The exploration of habitable environments on or inside icy moons around the gas giants in the solar system is of major interest in upcoming planetary missions. Exactly this theme is addressed by the JUpiter ICy moons Explorer (JUICE) mission of ESA, which will characterise Ganymede, Europa and Callisto as planetary objects and potential habitats.
We developed a prototype of the Neutral Gas and Ion Mass spectrometer (NIM) of the Particle Environment Package (PEP) for the JUICE mission intended for composition measurements of neutral gas and thermal plasma. NIM/PEP will be used to measure the chemical composition of the exospheres of the icy Jovian moons. Besides direct ion measurement, the NIM instrument is able to measure the inflowing neutral gas in two different modes: in neutral mode, where the gas enters directly the ion source (open source), and in thermal mode, where the gas gets thermally accommodated to the wall temperature by several collisions inside an equilibrium sphere, called antechamber, before entering the ion source (closed source).
We performed measurements with the prototype NIM using a neutral gas beam of
1 up to 4.5 km s
The NIM prototype was successfully tested under realistic JUICE mission conditions. In addition, we find that the antechamber (closed source) behaves as expected with predictable density enhancement over the specified mass range and within the JUICE mission phase velocities. Furthermore, with the open source and the closed source we measure almost the same composition for noble gases, as well as for molecules, indicating no additional fragmentation of the species recorded with the antechamber for the investigated parameter range.
JUpiter ICy moons Explorer (JUICE) is an L-class mission of ESA, which will
investigate and characterise Ganymede, Europa and Callisto as planetary
objects and potential habitats (JUICE Team, 2012; Grasset et al., 2013). The
current trajectory of the JUICE spacecraft foresees a flyby velocity of
4 km s
Photograph of CASYMIR calibration facility in clean room area (class 100) with attached UHV chamber for the NIM prototype in the rear.
The Particle Environment Package (PEP) carried by JUICE combines remote global imaging with in situ measurements to study the atmospheres, plasma environments, and magnetospheric interactions and to determine global surface composition and chemistry, especially as related to habitability (Barabash et al., 2013).
NIM, the Neutral Gas and Ion Mass spectrometer, is part of the PEP suite and will be used to measure the chemical composition of the regular atmosphere produced by sublimation, energetic particle bombardment and photon interaction with the surface of the icy Jovian moons (e.g. Wurz et al., 2014; Vorburger et al., 2015). The NIM measurements include volatile species, contributions from non-ice material on the surface and the isotopic composition of major species. In addition, the ion composition of the ionospheres will be measured by direct ion measurement of NIM (ion mode).
We developed a prototype of the NIM instrument, part of PEP, for the JUICE
mission and performed measurements with the prototype instrument using a
neutral gas beam of 1 up to 4.5 km s
To obtain a neutral gas beam at reasonable velocities, the CASYMIR
(calibration system for the mass spectrometer instrument ROSINA) calibration
facility has been used in combination with the UHV chamber, housing the NIM
prototype. The scope of the CASYMIR project was the development of a
calibration chamber with the aim of testing and calibrating the two ROSINA
mass spectrometers for the Rosetta mission (Graf et al., 2004). CASYMIR
consists of a vacuum system with several pumping stages leading to an
ultra-high vacuum of a few 10
Scheme of NIM prototype illustrating its three operational modes.
NIM is a time-of-flight instrument with heritage from the RTOF sensor of the ROSINA instrument on the Rosetta mission (Scherer et al., 2006; Balsiger et al., 2007) and the P-BACE instrument (Abplanalp et al., 2009). The NIM is designed to operate in three different modes:
Photograph of NIM prototype with cabling and rotation mechanism.
In the thermal mode (th-mode), the neutral gas is decelerated
from spacecraft velocity down to thermal energies by an equilibrium sphere,
called antechamber, and passed on into the ion source in thermal state
(th In the neutral mode (n-mode), the neutral gas (n In the ion mode (i-mode), thermal ions from the ambient plasma (i
These three different modes are illustrated in Fig. 2 together with the
entire ion-optical scheme of the NIM prototype instrument. In ion mode
(i-mode) the ions from the ambient plasma enter directly into the ion source
with spacecraft velocity in ram direction. In neutral mode (n-mode) the
neutral gas is also entering directly into the ion source with spacecraft
velocity in ram direction, which is a so-called open source. The neutral gas
is then ionised around the central axis of the ion source by an electron beam
produced by a filament and the generated ions are deflected by ion optics
towards ion source exit. In thermal mode (th-mode) the neutral gas is first
entering a sphere and then decelerated by hitting the interior wall many
times, thus establishing thermal equilibrium of the gas with the walls of the
antechamber. This is exactly the principle of an antechamber, which is a
so-called closed source. The thermalised neutral gas from the antechamber
enters the ion source, where it is ionised by the electron beam and the
generated ions are again deflected by ion optics towards ion source exit.
