# Moons and Jupiter Imaging Spectrometer (MAJIS) on Jupiter Icy Moons Explorer (JUICE)

F. Poulet, G. Piccioni, Y. Langevin, C. Dumesnil, L. Tommasi, V. Carlier, G. Filacchione, M. Amoroso, A. Arondel, E. D'aversa, et al.

► To cite this version:

F. Poulet, G. Piccioni, Y. Langevin, C. Dumesnil, L. Tommasi, et al. Moons and Jupiter Imaging Spectrometer (MAJIS) on Jupiter Icy Moons Explorer (JUICE). Space Science Reviews, 2024, 220 (3), pp.27. 10.1007/s11214-024-01057-2. hal-04519237

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# Moons and Jupiter Imaging Spectrometer (MAJIS) on Jupiter Icy Moons Explorer (JUICE)

F. Poulet¹ · G. Piccioni² · Y. Langevin¹ · C. Dumesnil¹ · L. Tommasi³,² · V. Carlier¹ ·

G. Filacchione² · M. Amoroso⁴ · A. Arondel¹ · E. D’Aversa² · A. Barbis³ · A. Bini³ ·

D. Bolsée⁵ · P. Bousquet⁶ · C. Caprini³ · J. Carter¹ · J.-P. Dubois¹ · M. Condamin¹ ·

S. Couturier¹ · K. Dassas¹,⁷ · M. Dexet¹ · L. Fletcher⁸ · D. Grassi² · I. Guerri³ · P. Haffoud¹ ·

C. Larigauderie⁶ · M. Le Du⁶ · R. Mugnuolo⁴ · G. Pilato³ · M. Rossi³ · S. Stefani² · F. Tosi² ·

M. Vincendon¹ · M. Zambelli² · G. Arnold⁹,¹⁰ · J.-P. Bibring¹ · D. Biondi² · A. Boccaccini² ·

R. Brunetto¹ · A. Carapelle¹¹ · M. Cisneros González⁵ · C. Hannou¹ · O. Karatekin⁵ ·

J.-C. Le Cle’ch¹ · C. Leyrat¹² · A. Migliorini² · A. Nathues¹³ · S. Rodriguez¹⁴ · B. Saggin¹⁵ ·

A. Sanchez-Lavega¹⁶ · B. Schmitt¹⁷ · B. Seignovert¹⁸ · R. Sordini² · K. Stephan⁹ ·

G. Tobie¹⁸ · F. Zambon² · A. Adriani² · F. Altieri² · D. Bockelée¹² · F. Capaccioni² ·

S. De Angelis² · M.-C. De Sanctis² · P. Drossart¹⁹ · T. Fouchet¹² · J.-C. Gérard²⁰ ·

D. Grodent²⁰ · N. Ignatiev²¹ · P. Irwin⁸ · N. Ligier¹ · N. Manaud¹ · N. Mangold¹⁸ ·

A. Mura² · C. Pilonget¹,²² · E. Quirico¹⁷ · E. Renotte¹¹ · G. Strazzulla²³ · D. Turrini²⁴ ·

A.-C. Vandaele⁵ · C. Carli² · M. Ciarniello² · S. Guerlet¹²,²⁵ · E. Lellouch¹² ·

F. Mancarella²⁶ · A. Morbidelli²⁷ · S. Le Mouélic¹⁸ · A. Raponi² · G. Sindoni⁴ · M. Snels²⁸

Received: 15 September 2023 / Accepted: 23 February 2024
© The Author(s) 2024

## Abstract

The MAJIS (Moons And Jupiter Imaging Spectrometer) instrument on board the ESA JUICE (JUpiter ICy moon Explorer) mission is an imaging spectrometer operating in the visible and near-infrared spectral range from 0.50 to 5.55 µm in two spectral channels with a boundary at 2.3 µm and spectral samplings for the VISNIR and IR channels better than 4 nm/band and 7 nm/band, respectively. The IFOV is 150 µrad over a total of 400 pixels. As already amply demonstrated by the past and present operative planetary space missions, an imaging spectrometer of this type can span a wide range of scientific objectives, from the surface through the atmosphere and exosphere. MAJIS is then perfectly suitable for a comprehensive study of the icy satellites, with particular emphasis on Ganymede, the Jupiter atmosphere, including its aurorae and the spectral characterization of the whole Jupiter system, including the ring system, small inner moons, and targets of opportunity whenever feasible. The accurate measurement of radiance from the different targets, in some case particularly faint due to strong absorption features, requires a very sensitive cryogenic instrument operating in a severe radiation environment. In this respect MAJIS is the state-of-the-art imaging spectrometer devoted to these objectives in the outer Solar System and its passive cooling system without cryocoolers makes it potentially robust for a long-life mission as JUICE is. In this paper we report the scientific objectives, discuss the design of the instrument including its complex on-board pipeline, highlight the achieved performance, and address the observation plan with the relevant instrument modes.

Extended author information available on the last page of the article
**Keywords** JUICE · Jupiter · Jovian satellites · Imaging spectroscopy · Visible · Infrared

**Acronyms**

ASI Agenzia Spaziale Italiana

ASIC Application Specific Integrated Circuit

BPF Band Pass Filter

CFRP Carbon-fiber-reinforced polymers

CNES Centre National d'Etudes Spatiales, the French space Agency

CPCU Command and Process Control Unit

CU Compression Unit

DPU Digital Processing Unit

EGSE Electrical Ground Segment Equipment

EMC Electro-Magnetic Compatibility

ESTEC European Space Research and Technology Centre

E2E End-to-End

FFT Full Functional Tests

FM Flight Model

FOV Field of view

FPA Focal Plane Array

FPGA Field-Programmable-Gate-Array

FPE Focal Plane Electronics

FPF Focal Plane Flex

FPU Focal Plane Unit

GCO Ganymede Circular Orbit

GEO Ganymede Elliptical Orbit

GRS Great Read Spot

HK HouseKeeping

IAS Institut d'Astrophysique Spatiale

IAPS Istituto Di Astrofisica e Planetologia Spaziali

IASB Institut royal d'Aéronomie Spatiale de Belgique

ICU Internal Calibration Unit

IDOC Integrated Data and Operation Center

ILS Instrument Line of Sight

INAF Istituto nazionale di astrofisica

ITF Instrument Transfer Function

JANUS Jovis Amorum ac Natorum Undique Scrutator

JIRAM Jovian Infrared Auroral Mapper

JUICE JUpiter ICy moons Explorer

JWST James Webb Space Telescope

LDO Leonardo Company

LESIA Laboratoire d'études spatiales et d'instrumentation en astrophysique

LOS Line-Of-Sight

LOPRI Low Priority

LTE Local Thermal Equilibrium

LVF Linear Variable Filter

ME Main Electronics

MISE Mapping Imaging Spectrometer for Europa

MLI Multi-Layer Insulation
MOC JUICE Mission Operation Center

MOS MAJIS Operation Simulator

MSW Mission Software

NEB North Equatorial Belt

NECP Near-Earth Commissioning Phase

NIMS Near-Infrared Mapping Spectrometer

NSDP Native Science Data Packet

NTB North Temperate Belt

NYS No Yaw Steering

OBC On-Board Computer

OH Optical Head

PCDU Power Conversion and Distribution Unit

PE Proximity Electronics

PFM Proto Flight Model

PSA Planetary Science Archive

RAM Random Access Memory

ROB Royal Observatory of Belgium

ROI Region Of Interest

ROIC Read-out Integrated Circuit

RON Read-Out Noise

S/C Spacecraft

SciRD Science Requirements Document JUI-EST-SGS-RS-149 001

SEBD South Equatorial Belt

SGS MAJIS Science Ground Segment

SIS Spacecraft Interface Simulator

SLI Single Layer Insulation

SpW SpaceWire

SSMM Solid State Mass Memory

STM Structural Model

SWI Submillimetre Wave Instrument

TC/TM Telecommand/Telemetry

TID Total Ionizing Dose

UARF Unit Alignment Reference Frame

URF Unit Physical Reference Frame

UVS UltraViolet Spectrometer

# 1 Introduction

The JUICE mission selected in 2012 is the first large-class mission under the Cosmic Vision Programme of the European Space Agency (Grasset et al. 2013). After successful launch on 14 April 2023 followed by a 8.25-year cruise and insertion into Jupiter's orbit in July 2031, it will provide a comprehensive survey of the Jupiter system, with special emphasis on the three ocean-bearing worlds, Ganymede, Europa, and Callisto as planetary bodies and potential "habitats" for the two first. The Galilean satellites exemplify solar system objects with shared origins and initial conditions, yet exhibiting an extreme diversity today, reflecting the complexity of the processes which shaped their evolution. Their detailed analysis therefore offers potential breakthroughs in the evolutionary processes of solar and stellar systems.
Ganymede has been selected for enhanced investigation with 12 flybys (including 8 close flybys for which the spacecraft altitude will be below 1500 km) followed by a 9-month orbital phase starting in December 2034; Ganymede will thus provide an unprecedented target for deciphering the nature, evolution and potential habitability of icy worlds, while enabling the study of its unique magnetic and plasma interactions with the surrounding Jovian environment. For Europa, with two planned flybys, the focus will be on the surface and sub-surface chemistry thought to be essential for life, with an emphasis on the detection of organic molecules and endogenic species. The characterization of specific surface features with a special attention on the most active zones could lead to the identification of potential candidate sites for future in situ exploration. By observing the ancient highly cratered surface of Callisto during 21 flybys, JUICE will also add new constraints on a possible subsurface ocean as well as on the physical processes that occurred in the early Jovian system, leading to the unique erosion and gardening of its densely cratered plains. The study of the diversity of the satellite system will be enhanced by additional remotely sensed information on Io, the smaller moons and the rings. During the nominal ~3.5-year duration of the Jovian tour, the mission will also focus on characterizing Jupiter's atmosphere (circulation, meteorology, chemistry and structure from the cloud tops up to the thermosphere) and magnetosphere (three-dimensional properties of the magnetodisc and coupling processes), and their interaction with the Galilean satellites. These extended observations of the planet will last long enough to monitor the evolving weather systems and the mechanisms that transport energy, momentum and material between the different layers. JUICE science objectives and an overview of the S/C and mission operations are described in detail in the respective other articles of this collection.

MAJIS is a visible and infrared hyperspectral imager operating from 0.5 µm to 5.5 µm. It is one of the four remote sensing instruments on the JUICE payload (see the respective other articles of this collection). This set of instruments will perform remote sensing observations from UV to millimeter wavelengths to characterize the geology, the surface composition and the exospheres of satellites as well as the Jovian atmosphere at high spatial and spectral resolution.

The MAJIS instrument is based on a heritage from similar instruments, most developed by the same partners, onboard previous and currently active planetary missions. However, MAJIS benefits from several technical improvements (in particular the number of spatial and spectral elements) customized to achieve the unique goals of the JUICE mission. A similar instrument (Mapping Imaging Spectrometer for Europa – MISE, Blaney et al. 2023) is part of the payload of the Europa Clipper mission of NASA. It will collect spectral measurements of the Europa surface in the same timeframe as MAJIS providing an unique opportunity to coordinate the analyses between MAJIS and MISE.

The scientific and technical overview of the MAJIS investigation is described in this review. The MAJIS instrument was built by a French-Italian consortium led by Institut d'Astrophysique Spatiale in Orsay, France with the support of CNES and ASI. Specifically, CNES has funded the MAJIS' detectors and their proximity electronics, provided ad hoc technical support (in particular for electronics design and flight software) and supported on a case-by-case basis the technical resolution of anomalies that occurred during the instrument development. The Italian contribution comes from the Istituto Nazionale di Astrofisica (INAF) with the support of the Italian Space Agency (ASI) and the industrial contract for the optical head from Leonardo S.p.A. in Florence. Belgian laboratories supported by Belspo were involved in the characterization of the MAJIS detectors including radiation effects.

