-track grids which introduce a transversal E-field able to filter out the bulk of the charged particles of both polarities. The unit defines intensity and direction of the incoming particles.

 

STROFIO (sTart from a ROtating Field mass spectrOmeter) is a highly sensitive neutral mass spectrometer designed to characterise the chemical composition of Mercury’s exosphere. It utilises a continuous time-of-flight (TOF) technique to determine the mass-to-charge (m/q) ratio of neutral particles entering its aperture. Strofio has a sensitivity of 0.14 counts/s at a density of 1 particle/cm3 and achieves a mass resolution (m/∆m) of 60 at mass peak 18 (H2O).


STROFIO technological highlights

Neutral particles enter the instrument through one of two apertures, where they are ionised via electron-impact ionisation. The resulting ions are then guided and accelerated into the detector region by an array of programmable electrodes. Upon reaching the detector, the ions encounter a rotating electric field that applies a Coulomb force, bending their trajectories. This bend occurs in a specific plane, marking their start time. The ions traverse the remainder of the detector region until they impact a microchannel plate (MCP) detector, which records both their position and stop time. The rotating electric field operates continuously, ensuring a 100% duty cycle for the detection system.

MIPA (Miniature Ion Precipitation Analyser) is an ion monitor sensor that measures the ion distribution function over the range of 10 eV to 13.5 keV with moderate mass resolution within a 80° x 360° (hemispherical) field-of-view. The size of the directional pixels varies from 5° x 15° to 40° x 20°. The geometrical factor of MIPA is optimised for monitoring very high fluxes (up to 10⁹ particles/cm²·s·sr). MIPA measures a single direction and energy instantaneously, with a sampling time of 7.2 ms. The instrument concept includes an electrostatic analyser selecting the energy per charge and a time-of-flight (TOF) cell providing velocity information. The full measurement cycle, covering all energy steps and directions, takes 20 seconds, including processing time.


 

MIPA technological highlights

Using innovative time-of-flight technology (advanced for the year 2003), based on surface–particle interaction, MIPA mass was reduced to 610 g. The deflector for directional analysis employs a novel design consisting of three funnel-like electrodes. The ingenious approach solves the problem of the thermal trap created by the deflector geometry. It incorporates a perforated conical radiator, a titanium decoupling tube, and a unique coating, enabling the electronics to operate at moderate temperatures even when the deflector temperature reaches up to 400°C.

 

PICAM (Planetary Ion CAMera) is a mass spectrometer operating as an all-sky camera for positively charged particles. On BepiColombo/MPO, it aims at studying the chain of processes by which neutrals are ejected from the surface of Mercury, ionised and transported through the environment surrounding the planet.

The design of its electrostatic analyser facilitates measurements for the 3D velocity distribution and mass spectrum for ions over a 1.5π Field of View (FoV). It covers ions up to ~ 3 keV energies, with mass range extending up to ~ 132 a.m.u.


 

PICAM technological highlights

PICAM has an instantaneous 1.5π FoV detection, which drastically improves the time-resolution of the measurement, in comparison with the conventional FoV scanning method. For mass spectrometry, PICAM measures the impact-free Time-of-Flight (ToF) of ions, using a single-pulse or Hadamard gating technique. The wide mass range and a mass resolution better than ~ 50 is a unique capability, which provides a superior performance in the frame of the MPO science objectives.

SCU (System Control Unit) is inside the ELENA box and manage all the interfaces of SERENA sensors with the spacecraft: TC and TM, power distribution and emergency signals.

 

FIRST RESULTS

During cruise, MIPA and PICAM could perform science measurements. They operated during the planetary flybys, and for the Heliospheric Solar Wind science campaigns.

 

 

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