Scheme of neutral gas beam entry in th-mode at CASYMIR measurements. View from top; see Fig. 2.
Scheme of neutral gas beam entry in n-mode at CASYMIR measurements. View from top; see Fig. 2.
The advantage of operation in th-mode is the large field of view for the neutral gas entering a small hole in the antechamber, which covers basically a half-sphere opening angle. This allows measurement not only near closest approach of a flyby but also at farther distances where the ram direction is outside the narrow field of view of the neutral mode. Furthermore, thermalised neutrals do not possess large energy spread (constant energy instead of constant velocity), which results in best possible performance in terms of mass resolution and transmission. The disadvantage of operation in th-mode is the complex interactions of the incoming species with the inner wall material of the sphere resulting in fragmentation and chemical alteration. However, the measurements obtained in th-mode can then be compared with those obtained by the n-mode at closest approach of a flyby or in orbit phase. Furthermore, inter-calibrations between th-mode and n-mode can be done in the laboratory.
Ratio of signal in thermal mode to signal in neutral mode as
function of beam velocity in
Figure 3 shows a photograph of the NIM prototype with cabling and rotation
mechanism, which covers a measurement range of 180
Because of the thermalisation of the incoming gas, the antechamber produces
a density enhancement compared to the open source, which can be calculated
as follows (Wurz et al., 2007):
The results of azimuthal rotation campaign conducted in CASYMIR
facility. Panels
Measurements with the NIM prototype, built into the UHV chamber, using a
neutral gas beam of realistic velocities in the n-mode (open source) and
th-mode (closed source) have been performed. To obtain a neutral gas beam at
realistic velocities, the CASYMIR facility has been used (see Sect. 2.1).
Different species and gas mixtures are used, such as noble gases Ne, Ar, Kr
as well as molecules like H
The used measurement characteristics are discussed in the following. On the
one hand, Fig. 4 shows the scheme of neutral gas beam entry in th-mode at the
CASYMIR measurements, where the neutral gas beam with Gaussian beam profile
impinges on the antechamber opening hole with 4 mm diameter, which can be
rotated
On the other hand, Fig. 5 shows the scheme of neutral gas beam entry in
n-mode for the CASYMIR measurements, where the neutral gas beam with Gaussian
profile enters the open (line-of-sight) entrance rings, which can be rotated
Generally, the Gaussian gas beam, with width of about 11 mm at instrument
entrance, has a flux of about
For the beam velocity campaign, measurements with all stated gas mixtures at
different velocities (1 up to 4.5 km s
Isotope analysis results of mass spectra from CASYMIR measurements, obtained by summing up 100 000 individual spectra.
The measured c–o ratio is in good agreement with the calculated density
enhancement for noble gases like Ne and Kr, as well as molecules like
CH
For the azimuthal rotation campaign, measurements of all stated gas mixtures
at different azimuthal entrance angles (
In the upper right panel b of Fig. 7, the th-mode area is plotted for
different species in the measured gas mixtures. The solid lines give again
the expected behaviour, which follows a cosine of the entrance angle
modulated by the Gaussian beam profile. In space, the curves are expected to
be purely cosine of the angle between entrance aperture of the antechamber
and the spacecraft velocity, since the incoming gas is equally distributed in
space (at least locally) and has in principle a flat profile. However, the
th-mode measurements are in good agreement with the expected profile around
Typical mass spectrum of a th-mode measurement of a gas mixture of
H
The measured species from the residual gas in the vacuum chamber are expected
to be independent of the azimuthal rotation angle for both the n-mode and
the th-mode, as is shown by the measurement of the residual gas species
N
For the isotope analysis, measurements with all stated gas mixtures at
velocities between 2 and 3 km s
In the middle panel b of Fig. 8, the fragmentation pattern normalised to the
total abundance is plotted for the molecule C
A typical mass spectrum, measured in th-mode with a gas mixture of H
The NIM prototype has been successfully tested under realistic JUICE mission conditions. In addition, the antechamber (closed source) behaves as expected with predictable density enhancement over the specified mass range and within the JUICE mission phase velocities. Moreover, n-mode (open source) and th-mode (closed source) measure almost the same composition for noble gases, as well as for molecules, indicating no additional fragmentation of the species inside the antechamber. This holds for both versions of the antechamber, the DLC-coated one and the gold-coated one, respectively, which were both successfully tested.
The data used in this paper can be found in the Supplement.
The authors would like to acknowledge the contribution of a number of people helping in technical preparation of the NIM prototype for the presented investigations, including Stefan Brüngger, Philippe Németh and mechanical workshop, as well as Matthias Lüthi and electronics workshop. The financial support from Swiss National Science Foundation and the PRODEX programme of the Swiss Space Office is acknowledged.Edited by: G. Kargl Reviewed by: J. De Keyser