The review article is structured in seven main sections: (1) the science background of the MAJIS objectives, and their translation into imaging spectroscopic measurements; (2) the
driving requirements for the instrument design and overview; (3) the detailed optical, me-
chanical and electronic design of the MAJIS instrument; (4) overview of instrument testing,
calibration, and pre-launch performances; (5) onboard processing of acquired data with the
MAJIS software; (6) the observing strategy taken into account the versatility of the JUICE
trajectory and the MAJIS capabilities; and (7) the ground support for the operations and data
analysis.

**2 Science Objectives**

**2.1 Investigation Overview**

The JUICE Science Study Team recommended several high-level investigations to charac-
terize the three icy moons and the Jupiter system (Grasset et al. 2013). MAJIS has been
designed to address several of the prime objectives (Table 1). In the table below, we in-
troduce the major investigations of MAJIS with respect to the icy satellites (both surfaces
and exospheres) as well as Jupiter’s atmosphere and its aurorae. More details on the science
objectives, related observations and retrieval of physical parameters are given in Sects. 2.2
(satellites) and 2.3 (Jupiter).

Do Ganymede, Europa and Callisto lie along a continuum of surface activity and compo-
sition resulting from their size or distance to Jupiter? Or must distinct endogenic or exogenic
processes/events be implied to account for their present diversity? Analyzing their surface in
the visible/infrared range will make it possible to address several key questions: how do they
interact with their environment, and how do their properties record endogenous processes
linked to the geological activity coupled to exogenous processes? Studying their compar-
ative composition at high spatial resolution will provide new insight into the geochemical
state of the surface and sub-surface as well as the space environment.

One of the major scientific requirements of JUICE is the spectral sampling needed to resolve the narrowest signatures in the laboratory spectra of the non-water ice materials expected on the surface of the icy Galilean satellites, in particular hydrated materials. With high spectral resolution, MAJIS can distinguish between different compounds showing similar spectral features and accurately map their abundance on the surface with reduced uncertainties.

The state of water ice and the average grain size of the icy surface can be inferred from
the analysis of near-infrared (NIR) spectra. Using spectral markers of ice crystallinity in the
NIR range such as the Fresnel peak at 3.1 µm, the distribution of crystalline versus amor-
phous water ice can be mapped on the surfaces of the icy Galilean satellites, particularly
Ganymede and Callisto. Average grain sizes of pure icy surfaces can be also inferred by
analyzing the 1.25-1.5-2.0 µm band depths and the spectral continuum profile between 3
and 4 µm, supported by radiative transfer modeling, with finer grains showing fainter band
depths and higher continuum level in this region (Filacchione et al. 2012). However, a major
difference in the icy Galilean satellites is the presence of hydrated minerals or other hydrates,
which can mimic the 1.5 and 2.0 µm water ice absorption bands, so that mixtures of pure
granular water ice and salts must be considered in computing spectral indices (Stephan et al.
2020). The four Galilean satellites are also known to have tenuous atmospheres/exospheres
(McGrath et al. 2004), and their properties are indicative of surface processes and composi-
tion. Characterizing the composition of the bound and escaping neutral atmospheres of the
Galilean satellites is one of the investigations of the JUICE mission.
<table><caption>**Table 1** MAJIS goal compliance to the high-level JUICE objectives as defined in the JUICE redbook (1994). Green box: Primary. Yellow box: Supporting. White: N/A</caption><thead><tr><th>Topic</th><th>JUICE high-level objectives</th><th>JUICE Investigations</th><th>MAJIS</th></tr></thead><tbody><tr><td rowspan="10">Icy Moons</td><td rowspan="5">Characterize Ganymede as a planetary object and possible habitat</td><td>Characterize the extent of the ocean and its relation to the deeper interior</td><td></td></tr><tr><td>Characterize the ice shell</td><td style="background-color: #ffff00;"></td></tr><tr><td>Determine global composition, distribution and evolution of surface materials</td><td style="background-color: #00b050;"></td></tr><tr><td>Understand the formation of surface features and search for past and present activity</td><td style="background-color: #ffff00;"></td></tr><tr><td>Characterize the local environment and its interaction with the Jovian magnetosphere</td><td></td></tr><tr><td rowspan="3">Explore Europa's recently active zones</td><td>Determine the composition of the non-ice material, especially as related to habitability</td><td style="background-color: #00b050;"></td></tr><tr><td>Look for liquid water under the most active sites</td><td style="background-color: #00b050;"></td></tr><tr><td>Study the recently active processes</td><td style="background-color: #ffff00;"></td></tr><tr><td rowspan="3">Study Callisto as a remnant of the early jovian system</td><td>Characterize the outer shell, including the ocean</td><td style="background-color: #00b050;"></td></tr><tr><td>Determine the composition of the non-ice material</td><td style="background-color: #00b050;"></td></tr><tr><td>Study the past activity</td><td style="background-color: #ffff00;"></td></tr><tr><td rowspan="10">Jupiter System</td><td rowspan="3">Characterize the Jovian atmosphere</td><td>Characterize the atmospheric dynamics and circulation</td><td style="background-color: #00b050;"></td></tr><tr><td>Characterize the atmospheric composition and chemistry</td><td style="background-color: #00b050;"></td></tr><tr><td>Characterize the atmospheric vertical structure</td><td style="background-color: #00b050;"></td></tr><tr><td rowspan="3">Explore the Jovian magnetosphere</td><td>Characterize the magnetosphere as a fast magnetic rotator</td><td style="background-color: #00b050;"></td></tr><tr><td>Characterize the magnetosphere as a giant accelerator</td><td style="background-color: #00b050;"></td></tr><tr><td>Understand the moons as sources and sinks of magnetospheric plasma</td><td style="background-color: #ffff00;"></td></tr><tr><td rowspan="2">Study Io and ring systems</td><td>Study Io's activity and surface composition</td><td style="background-color: #00b050;"></td></tr><tr><td>Study the main characteristics of rings and small satellites</td><td style="background-color: #00b050;"></td></tr></tbody></table>

MAJIS will acquire low spatial resolution data (between 10 and 5 km/px) during the in-bound and outbound phases of the flybys, mapping large areas to both characterize a large number of high-interest sites of the icy satellites and to reveal leading/trailing asymmetries potentially due to contamination by exogenous material. During the closest approach phases, the spatial sampling will be less than 100 m/px. Extended regional mapping of Ganymede is foreseen during the GCO5000 and GEO5000 phases (as defined in Sect. 7.1), complemented by high spatial resolution observations during the GCO500 phase. The compositional information from the Galilean satellites, obtained from high spatial resolution (see Table 2 to Table 4) and high Signal to Noise Ratio (SNR) observations of small spatial areas (targeting specific regions or geological features) will thus be entirely complementary to global mapping at reduced resolutions. The ability to correlate compositional units with geological structures at different scales shall be one of the key objectives of MAJIS. Moreover, the combination of the high spatial/spectral images acquired by MAJIS, with unprecedented high spatial resolution imagery with JANUS and the ice-penetrating radar exploration with RIME constitutes a unique feature of the JUICE combined science, key to unraveling the surface-subsurface exchange processes on Ganymede and the other two icy satellites (e.g., Tosi et al. 2023). Regarding to the characterization of their exosphere, MAJIS shall complement the UV imaging spectrometer and particle instruments in achieving this goal, by performing limb scans at high phase angles to highlight non-local thermodynamic emissions.
sions (non-LTE) of trace compounds. Spectral imaging of Ganymede's aurorae as well as stellar occultations in the VIS-IR range are also planned to complement similar observations carried out in the UV domain.

Besides the icy satellites, Io is a target of great interest because of its extensive volcanic activity, the strongest in the solar system, and its influence on the Jovian system, in particular its magnetosphere. Although close Io flybys of JUICE are not planned, disk-resolved observations carried out from distance (typically ~10⁶ km) can be achieved by MAJIS. These observations will monitor the volcanic activity of Io, its hemispheric thermal emission, and determine the composition of the different surface materials on a regional scale as well as their induced volatile cycle.

In summary, MAJIS will determine the surface composition of the icy Galilean satellites providing major improvement in terms of spectral (and thus compositional) and spatial resolution. Coverage of Ganymede and Callisto at scales up to a few km (which cannot be obtained from the ground even with the extremely large telescopes) will be much more extended than that of previous space missions. Targeted observations at high spatial and spectral resolutions coupled with a good SNR (>100 on average for Ganymede and Callisto, and >30 for Europa in the 3.2-3.6 µm range which is most critical for the detection of organics), will offer a reliable identification and discrimination of non-water-ice constituents on the icy satellite surfaces, and a consolidated quantification of their abundance to be coupled with geological features in a wide range of spatial scales (down to a spatial sampling of 100 m and possibly less on a number of well selected targets on Ganymede).

The visible and infrared spectrum spans both reflected sunlight, auroral emission, and thermal emission from the deep atmosphere, enabling sounding of the atmosphere from the cloud-forming weather-layer into the ionosphere and thermosphere. The MAJIS design is thus suited to study Jupiter's tropospheric meteorology, dynamics, clouds, chemistry, and its aurorae, and to generate a long-term database of atmospheric/auroral variability over the length of the Jupiter tour. The orbit of JUICE will allow MAJIS to study Jupiter's atmosphere in all local time conditions, improving on the limited local time coverage of the Jovian Infrared Auroral Mapper (JIRAM) due to the Juno mission profile. The Instantaneous Field-Of-View (IFOV) of MAJIS will provide a spatial resolution of ~150 km (at the reference 1-bar level that represents the zero-altitude surface of Jupiter's atmosphere) from the distance of Ganymede's orbit.

The MAJIS spectral range covers the $H_3^+$ emission signatures around 2, 3.5, 3.67 and 5 µm that are strongly enhanced in the Jovian aurora (Drossart et al. 1989). The MAJIS observations will extend the JIRAM dataset (Adriani et al. 2017a,b) in terms of temporal coverage and spectral resolution. The mapping capabilities of MAJIS are ideally suited to monitor the overall auroral activity and phenomenology of the polar atmosphere during the high-inclination phases and to correlate them to in situ measurements from the magnetometer and plasma/neutral packages. The observations will also be coordinated with those of UVS to study the morphological differences between the instantaneous mapping of the incident particle flux (UV, visible) and the slower atmospheric response of the $H_3^+$ emission (as exemplified with the Juno mission in e.g. Gérard et al. 2023). Whenever possible, MAJIS stellar occultations by Jupiter's disk will contribute to constrain the mixing ratio of methane in the stratosphere as well as the variability of atmospheric temperatures. Observations of the nightside hemisphere will be of interest for possible detections of lightnings (Smith et al. 1979; Becker et al. 2020), Transient Luminous Events (Giles et al. 2020) and nightglow emissions.

Jupiter's atmospheric circulation and cloud systems will be continuously monitored by synoptic tracking of features over large regions of the planet (disk scan, see Sect. 2.3.3).
Particular attention will be devoted to the study of dynamical regimes associated with circulations on the scale of Jupiter's belts and zones (e.g., the zonal winds, Fletcher et al. 2020), vortices of various scales, and the occurrence of plumes and storms (Sánchez-Lavega et al. 2018; Sindoni et al. 2017). The high latitudes reached during the Callisto flybys will provide mapping of reflective aerosols that cover the polar domain, as well as the morphology and dynamics of the circumpolar cyclones (Adriani et al. 2018). The high number of the spectral sampling points in MAJIS data will allow the effective application of statistical analysis methods (such as principal component analysis and various classification schemes) to detect, for example, subtle differences in aerosol properties, with special attention to the nature of chromophores.

