THE BEPICOLOMBO SERENA/ELENA INSTRUMENT: SENSOR

THE BEPICOLOMBO SERENA/ELENA INSTRUMENT: SENSOR AND TESTS DESCRIPTION,
SCIENTIFIC GOALS S. Orsini (1), S. Selci (2), A.M. Di Lellis (3), A. Mura (1), E. De Angelis (1), A. Milillo (1),
R. Leoni (4), I. Dandouras (5), J. Scheer (6), P. Wurz (6), C. Austin (5), M. Bassas(5) ,F. Bertani(2), D. Brienza(3),
F. Camozzi (7), S. Cibella (2), L. Colasanti (1), M. D’alessandro (2), M. Gerber (6), F. Lazzarotto (1), F. Mattioli
(4), D. Maschietti (3), S. Massetti (1), J.-L. Medale (5), H. Mischler (6), D. Piazza (6), R. Rispoli (1), F. Tominetti
(7), N. Vertolli (1), R. Wiesendanger (6). (1) INAF-IAPS via del Fosso del Cavaliere, 100, 00133 Roma, Italy
(contact: [email protected]), (2) ISC-CNR, ROME, Italy, (3) AMDL srl, ROME, Italy, (4) IFN-CNR, ROME,
Italy, (5) IRAP, Toulouse, France, (6) University of Bern, Switzerland, (7) CGS, Milano, Italy.
Introduction. The neutral sensor ELENA (Emitted
Low-Energy Neutral Atoms) for the ESA cornerstone
BepiColombo mission to Mercury (in the MPOSERENA instrument
package) is a new
kind of low energetic
neutral atoms instrument, mostly devoted
to sputtering emission
from planetary surfaces, from E=10 eV up
ELENA instrument schematics
to E=5 keV, within 1D (4.5°x76°). An
ENA detector is not included in the MESSENGER
payload; hence, the BepiColombo mission will provide
a unique opportunity to obtain images of the surface
escaping at Mercury and to perform useful investigations on the evolution of the planet induced by space
weathering.
Instrument. ELENA is a Time-of-Flight (TOF) system, based on oscillating shutter (operated at frequencies up to >50 kHz) and mechanical gratings: the incoming neutral particles directly impinge upon the
entrance with a definite timing (START) and arrive to
a STOP section after a flight path. The STOP consists
of 1-dimensional array composed by MCPs and a discrete anodes set corresponding to a Field of View
(FOV) of 4.5°x76°, allowing the reconstruction of
both velocity and direction of the incoming events.
The spacecraft footprint track will provide the second
dimension.
Shutter subsystem. The shutter consists of both fixed and
oscillating elements. In the
central region the self standing
silicon nitride membrane is
located. One membrane is
fixed while the second one is
moved respect to the other by
means of a piezoelectric actuaMembrane slits
tor, at a frequency up to >50
kHz. In this way the low-energy neutral particles are
directly detected, without using elements of interaction. This couple of nano-patterned self-standing silicon nitride membranes, one facing the other, are sepa-
rated by a distance between 1 and 5 μm, in order to
have the correct number
of time of flight channels. ELENA Si3N4
membranes are 10x10
mm2 wide, 1µm thick,
with slits of the order of
200nm and 1,4 μm
ELENA EQM shutter
pitch. The EQM ELEmounted in ELENA box with
NA shutter has been
Piezo-board
realized following several crucial steps: mechanical realization, membranes
production, mounting and alignment. The shutter has
been tested with both Ion and Neutral beams at several
frequencies.
Science goals. In the recent years, the ENA (Energetic
Neutral Atoms) detection technique has allowed new
scientific investigation in the solar system. The major
processes able to produce directional neutral atoms are
space plasma charge-exchange with exospheric gas
and ion-sputtering and -backscattering induced by
plasma impact on planetary surfaces. Among the surface release processes, the ion-sputtering is particularly
intriguing, since the involved energies induce escape
from the planet, with possible implications on its evolution. The ion-sputtering process is caused by the
impact of an energetic ion (in the keV range) on the
surface. The energy transfer generates a release of neutral atoms and in a minor part of ions [1] . The bulk of
neutral emission is in the few eV range, but has a significant high energy tail [2]. It is a localized and highly
variable release process, since the intensity of the released flux depends on plasma precipitating flux but
also on energy of impacting ions as well as on composition and mineralogy of the target [3].
Conclusions. The idea of space remote-sensing of the
released neutral particles in order to map the emission
from the surface was proposed for the first time for the
BepiColombo/MPO spacecraft to Mercury [4] [5].
Recent studies evidenced the role played by the solar
wind plasma interaction with the planet in the Hermean evolution [6]. In this perspective, the major
ELENA scientific objectives may be resumed as in the
following:
Simulation of the energy-integrated (between 20-1000 eV) sputtered O signal from vantage point MLT=1200, 45o elevation and
520 km altitude is shown in the upper panel. The horizon at in
this position is zoomed in the bottom-right panel and the slice of
ELENA FOV is evidenced in the bottom-left panel.
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Surface emission rate and release processes;
Particle loss rate from Mercury’s environment;
Remote sensing of the surface composition;
ENA imaging applications for comparative solarplanetary relationship.
The need to assess these ambitious scientific goals
within a so complex environment (strongly affected by
high albedo and thermal fluctuations) results in the
design of this innovative ENA instrument, not only in
the frame of the scientific goals achievable by ENA
detection, but also for the technology necessary to
detect the low energy neutral atoms sputtered from the
Hermean surface.
References. [1] Hofer, W.O. (1991), Sputtering by
Particle Bombardment, edited by R. Behrisch, K.
Wittmaack, 15–90. [2] Sigmund, P. (1969) Phys. Rev.
184, 383–416. [3] Lammer, H., P.Wurz, M.R. Patel,
R. Killen, C. Kolb, S. Massetti, S. Orsini, A. Milillo,
(2003), Icarus, 166, 238–247. [4] Massetti, S., S.
Orsini, A. Milillo, A. Mura, E. De Angelis, H.
Lammer, P. Wurz (2003), Icarus, 166, 229-237. [5].
Orsini, S., S. Livi, K. Torkar, S. Barabash, A. Milillo,
P. Wurz, A.M. Di Lellis, E. Kallio and the SERENA
team (2010), Planetary and Space Science, 58, 1-2,
166-181, 2010. [6]. Mura, A., P. Wurz, H., I.M.
Lichtenegger , H. Schleicher, H. Lammer , D.
Delcourt, A. Milillo, S. Orsini, S. Massetti, M. L.
Khodachenko (2009), Icarus 200, 1–11.