MAJIS extends to shorter wavelengths (0.5 µm) than JIRAM (which cuts on at 2 µm), giving access to the short-wave reflected sunlight that has been also previously used, besides Earth-based and space-based observations, by close investigations from Galileo/NIMS and Cassini/VIMS. These short wavelengths allow MAJIS to better constrain the composition and size distribution of aerosols within the different cloud decks, by comparing radiances observed in different CH$_4$ spectral windows and observations of the same region at different phase angles (Sromovsky and Fry 2002; Migliorini et al. 2023). The JUICE orbit will enable observations at a variety of emission, phase, and incidence angles (Fletcher et al. 2023, this collection), which will ultimately better constrain the scattering properties and locations of the upper tropospheric and stratospheric hazes, and thus provide insights on their chemical composition and origin, in particular the condensation and photochemical phenomena that produce potential cloud-condensation nuclei. Further constraints on the aerosol properties at higher altitudes will be derived from the modeling of stellar extinction during occultation measurements.

Observations in the 5 µm atmospheric window probe thermal emission from relatively cloud-free regions, providing insights into the composition of Jupiter's atmosphere from the upper troposphere down into the cloud-forming region, potentially down to approximately 7 bars (e.g., Bjoraker et al. 2018; Grassi et al. 2010, 2021). Thermal emission from this spectral region is shaped by both condensable volatiles (e.g., ammonia and water), as well as species that are in chemical disequilibrium, mixed upwards from the deeper and warmer troposphere (e.g., phosphine, arsine, germane, and carbon monoxide). Ammonia is sensed both above (where photochemical depletion occurs) and below its condensation cloud deck (where ammonia should be well-mixed), but has been shown to be depleted and to vary on the scales of the belts and zones). Water is sensed only above the clouds, following a saturated vapor pressure curve with a relative humidity that varies with location (Grassi et al. 2017). Whilst these cannot be used to place constraints on the bulk composition of Jupiter, they can be used to investigate the microphysical and dynamical processes at work in the cloud-forming region, particularly in association with maps of lightning activity in Jupiter's troposphere. These are supported by the use of disequilibrium species as tracers for vertical mixing and photochemical destruction (e.g., Irwin et al. 1998; Fletcher et al. 2009; Grassi et al. 2017, 2021). Both the volatiles and the disequilibrium species vary from belts to zones, and exhibit sharp contrasts associated with vortices, convective plumes and wave-like patterns such as the 5-µm hotspots on the boundary between Jupiter's Equatorial Zone and North Equatorial Belt (NEB). MAJIS will determine the spatial variability of vertical mixing processes throughout Jupiter's weather layer, and above by mapping aerosols using reflected light and thermal emission, combined with mapping of gaseous composition (particularly with the extension of MAJIS to large IR wavelengths, further than any previous space-based mapping spectrometer).
## 2.2 Icy Satellites: Key Questions and Measurement Objectives

MAJIS will target the surface and the thin atmosphere/exosphere of each of the icy satellites at various spatial resolutions to determine their compositional properties and to place these properties into geological context. Following an introduction to the scientific background, the relationship of these investigations and the major JUICE science objectives is described below and compiled in Table 2 to Table 4.

### 2.2.1 Surfaces

**Ice Characterization** Most of the high-resolution compositional information available to date on the surface composition of the icy moons has been collected from 1995 to 2003 by the Near-Infrared Mapping Spectrometer (NIMS) onboard Galileo or the Visible and Infrared Mapping Spectrometer (VIMS) Cassini, which operated in the 0.7-5.2 µm range with a spectral sampling of 25 nm longward 1 µm and a spatial resolution varying from >100 km/pix to 2 km/pix (Carlson et al. 1992; McCord et al. 2010; Stephan et al. 2020). NIMS was one of the first imaging spectrometers to fly aboard a planetary exploration mission and was a major improvement over ground-based observations which could barely resolve the icy satellites. Due to the technology at that time, the spectral resolution was relatively low compared to today's space instruments, and nearly full coverage of the satellite was achieved with only >50 km/pix resolution due to the very low downlink capability (the high gain antenna of Galileo could not be deployed successfully). Scientific exploitation of NIMS data has been also disadvantaged by low instrumental SNR due to various sources of noise including the radiation from Jupiter's environment (Greeley et al. 2009).

More recently, broadly regional surface coverage has been acquired using large Earth-based telescopes providing complementary data with higher spectral resolution and lower noise level than NIMS data, but at spatial resolution still limited to several 10 s of km (e.g. Ligier et al. 2016, 2019), precluding the study of local-scale geological targets. Although it is not the primary objective of NASA's Juno Mission, several observing opportunities were available to observe Europa and Ganymede with JIRAM (Filacchione et al. 2019; Mura et al. 2020a; Tosi et al. 2024), with spatial sampling down to 80 km/px and <1 km/px respectively. At the time of writing, low spatial resolution, but high spectral resolution mapping of the Galilean satellites by James Webb Space Telescope (JWST) were acquired using the NIRSpec (2.9-5.3 µm) instrument with their analyses soon to be published.

These observations indicate that water ice is the dominant component of the surface of Ganymede, Europa and Callisto. Analyses of NIR spectra made it possible to infer the physical state of water ice and the average grain size of the surface regolith. Ganymede's water ice particles are generally smaller on the leading hemisphere than on the trailing hemisphere and towards the equator (Calvin et al. 1995), which is probably the result of micrometeorite gardening combined with sputtering effects on the trailing side. Sputtered ice molecules/particles easily reach the cold polar regions along Ganymede's magnetic field lines and form a thin layer of frost; the surface boundary between the open and closed field lines is clearly visible in color images of Ganymede (Khurana et al. 2007), with material mantling the polar regions that shows a higher albedo. In addition, the interaction with the Jovian magnetosphere was found to cause spectral differences with respect to the crystallinity and grain size of H₂O ice between the equatorial and the polar regions of Ganymede, with a general trend from more crystalline ice at low latitudes to more amorphous ice in the polar regions (Hansen and McCord 2004). This is believed to be caused by the configuration of Ganymede's internal magnetic field, which at least partially protects the surface regions between about ±40°
latitude from impacting magnetospheric particles. Alternatively, the spectral signature of H₂O is very sensitive to the external environment, including the surface temperature. Small H₂O ice grains at low polar temperatures on Ganymede of about 70 K could also explain the observations (Stephan et al. 2021a; Mura et al. 2020a). Furthermore, grain sizes of H₂O ice on both Ganymede and Callisto apparently vary with the local surface temperature (Stephan et al. 2020). Both have the largest grains at the equator, which continuously decrease in size towards the poles. Therefore, thermal migration of H₂O ice molecules to colder regions has a large effect on the grain size distribution (Spencer 1987). A better knowledge of grain sizes distribution and crystallinity is required for achieving a detailed mapping of the effects of the magnetic field and for distinguishing them from temperature effects (Stephan et al. 2021a). On Callisto, data returned by the Galileo S/C showed that the ice is crystalline in all areas and is abundant enough to be detected (Hansen and McCord 2004), as expected from the average temperature of the satellite and the much lower radiolytic fluxes than on Ganymede. Ice grain sizes near the equator of Callisto are the largest of the three icy Galilean satellites (Calvin and Clark 1993; Roush et al. 1990; Stephan et al. 2020), a small amount of fine grains being in the leading hemisphere (Calvin and Clark 1993). On Europa, where cold temperatures and high radiation fluxes would favor amorphous ice, the resemblance to the crystalline ice model is poor (Hansen and McCord 2004), and although the amorphous ice model is not an exact fit, the spectrum is very sensitive to the precise morphological state of the amorphous deposit. Reflectance spectra taken by the JIRAM instrument suggest both amorphous and crystalline ice phases on Europa's surface (Filacchione et al. 2019).

The distribution of crystalline versus amorphous water ice on the surfaces of the icy Galilean satellites, in particular Ganymede and Callisto, will be mapped by MAJIS with improved spectral and spatial samplings (objectives GE.1b, GE.3b, CB.1b listed in Tables 2, Table 3 and Table 4) using specific spectral markers of ice crystallinity (strength of the temperature-sensitive 1.65 µm band; position of the minimum of the 2.0 µm band; intensity and shape of the 3.1 µm Fresnel reflectance peak – in this order of importance, driven by SNR). MAJIS will be able to characterize possible longitudinal variations due to weathering effects as well as latitudinal gradients due to radiation and local conditions. Average grain sizes can be inferred by analyzing the 1.25, 1.5 and 2.0 µm band depths (Stephan et al. 2020) and the spectral continuum profile in the 3 and 4 µm range, with finer grains showing weaker band depths and higher continuum level in this region (GD.3b, GE.1b, GE.3b, CB.1b) (Filacchione et al. 2012) and by radiative transfer modeling (Poulet et al. 2002). To achieve these goals, all MAJIS observations must be considered in a range as wide as possible of spatial scales and illumination conditions (solar phase angles).

**Non-ice Compounds** The identification and the spatial distribution of non-water-ice compounds is a major science objective of MAJIS. Visible and near-infrared water absorption features (e.g. “dark” features ones that cover most of the Europa surface) appear to be asymmetric and distorted. McCord et al. (1998a, 1998b, 1999, 2002, 2010) interpreted these distortions as being due to the presence of salts and/or sulfuric acid hydrate (Carlson et al. 1999a, 2009), and perhaps carbonates, which could originate from both exogenic and endogenic processes. Five absorption features are also reported at wavelengths of 3.4, 3.88, 4.05, 4.25, and 4.57 µm in the surface materials of Callisto and Ganymede from analysis of reflectance spectra returned by NIMS (McCord et al. 1997). Candidate materials include CO₂, organic materials (among these, tholins are exogenically-driven organics containing C≡N and C-H), SO₂, and compounds containing an SH-functional group; CO₂, SO₂, and perhaps cyanogen [(CN)₂] may be present within the surface material itself as collections of a few molecules each. The analysis of data returned by the JIRAM instrument during a Juno
<table><caption>**Table 2** MAJIS traceability to JUICE science objectives for Ganymede surface based on the science matrix of the ESA JUICE Science Requirement Document (SciRD) by Titov (2014)</caption><thead><tr><th>JUICE Science Objectives</th><th>JUICE Investigations</th><th>Measurement requirements (see SciRD)</th><th>Science Targets / Regions of Interests (ROI)</th><th>Detailed explanations campaign description and associated conditions</th><th>MAJIS Products (nominal to enable resource budgets; flexibility should be brought in, to feedback from discoveries or contingencies)</th></tr></thead><tbody><tr><td>GB. Characterize the ice shell</td><td>GB.2. Correlate surface features and subsurface structure to investigate near-surface and interior processes</td><td>GB.2c</td><td>Extended areas of tight terrain with complex networks of tectonically formed bands with potential interaction with subsurface ocean material (such as Babylon Sulcus on the leading hemisphere, Sippar Sulcus/ Munmu Sulci at the anti-Jovian hemisphere and Harpaga Sulcus on the sub-Jovian hemisphere)</td><td>Compositional mapping on specific targets. Observations spread during the entire 132 days GCO500 duration (dayside) preferentially limited to low latitudes (&lt; ~50°) for high SNR</td><td>Spectral cubes with a resolution of 75 m/px (30 km cross-track × 8.7 km along-track) or 150 m/px with spatial binning by 2 (30 km cross-track × 17.4 km along-track); two cubes selected for downlink out of six acquisitions per day (8 orbits) correspond to ROI at 75 m resolution or ROI at 150 m resolution</td></tr><tr><td>GD. Understand the formation of surface features and search for past and present activity</td><td>GD.2. Constrain global and regional surface ages</td><td>GD.2b</td><td>Characterize spectral differences between geologic features such as impact craters similarly in size and morphology and environmental conditions but of different geologic age and the corresponding surrounding areas, in order to investigate changes in the surface composition with time</td><td>see GB.2c + Global compositional mapping of the surface of Ganymede (&gt; 50%; goal: 100%) during GCO500 (~ 120 days) and close flybys</td><td>see GB.2c + Spectral cubes with a resolution of 3 km/px (spatial binning × 4), 300 × 300 km swaths for GCO5000 and variable swaths during flybys</td></tr><tr><td></td><td>GD.3. Investigate processes of erosion and deposition and their effects on the physical properties of the surface</td><td>GD.3e</td><td>Investigate the ancient dark terrain and monitor thermal segregation process by mapping the distribution of dark non-ice material and water ice and compare with illumination conditions, geologic context and topography</td><td>During GCO5000, systematic and regular observations providing &gt;50% coverage at equatorial latitudes (target: 100%)</td><td>Spectral cubes of 3 km/px (spatial binning × 4), 300 × 300 km swaths + Spectral cubes of 750 m/pix (no spatial binning), 300 × 300 km swaths + Spectral cubes at intermediate spatial resolution of 50 to &lt;750 m/pix, bridging the gap in resolution between systematic mapping and GCO500 ROIs + Spectral cubes of variable pixels and swath sizes depending on the flyby geometries</td></tr></tbody></table>

<figure><img src="image_3.png" /></figure>
<table><caption>**Table 2** (Continued)</caption><thead><tr><th>JUICE Science Objectives</th><th>JUICE Investigations</th><th>Measurement requirements (see SciRD)</th><th>Science Targets / Regions of Interests (ROI)</th><th>Detailed explanations campaign description and associated conditions</th><th>MAJHS Products (nominal to enable resource budgets; flexibility should be brought in, to feedback from discoveries or contingencies)</th></tr></thead><tbody><tr><td rowspan="2">GE. Determine global composition, distribution and evolution of surface materials</td><td>GE.1. Characterize surface organic and inorganic chemistry, including abundances and distributions of materials</td><td>GE.1a</td><td>Global, regional and targeted mapping including polar regions</td><td>Same as GD.3e +<br>Compositional mapping on specific targets. Observations spread during the entire 132 days GCO500 duration (dayside) preferentially limited to low latitudes (&lt; ~50°) for high SNR</td><td>Same as GD.3e +<br>Spectral cubes with a resolution of 75 m/px (30 km cross-track × 8.7 km along-track) or 150 m/px with binning by 2 (30 km cross-track × 17.4 km along track); two cubes selected for downlink out of six acquisitions per day (8 orbits) correspond to ROI at 75 m resolution or ROI at 150 m resolution</td></tr><tr><td>GE.2 Relate composition and properties and their distributions to geology</td><td>GE.2g</td><td>Compare composition of geologic units such as dark and light terrain, subunits of the light terrain and surface features such as impact craters of different albedo (dark ejecta, floor or halo of fresh impact craters) and morphology</td><td>Same as GD.3e</td><td>Same as GD.3e</td></tr><tr><td rowspan="2">GE.3 Investigate surface composition and structure on open vs. closed field line regions</td><td>GE.3b</td><td>GE.3b</td><td>Global mapping as well as regional/local mapping of the surface where the transition between open and close field lines at mid-latitudes (~+/−40°) occurs</td><td>During GCO5000, systematic and regular observations providing &gt;50% coverage at equatorial latitudes (target: 100%) +<br>During GEOS5000, observations near percenter if the local daytime is between 8 am to 4 pm +<br>Observations during the 8 close flybys</td><td>Spectral cube of 3 km/px (spatial binning × 4), 300 × 300 km swaths (Fig. 38) +<br>Spectral cubes at intermediate spatial resolution of 50 to &lt;750 m/pix, bridging the gap in resolution between systematic mapping and GCO5000 ROIs +<br>Spectral cubes of variable spatial resolutions and swaths depending on the flyby geometries</td></tr><tr><td>GE.4 Determine volatile content to constrain satellite origin and evolution</td><td>GE.4d</td><td>Global as well as regional mapping of the surface to Characterize hemispherical differences. Investigate fresh impact craters and relationships to the composition of the upper crust depending on excavation depth of the crater materials</td><td>Same as GE.1a</td><td>Same as GE.1a</td></tr></tbody></table>

<figure><img src="image_4.png" /></figure>
<table><caption>**Table 3** Same as Table 2 but for Europa's surface</caption><thead><tr><th>JUICE Science Objectives</th><th>JUICE Investigations</th><th>Measurement requirements (see SciRD)</th><th>Science Targets / Regions of Interests (ROI)</th><th>Detailed explanations campaign description and associated conditions</th><th>MAJIS Products</th></tr></thead><tbody><tr><td rowspan="5">EA. Determine the composition of the non-ice material, especially as related to habitability</td><td>EA.1 Characterize surface organic and inorganic chemistry, including abundances and distributions of materials, with emphasis on essential elements for habitability and potential biosignatures</td><td>EA.1a</td><td>Characterize Europa globally with a special focus on its large variety of endogenic surface features such as bands, double ridges and chaotic terrain</td><td>During the 2 flybys, perform dayside full-disk scan at long distance (&lt;20000 km), ROIs mapping at medium resolution (1 km/px) and ROIs mapping at high resolution (up to 100 m/px) with motion compensation during the closest approach</td><td>Spectral cubes of various sizes (from 1000 to ~50 samples) depending on the distance of the observations</td></tr><tr><td>EA.2 Relate material composition and distribution to geological features and geological processes, especially material exchange with the interior</td><td>EA.2a</td><td>Characterize composition of surface features within the chaotic terrain on Europa's trailing hemisphere such as the lenticulae and maculae</td><td>Same as EA.1a</td><td>Same as EA.1a</td></tr><tr><td>EA.3 Characterisation of the backscattered and sputtered material from the surface</td><td>EA.3g</td><td>Characterize Europa globally with a focus on spectral differences between different hemispheres</td><td>Same as EA.1a</td><td>Same as EA.1a</td></tr><tr><td>EC.3 Look for surface changes w.r.t. Galileo observations</td><td>EC.3c</td><td>Study the most recently formed surface units such as the chaotic terrain, with its lenticulae</td><td>Same as EA.1a</td><td>Same as EA.1a</td></tr></tbody></table>

<figure><img src="image_5.png" /></figure>
<table><caption>**Table 4** Same as Table 2 but for Callisto's surface</caption><thead><tr><th>JUICE Science Objectives</th><th>JUICE Investigations</th><th>Measurement requirements (see SciRD)</th><th>Science Targets / Regions of Interests (ROI)</th><th>Detailed explanations campaign description and associated conditions</th><th>MAJIS Products</th></tr></thead><tbody><tr><td rowspan="2">CB. Determine the composition of the non-ice material</td><td>CB.1 Characterize surface organic and inorganic chemistry, including abundances and distributions of materials and volatile outgassing</td><td>CB.1a</td><td>Global imaging of Callisto including the investigation of hemispherical differences</td><td>During the 21 flybys, perform dayside full-disk scan at long distance (&lt;20000 km), ROIs mapping at medium resolution (1 km/px) and high-resolution mapping (&lt;100 m/px) with motion compensation during the closest approach</td><td>Spectral cubes of various sizes (from 1000 to ~50 samples) depending on the flyby geometry</td></tr><tr><td>CB.2 Relate material composition and distribution to geological and magnetospheric processes</td><td>CB.2c</td><td>Global imaging of Callisto including the investigation of hemispherical differences.</td><td>Same as CB.1a</td><td>Same as CB.1a</td></tr><tr><td rowspan="2">CC. Study the past activity</td><td>CC.1 Determine the formation and characteristics of tectonic and impact landforms</td><td>CC.1c</td><td>Target impact craters of different morphologies, size and ages including impact basins and palimpsests</td><td>Same as CB.1a</td><td>Same as CB.1a</td></tr><tr><td>CC.3 Constrain global and regional surface ages</td><td>CC.3a</td><td>Study spectral differences between geologic features (multi-ring basins, palimpsests, impact craters) and the surrounding areas</td><td>Same as CB.1a</td><td>Same as CB.1a</td></tr></tbody></table>

<figure><img src="image_6.png" /></figure>
<figure><img src="image_7.png" /><figcaption>**Fig. 1** (A) Laboratory spectra of hydrated and non-hydrated various salts (Na- carbonates, Mg/Na-sulfates, chlorides) of 75-100 µm sized grains acquired at cryogenic temperature (110 K) with a spectral sampling of 20 nm compared to spectra of frost and snow at different grain sizes. (B, C, D) Spectral variations resulting from the temperature (from room temperature down to 90 K) of three selected hydrated salts (mirabilite, natron, epsomite) of 75-100 µm sized grains (De Angelis et al. 2017, 2021, 2022)</figcaption></figure>

flyby of Ganymede that took place on June 7, 2021, achieving pixel resolution <1 km at low latitudes in the sub-Jovian hemisphere, suggests the presence of a mixture of chloride salts, carbonates and aliphatic organics, possibly aldehydes, associated with specific geological features (Tosi et al. 2024).

One of the scientific requirements induced is the spectral sampling needed to detect the narrow and/or subtle signatures of these materials expected to exist on the surface of the icy Galilean satellites, particularly hydrated materials, by reference to laboratory spectra of potential proxies. This capability will make it possible both to discriminate between different compounds with similar spectral features and to accurately map their abundance across the surface. A priority science goal for Europa and Ganymede is to discriminate between exogenic products, such as sulfur dioxide, sulfuric acid hydrate and hydrogen peroxide, and endogenic hydrated salt minerals. The spectral sampling of MAJIS (Sect. 3.1) is similar to that of laboratory spectra of several non-ice materials measured at temperatures representative of the icy Galilean satellites, which will be used as a comparison to properly interpret the data (Fig. 1 and Fig. 2).

Table 5 summarizes the expected compounds based on formation models, tentative or confirmed detections, and the key spectral features that we would expect in the spectral ranges of the instrument for all Galilean satellites. Spectra of the surface will be compared to laboratory analogues (Sect. 8.3) and modeled using linear un-mixing and non-linear, radiative transfer-based modeling (Hapke 1993; Shkuratov et al. 1999) for quantitative analysis. These modeling approaches have been successfully applied to spectra of various icy surfaces (Quirico et al. 1999; Ligier et al. 2016). Hydrated sulfates have proven to be a good match to some surface spectra of Europa (McCord et al. 1999; Brown and Hand 2013;
<figure><img src="image_8.png" /><figcaption>**Fig. 2** (A) Spectra of Cl-salts (Hanley et al. 2014) compared to hydrated sulfuric acid (Carlson et al. 1999a). Spectra of typical bitumen kerite (grain sized range 0-25 µm in (B) and 45-63 µm in (C)) measured at 173 K with varying portions of water ice (Ciarniello et al. 2021). Spectra illustrate the detectability of the typical absorptions related to aromatic CH, and CH₂/CH₃ aliphatic/aromatic symmetric and asymmetric stretch (contributing to complex broad band between 3.18 and 3.64 µm) as indicated by the dashed lines depending on grain size of kerite and varying portions of water ice</figcaption></figure>

Ligier et al. 2016), but there remains a dark reddish component in the visible to near-infrared. Mg-bearing chlorinated species (chloride, chlorate, and perchlorate) appear to correlate with large-scale geomorphologic units such as chaos and darker areas (Ligier et al. 2016; Trumbo et al. 2019; King et al. 2022; King and Fletcher 2022; Cruz Mermy et al. 2023) and provide good spectral fits between 1 and 2.5 µm. It is important to note that the detection is limited by a lack of comparison data in the spectral libraries, so that additional laboratory work is planned for ices and salts of interest (Sect. 8.3.2). In addition, at low temperatures, the spectral characteristics of many H₂O-bearing species differ from their room temperature behavior (Hanley et al. 2014; De Angelis et al. 2017, 2021, 2022; Fastelli et al. 2022, Fig. 1). Finally, the coverage of the visible spectral range shall allow photometric investigations of the icy surfaces, estimation of the contamination by different “chromophores” and cross-correlation of MAJIS data with JANUS images.

The ability to correlate compositional units with geological structures at a wide range of spatial scales, down to unprecedented, with very high pixel resolution (<100 m/px) during close flybys and over selected sites of interest during the GCO500 phase will be another key scientific implementation of MAJIS. The experience gained from the Galileo and Cassini missions suggests that the unique compositional information of the icy bodies such as the Galilean satellites, obtained from high spectral resolution, high SNR observations of small spatial areas (targeting specific regions or geological features) is essential and fully complementary to global mapping at reduced resolutions. This will allow to test whether the Galilean satellites are distinct and not united as “icy”, and that the processes involved in their surface structures and composition are fundamental in providing insight into what is going on in planetary systems.

Specifically, the global context for Ganymede’s surface composition will be provided by covering at least 50% of the surface at a spatial resolution comprised between 1 and 5
<table><caption>**Table 5** Chemical compounds of interest for the icy satellites and corresponding spectral features positions in the spectral range of MAJIS. The species in italics are the one not yet identified</caption><thead><tr><th>Compound spectrally active bond</th><th>Chemical composition</th><th>Diagnostic feature (µm) and rationale</th><th>Required spectral resolution</th><th>Related physical processes/origins</th></tr></thead><tbody><tr><td>Water ice</td><td>H<sub>2</sub>O</td><td>– Crystalline: 1.04, 1.25, 1.50, 1.65, 2.0, 3.0, 3.1 (triple Fresnel peak)<br/>– Amorphous: 1.05, 1.25, 1.50, 2.05, 3.0, 3.1 (single Fresnel peak)<br/>– Continuum, band depths and 3.0-3.6 µm region markers for regolith grain sizes<br/>– VIS spectral slopes indicators for ice contamination</td><td>from 1 nm to 20 nm depending on the signatures</td><td>Composition of crustal material excavated by recent impacts, sublimation, re-deposition and sintering due to diurnal and seasonal temperature variations, sputtering, amorphization due to impacting magnetospheric particles</td></tr><tr><td>Na/Mg hydrated sulfates, Na/Mg hydrated chlorides</td><td>MgSO<sub>4</sub>6(H<sub>2</sub>O)<br/>MgCl<sub>2</sub>nH<sub>2</sub>O, (Na,Mg)(ClO<sub>3</sub>,<sub>4</sub>)<sub>2</sub>nH<sub>2</sub>O</td><td>1.0, 1.2, 1.45, 1.8, 1.95, 2.5, 2.8</td><td>&lt;10 nm</td><td>Possible interaction with subsurface ocean</td></tr><tr><td>Mg-phylosilicates</td><td>Mg<sub>3</sub>Si<sub>2</sub>O<sub>5</sub>(OH)<sub>4</sub></td><td>1.39, 1.93, 2.32, 2.51</td><td>&lt;10 nm</td><td>Surface alteration due to micro-meteoritic impacts</td></tr><tr><td>Fe-phylosilicates</td><td>(Ca<sub>0.5</sub>, Na)<sub>0.3</sub>Fe<sub>2</sub><sup>3+</sup>(Si, Al)<sub>4</sub>O<sub>10</sub>(OH)<sub>2</sub></td><td>1.42, 1.93, 2.28</td><td>&lt;10 nm</td><td>Surface alteration due to micro-meteoritic impacts</td></tr><tr><td>Mg/Fe-carbonates</td><td>Mg<sub>5</sub>(CO<sub>3</sub>)<sub>4</sub>(OH)<sub>2</sub>4H<sub>2</sub>O</td><td>1.39, 1.93, 2.3, 2.5, 3.45, 3.9</td><td>&lt;10 nm</td><td>Possible interaction with subsurface ocean</td></tr><tr><td>Sulfur rich, carbonic acid</td><td>S-H bearing, H<sub>2</sub>CO<sub>3</sub></td><td>3.88</td><td>&lt;10 nm</td><td>Interaction with charged magnetospheric particles</td></tr><tr><td>CO<sub>2</sub> clathrate</td><td>CO<sub>2</sub>nH<sub>2</sub>O</td><td>2.0, 2.71, 4.28</td><td>&lt;10 nm</td><td>Possible crustal material as a result of the internal differentiation</td></tr><tr><td>CH<sub>4</sub> clathrate</td><td>CH<sub>4</sub>nH<sub>2</sub>O</td><td>2.0, 2.38</td><td></td><td></td></tr><tr><td>Tholins</td><td>triple CN</td><td>2.2, 3.4, 4.46-4.76</td><td>&lt;15 nm</td><td>Possible crustal material or residual of meteoritic or dust impacts</td></tr><tr><td rowspan="2">Sulphur dioxyde</td><td>N-H</td><td>1.49-1.54, 2.89-3.13</td><td>&lt;15 nm</td><td>“</td></tr><tr><td>C-H</td><td>1.19, 1.70-1.74, 3.40, 3.50</td><td>&lt;15 nm</td><td>“</td></tr><tr><td>Dioxygene</td><td>SO<sub>2</sub> frost</td><td>1.35, 2.54, 2.80, 2.92, 3.56, 3.79, 4.05, 4.37</td><td>&lt;10 nm</td><td>Interaction with charged magnetospheric particles</td></tr><tr><td></td><td>O<sub>2</sub> ice</td><td>0.577, 0.627</td><td>&lt;5 nm</td><td>Sublimation and redeposition during diurnal temperature variations, impact events, sputtering</td></tr><tr><td></td><td>O<sub>2</sub> gas</td><td>0.684, 0.760, 1.27</td><td>&lt;5 nm</td><td>“</td></tr></tbody></table>
<table><caption>**Table 5** (Continued)</caption><thead><tr><th>Compound spectrally active bond</th><th>Chemical composition</th><th>Diagnostic feature (µm) and rationale</th><th>Required spectral resolution</th><th>Related physical processes/origins</th></tr></thead><tbody><tr><td>Hydrogen peroxide</td><td>H<sub>2</sub>O<sub>2</sub></td><td>3.5 µm</td><td>&lt;20 nm</td><td>“</td></tr><tr><td>Ozone</td><td>O<sub>3</sub> gas</td><td>3.28, 3.59, 4.74</td><td>&lt;10 nm</td><td>Interaction with charged magnetospheric particles, sputtering</td></tr><tr><td>Ammonia</td><td>NH<sub>3</sub></td><td>1.54, 1.63, 2.01, 2.23, 2.3, 2.97, 3.03, 3.12</td><td>&lt;10 nm</td><td>Possible crustal material or residual of meteoritic or dust impacts</td></tr><tr><td>Carbon, iron</td><td>C, Fe</td><td>visible slopes</td><td>&lt;50 nm</td><td>Possible crustal material or residual of meteoritic or dust impacts</td></tr></tbody></table>

km/px (3 km/px on average), in order to correctly interpret the nature of large regional features and general latitudinal and longitudinal trends produced by magnetospheric weathering (Table 2). Furthermore, the combination of the high spatial and spectral resolution data acquired by MAJIS, with the unprecedented characterization of the icy envelope from JANUS, RIME and GALA, will be key to unraveling the surface-subsurface exchange processes on Ganymede and the other two icy satellites (GB.2c). On Europa MAJIS will acquire medium spatial resolution data (between 10 and 3 km/px) during the inbound phases of the fly-bys, mapping large areas in order to characterize as many sites of high interest as possible and to reveal lead/lag asymmetries due to contamination by exogenic material. High priority sites such as chaos terrains, fresh craters, linea or ridges will be investigated during the two closest approach phases, achieving a pixel resolution as good as ~1 km/px (or possibly smaller, i.e. better) and emphasizing the search for organic materials, not yet identified on Europa (Table 3). Measurements for Callisto are similar to those of Ganymede and Europa, but with a stronger emphasis on the distribution of volatiles (Hibbitts et al. 2000, 2002), which may shed light on the mechanism that allows continuous CO₂ replenishment at the surface (Table 4).

## 2.2.2 Exospheres

**Science Background** The characterization of exospheres is critical to understand the processes of ice alteration induced by the radiative environment. Spaceborne UV observations showed that Ganymede has a thin O₂ atmosphere and a hydrogen exosphere. Atomic hydrogen Lyman-α emission (121.6 nm) was firstly measured in limb scans by the ultraviolet spectrometer UVS onboard the Galileo S/C (Barth et al. 1997). After that, hydrogen Lyman-α has been monitored using Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph instrument (Feldman et al. 2000; Alday et al. 2017), and also observed in transmission using Lyman-α emission by Jupiter's atmosphere as background during transits (Roth et al. 2023). The first oxygen detection dates back to HST observations in 1996 of O I airglow emissions at 130.4 nm and 135.6 nm obtained through the GHRS instrument (Hall et al. 1998). The intensity ratio of the detected emission lines suggests atomic oxygen to be diagnostic of dissociative electron impact excitation of O₂, whose abundance can therefore be constrained by such observations. The auroral nature of these emissions was confirmed by successive HST-STIS observations (Feldman et al. 2000; McGrath et al. 2013), whose
enhanced spatial resolution enabled locating their sources near both poles of the satellite. More recently, oxygen aurora has also been confirmed by other O I emission lines observed from the ground at 630/636.4, 557.7, 777.4, and 844.6 nm (de Kleer et al. 2023).

Sublimation of surface ice is considered to be the main process producing an exospheric water component, the abundance of which has been inferred indirectly from oxygen, hydrogen, and ionospheric measurements (e.g. Roth et al. 2021). The question of whether O₂ or H₂O is the most abundant species in Ganymede's exosphere is still debated (Roth et al. 2021; Leblanc et al. 2023; de Kleer et al. 2023), and it is likely to be related to the different response of the efficiency of the source processes to the global illumination condition of the satellite. While sublimation is strongly temperature dependent, the presence of an intrinsic magnetosphere on Ganymede (Kivelson et al. 1997) complicates the interaction between the surface and the particles in the Jovian environment, modulating the sputtering intensity and therefore the radiolysis efficiency. At latitudes encompassed by closed magnetic field lines (within about 30° from the equator) the surface is partially shielded from ion impacts, while the opposite likely happens in polar regions. The most effective surface sputtering appears to occur at the boundary region between open and closed magnetic field lines (Jia et al. 2010), whose latitude is controlled by the interaction of Ganymede's magnetosphere with the Jovian plasma sheet, which is variable along Ganymede's orbit due to the inclination of Jupiter's magnetic field. Although this configuration is capable of producing different surface precipitation patterns for different ion populations (Plainaki et al. 2020), the spatially resolved HST observations locate the UV auroral emissions high in latitudes (40-50°) of the trailing hemisphere and as low as 10° in the leading hemisphere (McGrath et al. 2013; Greathouse et al. 2022). Furthermore, the state and distribution of O₂ molecules embedded in the surface, identified through the small absorption band around 577 nm but still poorly understood in nature, may also be related to such processes (Spencer et al. 1995; Trumbo et al. 2021; Migliorini et al. 2022). Determining the characteristics of the near-surface H₂O environment along Ganymede's orbit around Jupiter will allow the identification of its source regions and also the efficiency of the particle release from the ice, providing constraints for estimating the energy exchange between the Jovian system and the moon itself.

The presence of an exosphere around Callisto was first inferred from the detection of CO₂ emissions in Galileo NIMS limb scans up to 100 km above the surface (Carlson 1999b) and that of a significant ionospheric layer 25-50 km above the surface (Kliore et al. 2002). Similarly, to Ganymede, subsequent UV observations were successful in detecting also on Callisto O I emissions at 130.4 and 135.6 nm (Roth et al. 2017a), through HST-STIS instrument, and hydrogen Lyman-α emission thanks to HST-COS instrument (Cunningham et al. 2015, after previous non-detections with HST-STIS reported by Strobel et al. 2002). The measured intensities favored dissociative excitation of O₂ by photoelectrons as the most likely source of atomic oxygen, establishing O₂ as a major component of Callisto's atmosphere (Cunningham et al. 2015). On the other hand, the presence of an extended hydrogen corona indicated by Lyman-α observations is suggestive of a substantial H₂ component (Carberry Mogan et al. 2021, 2022). Another possible source for the hydrogen corona is represented by exospheric H₂O, whose presence is predicted by theoretical models of Callisto environment, although its expected abundance is debated by as much as 1-2 orders of magnitude (Liang et al. 2005; Carberry Mogan et al. 2022). Considering the composition of the ice and non-ice materials on the Callisto's surface, the presence of other minor neutral compounds (such as CO, OH, N₂, CH₃OH, H₂S) is also possible within 100 km above the surface, but estimates of their abundances are highly model-dependent (Vorburger et al. 2015). The detection of CO₂ and SO₂ molecules trapped in gaseous form in the regolith's upper layer (Carlson et al. 1996; McCord et al. 1997) indicates that the sublimation of the surface ice is
a major process for gas release and exosphere support. As a consequence, significant spatial and temporal variability of the exosphere composition can be expected, depending on the illumination conditions of different regions of the surface.

UV emissions have shown that Europa has also a tenuous O₂-rich atmosphere, again inferred from O I emissions at 130.4 and 135.6 nm triggered by intense radiation bombardment (Hall et al. 1998). The observed spatial and temporal pattern of emission lines' intensity ratio (135.6/130.4) suggests a stable H₂O abundance in the central sunlit trailing hemisphere with an H₂O/O₂ ratio of 12-22, and a pure O₂ atmosphere on the leading hemisphere (Roth 2021). On the other hand, evidence for oxygen and hydrogen auroral emissions in the visible range from ground-based observations suggests a global 0.25 H₂O/O₂ abundance ratio (de Kleer et al. 2023), while the modeling suggests H₂O is probably dominant at subsolar point (Leblanc et al. 2017). Hence, also for Europa, a H₂O gas component is expected to contribute to the exosphere significantly but in a variable amount, making many further observations needed to properly constrain its abundance and source/sink processes (Vorburger and Wurz 2018; Plainaki et al. 2018; Oza et al. 2019; Johnson et al. 2019). Furthermore, although the main source of exospheric oxygen is attributed to sputtering processes, it is not yet clear which are the particles mostly contributing to its formation, H, He, heavy ions or electrons (Davis et al. 2021). Lyman-α observations were demonstrated effective also in Europa for detecting an atomic hydrogen envelope of the satellite, consistent with a radially escaping H corona (Roth et al. 2017b). Other exospheric minor species have also been detected, such as atomic sodium through its 589 nm line (Brown and Hill 1996) and atomic potassium through its 769.9 nm line (Brown 2001), both populating an extended atmosphere around the satellite. Although their source process is still under investigation, the sputtering of Na- and K-bearing endogenic molecules may be favored with respect to an exogenic origin from Io's volcanic plumes (Ozgurel et al. 2018).

Of particular interest on Europa is the detection of large of water vapor plume(s), up to 200 km high, from coincident UV emissions of H and O with an estimated water vapor column density of ~10²⁰ m⁻², provided by HST imaging of far-UV emissions (Roth et al. 2014, 2017a). Reanalysis of the in-situ observations of Galileo's magnetic field and plasma wave measurements has revealed a potential detection near the equator and one possible polar detection, with a similar column (Blöcker et al. 2016; Jia et al. 2018). This identification has been interpreted as the evidence for active processes at Europa that allows the characterization of the subsurface liquid reservoir, although it is still not known whether such phenomenon is involving deep ocean sources or is limited to surficial crustal sections (Vorburger and Wurz 2021).

**Key Investigations** While the spectral range of MAJIS cannot cover the strongest exospheric emissions, spectrally located in the UV, MAJIS is unique in covering the non-LTE emissions of several expected neutral species (CO₂ at 4.23 µm and 2.7 µm, H₂O at 2.7 µm, O₂ at 1.27 µm, CO at 4.6 µm) allowing their abundances to be constrained directly at low altitudes by limb scans (Table 6).

For Callisto, given the measured vertical scale height for CO₂ (H = 23 km, Carlson 1999b), MAJIS limb observations will be most effective if performed at the highest possible spatial resolution (2-10 km/pixel), achievable during the several Callisto's flybys occurring during the Jovian tour mission phase. This level of resolution would make possible to retrieve the vertical abundance profiles of the detected species through limb scans, provided that the emission is strong enough. In the cases of CO₂ and H₂O, the MAJIS spectral resolution is high enough to study the shape of the emission bands (at 4.26 µm and 2.7 µm respectively), enabling the retrieval of vibrational temperature profiles and thus further constraining the environmental conditions. The detection of O₂ and especially of CO is more
<table><caption>**Table 6** MAJIS traceability to JUICE science objectives for exospheres and auroras investigation based on the science matrix of the SciRD</caption><thead><tr><th>JUICE Science Objectives</th><th>JUICE Investigations</th><th>Measurement requirements (see SciRD)</th><th>Science Targets / Regions of Interests (ROI)</th><th>Detailed explanations campaign description and associated conditions</th><th>MAJIS Products</th></tr></thead><tbody><tr><td>GC. Characterize the local environment and its interaction with the jovian magnetosphere</td><td>GC.3. Investigate the generation of Ganymede's aurorae</td><td>GC.3d</td><td>Monitor mid-latitudes (+/- 40°) where lines of Ganymede's magnetic field change from closed to open ones</td><td>During GCOS000 and GEO5000, perform aurora mapping in the nocturnal limb spatial resolution of about 2 km latitude (latitudes 30-35° N-S, minimum 2 latitudes) near apocenter, at least in the dawn and dusk sides of Jovian magnetosphere; no requirements on the slit orientation</td><td>Spectral cubes (100 lines) with a resolution of ~2 km</td></tr><tr><td>GC. Characterize the local environment and its interaction with the jovian magnetosphere</td><td>GC.4. Determine the sources and sinks of the ionosphere and exosphere</td><td>GC.4b</td><td>Monitor Ganymede limbs at different latitudes</td><td>During flybys and Ganymede phases, map the dayside limb at resolution of about 1 km or less at different latitudes; study the variability of the exospheric processes (sputtering, photodissociation, sublimation). Slit tangent to the limb is preferable</td><td>Spectral cubes of 100-200 lines; a minimum of 3 (north, equator, south) × 2 (dawn, dusk) positions considered. Spectral editing is effective in data volume reduction</td></tr><tr><td>EC. Study the active processes</td><td>EC.2 Observe the limb for activity</td><td>EC.2b</td><td>Monitor Europa limbs at different latitudes</td><td>Observations during flybys optimal for phase angle while observations during Ganymede phase optimal for spatial resolution</td><td>Spectral cubes (150 lines max) with 1-10 km/px sampling resolution for each latitude</td></tr><tr><td></td><td>CB.3 Characterize the ionosphere and exosphere of Callisto</td><td>CB.3a</td><td>Monitor Callisto limbs at different latitudes</td><td>Perform MAJIS limbs especially during outbound phase of the two flybys at high solar phase angles; to cover all latitudes thanks to a sequence of 6-8 limb pointings at several latitudes; possible placeholder low resolution limbs in backscattering during ingress</td><td>Same products as EC.2b. but during Callisto flybys</td></tr></tbody></table>

<figure><img src="image_9.png" /></figure>
<figure><img src="image_10.png" /><figcaption>**Fig. 3** (A) Example of a MAJIS sequence dedicated to exospheric observations during a Callisto flyby (C18 outbound phase in recently predicted JUICE mission profile (CReMA 5.0)). Green boxes represent MAJIS FOV (400 × 200 pixels) at different distances (spatial resolution) and orientations, covering almost all latitudes of the diurnal limb. Medium purple shaded area represents Callisto's nightside, while the red curves indicate isolines of solar incidence angle. The dotted circle line indicates a 100 km altitude envelope above Callisto surface. (B) Same as (A) but for a flyby of Ganymede. Aurora observations are illustrated as green boxes on the nightside</figcaption></figure>

challenging due to their lower expected emission intensities. However, the occurrence of several flybys offers some coverage of Callisto's orbital phase, allowing the investigation of the response of exospheric densities to different illumination conditions of the satellite (Fig. 3A). Assessing the abundances of neutral species is also the driver of the MAJIS observing strategy for Ganymede. No evidence of exospheric CO₂ has been found on this satellite, hence H₂O and O₂ are the main target species through their emission bands at 2.7 µm and 1.27 µm respectively. Observations performed during flybys and from GCO orbits will allow vertical and horizontal mapping of such emissions at different latitudes and orbital position (Fig. 3B). While a maximum resolution of 2 km/px can be envisaged during flybys, higher spatial resolution can be achieved from GCO orbits (~1.2 and 0.6 km/px from an altitude of 5000 and 500 km, respectively), as well as better local time coverage. Recent ground-based observations detected auroral emissions from oxygen and hydrogen also in the visible spectral range, during eclipses of the satellite (de Kleer et al. 2023). For oxygen, the reported full-disk intensity of auroral emissions (OI at 630 nm) appears to be the same order of magnitude as that expected from non-LTE conditions. Although not spectrally resolved, such emissions may provide an opportunity for MAJIS to detect this kind of phenomena in vertical limb scans at different latitudes (requirement GC.3d from Table 6) (Fig. 3B).

Detection of exospheric oxygen and water from their non-LTE emissions around Europa is also a major objective for MAJIS observations. As with Callisto and Ganymede, such observations are best performed by scanning the limb of the satellite at the highest possible spatial resolution, which, in this case, can only be carried out during the two scheduled flybys of Europa. Also, the detection of possible plumes is more effective in limb viewing geometries: the low reflectance level of a plume, determined by the scattering of solar light by solid particles in the rarefied plume can only be seen against a dark background (deep space in limb observations, dark surface of the satellite in terminator observations). Since the plume constituents are thought to be small grains of water ice, the plume reflectance varies significantly with the scattering angle, increasing especially in the forward direction.
The lesson learned from the Cassini VIMS observations of Enceladus plumes, in the same spectral range as MAJIS, is that the signal from plumes will be maximized if observed at scattering angles greater than ~140°. Such observations, even if at a lower spatial resolution than the imaging camera, would make possible to obtain information on the composition, density and microphysics of the particles in the plume, thanks to the coverage of water ice bands in the NIR, whose the spectral shapes can provide invaluable information on the diffusion process (e.g., Hedman et al. 2009). It should be noted that the solid particles should be associated with a gaseous water component in a plume. In this case, a local enhancement of the limb 2.7 µm water emission could be observable by MAJIS as well, and may represent an independent way to detect the plume (Danny et al. 2022).

Finally, an alternative and foreseen geometry for plume detection occurs when Europa is viewed transiting a very bright source, e.g. Jupiter. In this case, a plume is observed as a shallow dimming of the Jovian light just off the satellite's limb. However, in addition to being severely constrained by geometry (three-bodies alignment required), these opportunities are not ideal for observing the solid component, that would scatter most of the Jovian light in the direction of the observer. On the other hand, it can be effective for measuring the gaseous component through the water absorption bands, as long as the spatial resolution is sufficient to separate it from the strong surface signal.

Although reduced spectral coverage can be used in most cases to trace specific exospheric emissions, the search for other unpredicted exospheric features is also valuable for all three targets, and can take advantage of the full spectral coverage of the instrument in limb-viewing geometry. The search for auroral signatures and their possible mapping may also be based on observations during several eclipses of satellites observable from the JUICE's position, albeit at a lower spatial resolution. About twenty of such events are expected to occur during the Jovian tour mission phase at resolutions of ~60-100 km/px.

Note that detailed operational scenarios of the satellites are presented further in the paper in Sect. 7.1 and Table 20.

## 2.3 Jupiter: Key Questions and Measurement Objectives

### 2.3.1 Science Background

One of the goals of the JUICE is to characterize the dynamics, chemistry, and vertical structure of the aerosols both within and above Jupiter's cloud-forming region, commonly known as the weather layer. The weather layer exhibits planetary banding (reflective, cool and cloudy zones, contrasted with cloud-free and visibly-dark belts) on the scale of the zonal jets; a diverse collection of giant vortices (from the anticyclonic Great Red Spot and Oval BA, to the circumpolar cyclones); and vigorous moist-convective plumes and associated lighting phenomena. The weather layer of Jupiter varies considerably with time, in response to local meteorology (e.g., eruptions of convective plumes), as well as longer-term cycles of activity that cause considerable changes to the belts and zones (e.g., fades, revivals, etc.). The circulation patterns shaping the distributions of gases and aerosols also appear to vary with height, suggestive of layering within the upper troposphere. Fletcher et al. (2023, this collection) provide a full review of these time- and spatially-variable phenomena. Below we remind the characteristics of the signal of Jupiter that MAJIS will be able to capture in its spectral range.

The visible and near-infrared spectrum of Jupiter at wavelengths shorter than 4 µm is dominated by the scattering of solar photons by the different types of aerosols found in Jupiter's atmosphere or directly by atmospheric gas molecules (Rayleigh scattering). Over
<figure><img src="image_11.png" /><figcaption>**Fig. 4** Levels at which the 2-way optical depth reaches unit value for the different wavelengths in the MAJIS range, neglecting scattering by clouds and hazes. The major cloud decks of NH₃ ice, NH₄SH ice, and H₂O ice predicted by equilibrium thermochemistry, are indicated by thick horizontal colored lines. Adapted from Sromovsky and Fry (2018)</figcaption></figure>

the same spectral range, methane absorption bands induce strong variations in the opacity of the atmosphere with wavelength: in the most opaque spectral regions, a photon can only be scattered in the uppermost aerosol layers; conversely only the most transparent regions can bring information on the deepest clouds. Figure 4 shows where the two-way optical thickness from the top of the atmosphere reaches unity, as an approximate guide to the vertical sensitivity of MAJIS in the absence of aerosols. Thanks to the modulation of the shape of the spectrum resulting from the variation of the cloud albedo with wavelength, ultimately related to aerosol composition and size distribution, the particle properties and their vertical distribution are obtained using state-of-the-art radiative transfer codes. Furthermore, the JUICE orbital tour provides access to the scattering phase functions of aerosols over a wide variety of illumination conditions. Past applications of such investigations to interesting areas were the large anticyclones such as Oval BA (Pérez-Hoyos et al. 2009; de Pater et al. 2010; Grassi et al. 2021) and the Great Red Spot (GRS, Baines et al. 2019; Anguiano-Arteaga et al. 2021), and cloud changes produced by large planetary disturbances such as the North Temperate Belt Disturbance (NTBD) (Pérez-Hoyos et al. 2020) and the South Equatorial Belt Disturbance (SEBD) (Pérez-Hoyos et al. 2012).

At wavelengths longer than 4 µm the influence of reflected sunlight lessens, and becomes overwhelmed by thermal emission from the deeper altitudes within the weather layer. The so-called “5- µm window” is where an absence of strong methane or hydrogen opacity allows thermal emission from the 4-6 bar level to escape. Absorption and scattering by aerosols reduce this 5- µm emission such that clouds appear dark in 5- µm images (e.g., Antuñano et al. 2018), and long-term variability of 5- µm emission reveals quasi-periodic cycles of cloud coverage (sometimes called global upheavals) associated with thinning and thickening of cloud decks over time. Spectroscopically, the 5- µm window provides access to both condensable volatiles on the longward side (ammonia and water), and to a host of species in chemical disequilibrium on the shortward side (phosphine, arsine, germane, and carbon monoxide). These latter species can be used to trace the vertical mixing processes and chemical destruction at work in Jupiter's cloud-forming region, providing a window onto the circulation of the deeper atmosphere. The time-variable distribution of cloud opacity, volatiles, and disequilibrium species can therefore be used to study belt/zone dynamics.
ics and global circulation (e.g., Irwin et al. 1998; Giles et al. 2016; Grassi et al. 2017), the properties of giant vortices (e.g., Bjoraker et al. 2018), and perturbations from wave propagation and moist convection. Furthermore, by mapping the 5- µm brightness we gain access to dynamical phenomena at depth, particularly on the nightside where there is no complicating influence of reflected sunlight (e.g., for mapping the circumpolar cyclones or folded-filamentary regions that dominate the mid- and high-latitudes, Adriani et al. 2017b).

The transitional spectral region between 3.1 and 3.8 µm contains the most intense methane absorption band in the MAJIS range, limiting any reflection from the atmosphere itself. The most significant source of radiation here are the emission lines of H<sub>3</sub><sup>+</sup>, which are produced on the dayside by the reaction of H<sub>2</sub> with H<sub>2</sub><sup>+</sup> produced by the ionization of H<sub>2</sub> under solar Extreme Ultraviolet (EUV) radiation. Another major source of H<sub>3</sub><sup>+</sup> emission is particularly evident in the polar regions, where it is associated with the precipitation of energetic magnetospheric electrons. Nadir observations of the polar regions can suitably be used to investigate the H<sub>3</sub><sup>+</sup> emissions, which have very strong bands at 3.42, 3.53, 3.62 and 3.66 µm. These emissions provide information about the specific electromagnetic interaction between Jupiter's ionosphere and the moons (Io, Europe and Ganymede). A spectral resolution of the order of 10 nm is sufficient to separate and identify the contribution of each transition band, as shown in the analysis of Juno/JIRAM data reported in Adriani et al. (2017b), Dinelli et al. (2017, 2019) and Migliorini et al. (2019, 2023). H<sub>3</sub><sup>+</sup> has a maximum of emission at 650-950 km above the 1-bar level, as obtained from ground-based measurements (Uno et al. 2014; Kita et al. 2018), while it shows a maximum at about 650 km above the 1-bar level, as observed at the limb by JIRAM at equatorial latitudes (Migliorini et al. 2023). An inversion method based on a global fit technique and a Bayesian approach can help to retrieve the concentration and temperature of the species (Dinelli et al. 2017, 2019; Adriani et al. 2017b; Migliorini et al. 2019, 2023). Better spectral resolution would allow the separation of the CH<sub>4</sub> emission at 3.32 µm, which is partially superimposed on the left tail of the H<sub>3</sub><sup>+</sup> emission at 3.42 µm, thus allowing the study of the CH<sub>4</sub> concentration at these heights. Limb measurements across to the auroral oval are suitable to investigate wavy activity, which is responsible for fluctuations in the temperature and concentration of H<sub>3</sub><sup>+</sup> at mid- to equatorial latitudes (O'Donoghue et al. 2021; Cosentino et al. 2017) and possibly CH<sub>4</sub>.

Besides compositional investigations, the extraction of wind velocities at different alti-
tudes is possible with repeated views of the same region over one or more Jovian rotations.
These provide access to the circulation within the cloudy regions, such as the strength of
the zonal jets, or the vorticity of cyclones and anticyclones. These velocities can be used to
calculate the divergence and vorticity fields of the horizontal wind, as well as their fluctua-
tions, in order to obtain the turbulent momentum and energy transfers between the different
dynamical elements of the atmosphere (Sánchez-Lavega et al. 2019). They can also be used
to explore the turbulent spectra of the cloud brightness as a function of latitude and wind
distribution (Barrado-Izagirre et al. 2009). The elevated, dense hazes can exhibit textured
patterns that can potentially be used to track motions and infer winds in the hazes and then
vertical wind shear relative to the background cloud motions. By comparison to features
tracked at higher pressures (e.g., from 5 µm emission) and intermediate altitudes (e.g., from
JANUS), MAJIS could then estimate the vertical wind shears associated with these dynamic
phenomena.

2.3.2 MAJIS Key Investigations

The review by Fletcher et al. (2023, this collection) illustrates how these science objectives related to the study of Jupiter's atmosphere will be accomplished at mission level. In this
section, we aim to provide more information on the specific issues that can be addressed using MAJIS data. We will demonstrate that the instrument design and planning can make a substantial contribution for achieving these scientific objectives, with the MAJIS traceability to the JUICE science objectives for Jupiter science cases summarized in Table 7. Specifically, the broad spectral range of MAJIS allows several different physical processes, described below, to be recorded in a single spectrum of Jupiter's atmosphere.

For nadir viewing conditions, solar radiation recorded by MAJIS can be scattered from levels down to 10-bars, in the absence of substantial overlying clouds. Observations at wavelengths with different methane opacity can be exploited to constrain the vertical distribution of aerosol densities. MAJIS daytime cubes will thus allow the mapping of cloud properties such as effective altitude and scale height, by extension of the methodologies presented in Grassi et al. (2021) for the JIRAM-Juno spectra, Sromovsky and Fry (2018) for Galileo/NIMS and New Horizons/LEISA measurements and Sromovsy and Fry (2010) for the Cassini/VIMS data. Retrieval of density profiles and particle radius from ground-based measurements in the visible range of MAJIS was described by Braude et al. (2020). The same study provided constraints on the spectral properties of chromophores that give Jupiter's clouds their characteristic color. In general terms, retrievals of aerosol properties are made difficult by the high degeneracy of the inverse problem, i.e. the same spectral shape can be created by very different combinations of vertical aerosol densities, particle sizes and spectral properties. This degeneracy is mainly linked with the very broad spectral features associated with aerosol scattering or absorption. The large spectral range of MAJIS is particularly beneficial for coping with these ambiguities, and determine composition, vertical structure and optical properties of clouds and hazes. In addition to obtaining the particle parameters by fitting the spectral radiance, the spatial information, i.e., the variation of radiance with the observation geometry (scattering angle), allows discrimination in the degeneracy introduced by a variety of models due to the multi-parametric nature of the problem. The daytime MAJIS cubes will also allow the retrieval of ammonia content in the upper troposphere at the level of 0.5-bar from the band at 2.8 µm, as demonstrated by Grassi et al. (2021) with JIRAM data. Similar studies based on MAJIS data are relevant for JUICE science investigations (Table 7). MAJIS spectra of a given region obtained under several different observing geometries will provide constraints on the properties of the uppermost cloud (ammonia mixed with chromophores) and the stratospheric haze layers above it up to 20-30 mbar. Of high interest will be also the study of polar regions where the haze layers extend to high altitudes (West et al. 2004), providing a potential connection between auroral chemistry and the production of polar aerosols, coupling the atmosphere to the external magnetospheric environment (Sinclair et al. 2019, 2023; Giles et al. 2023). Cavalier et al. (2021) also reported a wind vortex at polar latitudes that may help to increase the efficiency of chemical complexification by confining the photochemical products in a region of large energetic electron precipitation.

MAJIS cubes in the thermal range allow the mapping of mixing ratios of the minor species at the level of a few-bars by extension of the methodologies presented in Grassi et al. (2020) for JIRAM spectra, Giles et al. (2015) for the VIMS data and Irwin et al. (1998) for NIMS measurements. The extended spatial coverage available with MAJIS will enable the study of the distribution and potential correlation of contents of condensable species – ammonia and water vapor – at different spatial scales. As discussed in Grassi et al. (2010), MAJIS spectral coverage of the 5 to 5.4 µm range will be crucial to constrain the ammonia content in the 1-3-bar pressure range, where NH₄SH cloud forms from a chemical reaction of NH₃ and H₂S (Fig. 5). The large MAJIS dataset will provide strong constraints on the contents of disequilibrium species and their mutual relationships, clarifying the conflicting
<table><caption>**Table 7** MAJIS investigations related to mission Jupiter Science Requirements as described in the science matrix of the ESA JUICE Science Requirement Document by Titov (2014). See text (Sect. 2.3.3) for detailed physical parameters that will be retrieved</caption><thead><tr><th>JUICE Science objectives</th><th>JUICE investigations</th><th>Measurement requirements (limited to MAJIS)</th><th>Implementation</th><th>MAJIS related products</th></tr></thead><tbody><tr><td>JA. Characterize the atmospheric dynamics and circulation.</td><td>JA.1 Investigate the dynamics and variability of Jupiter's weather layer</td><td>JA.1c</td><td>Perform<br>1) regular scans of Jupiter disk covering the visible hemisphere in the mid/high latitude range (minimal 30°S, 30°N) and using the full spectral coverage<br>2) feature tracking with limited latitudinal coverage, acquired over the same geographical region at time intervals of about 2 h. Typically, about 2 pairs per peri-jove passage, mostly following the target list by JANUS<br>Combined measurement requirements regarding the spectral coverage and resolution, spatial resolution ("150 km/px") and global or near-global daily coverage are not compatible with the available data volume. Trade-offs will be necessary.<br>Cloud top winds speed estimates will be possible from planned "feature tracking" observations, albeit with a velocity resolution of 20 m/s for image pairs intercalated of 2H</td><td>1) Sets of 2-4 spectral cubes (between 400 to 800 lines depending on the distance and the MAJIS Internal scan exploration angle) in the spectral range 0.5-5.55 µm. Time gap between individual cubes is typically about 70 min, to ensure longitudinal coverage and similar emission angles.<br>2) 2-4 cubes (150 lines) covering the spectral range 0.5-5.56 µm</td></tr><tr><td>JA.2 Determine the thermodynamics of atmospheric meteorology</td><td>JA.2b</td><td></td><td>Perform occultations of a bright star while it rises/falls above/below Jupiter limb and preferentially away from peri-jove to exploit longer dwell time. Solar occultations are not currently envisaged</td><td>Sets of 10-20 spectral cubes of limited spatial extensions (typically 6 lines and 32 pixels close to the center of the slit) acquired around the nominal position cubes cover the full spectral range</td></tr></tbody></table>

<figure><img src="image_12.png" /></figure>
<table><caption>**Table 7** (Continued)</caption><thead><tr><th>JUICE Science objectives</th><th>JUICE investigations</th><th>Measurement requirements (limited to MAJIS)</th><th>Implementation</th><th>MAJIS related products</th></tr></thead><tbody><tr><td></td><td>JA.3 Quantify the roles of wave propagation and atmospheric coupling on energy and material transport<br>JA.4 Investigate auroral structure and energy transport mechanisms at high latitudes</td><td>JA.3a<br>JA.4a</td><td>Same as JA.1c (features tracking excluded) and JA.2b</td><td>Same as JA.1c and JA.2b</td></tr><tr><td></td><td>JA.5 Understand the interrelationships between the ionosphere and thermosphere</td><td>JA.5b</td><td>Perform scans of Jupiter's disk between the visible pole and 50° latitude (Jupiter's auroras) once every perijove passage. Time gap between individual cubes is 50 min.<br>Spectral coverage includes the main H<sub>3</sub><sup>+</sup> bands and the optically-thick CH4 band (2.2-2.7 µm), to constrain stratospheric aerosols. Given the increase of thermal signal beyond 4 µm, spectral coverage of auroral measurements is not envisaged there</td><td>Sets of 4 spectral cubes (150 lines maximum) covering the region between the visible pole and 50° latitude<br>1) Same as JA.4a<br>2) 4-5 cubes of about 150 lines each, with spectra coverage including the main H<sub>3</sub>+ bands and optically-thick CH4 bands<br>3) See JA.2b<br>4) 8 cubes (maximum) at different positions around the limb of the planet. Only the central 40 pixels of each line (about 100 lines) are transmitted to save data volume</td></tr></tbody></table>

<figure><img src="image_13.png" /></figure>
<table><caption>**Table 7 (Continued)**</caption><thead><tr><th>JUICE Science objectives</th><th>JUICE investigations</th><th>Measurement requirements (limited to MAJIS)</th><th>Implementation</th><th>MAJIS related products</th></tr></thead><tbody><tr><td rowspan="5">JB. Characterize the atmospheric composition and chemistry</td><td>JB.1 Determine Jupiter's bulk elemental composition to constrain formation and evolution</td><td>JB.1a</td><td>See disk scan implementation (JA.1c)</td><td>Same as JA.1c</td></tr><tr><td>JB.2 Investigate upper atmospheric chemistry and exogenic inputs from the stratosphere to the thermosphere</td><td>JB.2b</td><td>See Jupiter's aurorae implementation (JA.4a)</td><td>Same as JA.4a</td></tr><tr><td>JB.3 Study spatial variation in composition associated with discrete phenomena and polar vortices</td><td>JB.3b</td><td>See Jupiter's disk scan implementation. Disk scan preferentially performed during closest approach for being compatible with spatial resolution requirement</td><td>Same as JA.1c</td></tr><tr><td>JB.4 Determine the importance of moist convection in meteorology, cloud formation, and chemistry</td><td>JB.4a</td><td>Same as JA.1c</td><td>Same as JA.1c</td></tr><tr><td>JC.1 Determine the three-dimensional temperature, cloud and aerosols structure from Jupiter's upper troposphere to the lower thermosphere</td><td>JC.1a</td><td>1) Same as JA.1c<br>2) Perform feature tracking for a limited number (5-10) of cases along the mission at different longitudes and therefore different phase angles</td><td>1) Same as JA.1c<br>2) 4-5 cubes of about 150 lines each, covering the spectral range 0.5-3 µm</td></tr><tr><td>JC.2 Study coupling by waves, eddy mixing and global circulation across atmospheric layers</td><td>JC.2b</td><td>Perform Feature tracking observations for wind shear. Perform Scans of Jupiter's limb and star occultations for vertical study of the atmosphere</td><td>See JA.1c for feature tracking mode, JA.5b for limb observation, JA.2b for star occultation</td></tr></tbody></table>

<figure><img src="image_14.png" /></figure>
<figure><img src="image_15.png" /><figcaption>**Fig. 5** Jacobian functions (partial derivatives) of the measured MAJIS radiance for the mixing ratio of ammonia at different pressure levels and under aerosol-free conditions. Modified from Grassi et al. (2010)</figcaption></figure>

findings of previous studies (Grassi et al. 2020; Giles et al. 2017). These techniques are applied to radiance observed in the 5 µm methane transparency windows, and can therefore be pursued on both dayside and nightside data. These are complemented by retrievals of ammonia content at the approximate level of 0.5 bar from observations of its 3 µm band in daytime spectra, particularly valuable in relation to simultaneous and co-located cloud retrievals.

Finally, MAJIS images at selected wavelengths will reveal the variable morphology of the deep cloud layers, and allow the extraction of velocities at different altitudes obtained from the motion of atmospheric features. The most likely regions of interest in which studying wind variations with height are those with high-altitude hazes over a rich cloud field, such as the large anticyclones BA and the GRS (Fletcher et al. 2010; Sánchez-Lavega et al. 2018, 2021) and turbulent structures such as folded-filamentary regions and the ‘wake’ of the GRS; the tropics (Equatorial Zones and Belts) between latitudes of about 20°N and 20°S (García-Melendo et al. 2011; Antuñano et al. 2021), and the polar hazes (Barrado-Izagirre et al. 2008; Orton et al. 2017). In addition, the structure and motions within the convective plumes behind the major planetary-scale disturbances such as the NTBD (Sánchez-Lavega et al. 2017) and the SEBD (Fletcher et al. 2017) are also targets of high interest, if such disturbances occur during the JUICE mission. Finally, in the event of impacts by asteroidal or cometary bodies, MAJIS would be able to investigate the resulting distributions of aerosols and gases surrounding the impact scars. The wavelengths of greatest interest for the study of vertical wind-shear from imaging cloud-tracking will be (Fig. 4): 5 µm (P~1-2-bar), 750-850 nm (P~0.5-1-bar), 890 nm (P~0.2-0.3-bar), 1.7-1.8 µm (P~0.15-bar), 2.3 µm (P~0.05-bar), 3.35 µm (P~0.05-bar).

In addition to the spectral information, the MAJIS pushbroom capabilities will allow the generation of images at each of the spectral sampling points by spatial stacking of individual slits acquired in temporal sequence. At the Ganymede orbit distance, the MAJIS IFOV corresponds to 150 km above the reference 1-bar surface (approximately 0.1° in latitude or longitude), with a full FOV spanning over 50° in longitude at the equator and 85° in longitude at 50° latitude. Exploiting the full span of its own pointing mirror MAJIS can cover in the same cube latitudes between 70°S and 70°N.

The imaging capabilities of MAJIS are particularly valuable for monitoring phenomena occurring on Jupiter's nightside, given the instrument's coverage of the thermal infrared range. Of especial note is MAJIS IR data will assess the long-term evolution of the circum-polar cyclones (Adriani et al. 2018) since the Juno epoch. Furthermore, observations during
<figure><img src="image_16.png" /></figure>

**Fig. 6** Distribution of MAJIS scans of Jupiter's disk in terms of planetary phase angle and distance. Each triangle represents an individual MAJIS cube. The plan includes a total of 710 cubes (from CReMA 5.1). Of these, 346 are expected to be acquired at distances below $2\times10^6$ km providing a spatial resolution of 300 km. Given the much smaller overall disk size, the total data volume production is nevertheless dominated by observation at close distance. (B) Same as (A) but for the MAJIS scans of Jupiter's auroras, in terms of planetary phase angle and distance. A total of 228 cubes is planned. Since aurora observations are requested only within two days from pericenter passage, they seldom exceed a JUICE-Jupiter distance of $1.4\times10^6$ km. (C) Same as (A), but for the MAJIS scans of Jupiter's limb in terms of planetary phase angle and distance. The current plan includes a total of 1176 cubes. As per planning criteria, these observations are performed only when the JUICE-Jupiter distance remains below $2\times10^6$ km and as close as possible to passages at 0° and 90° planetary phase angle

the high-altitude phase of the JUICE Jupiter tour over multiple days will allow to study the short-term evolution of these structures, a task not achievable with the 2-month orbit of Juno (Mura et al. 2022; Scarica et al. 2022) and of high importance to reveal the ultimate mechanisms maintaining these patterns (Mura et al. 2021; Gavriel and Kaspi 2022). MAJIS imaging of the polar region is also essential for monitoring phenomena such as the auroral morphology (Adriani et al. 2017b) and to assess the long-term variability of subtle features such as satellites' auroral tails (Mura et al. 2018; Moirano et al. 2021). Studies on the basis of MAJIS data are relevant for JUICE science investigations JA.4 and JA.5 (SciRD and Table 7).

### 2.3.3 Typical Observational Strategy

Using the capabilities of MAJIS, a set of observations of Jupiter was defined to meet the science objectives and classified according to the following types:

- **Scans of Jupiter's disk** (see Fig. 6 for planning statistics and Fig. 18 in Fletcher et al. (2023, this collection) for a conceptual representation and example coverage). These observations are performed to study Jupiter's troposphere in the vertical range between 0.02 and 10-bars, by measuring emerging radiance in the entire MAJIS spectral range (with reduced spectral resolution for $\lambda < 4.1$ µm). They consist of a series of cubes acquired around the sub-S/C point. The distance between two adjacent cube lines is achieved by the motion of the MAJIS pointing mirror benefiting from the synchronized scan procedure (see Sect. 7.1). The rotation of the planet occurring during the time elapsed between two consecutive acquisitions (typically about 70 minutes) will create the longitudinal gap between the individual cubes. Exposure times and scan rates will be optimized to avoid saturation of the 5 µm methane transparency windows. Physical parameters retrievable from the spectra include the content of minor gases (JB.3), and, for spectra acquired in daytime, the aerosol density profiles (JC.1). These parameters will be mapped in latitude and longitude, enabling reciprocal intercomparing (JB.4). The number of cubes (that translate directly in the overall amplitude of longitudinal coverage) in each scan series and the temporal frequency of series will be very likely limited only by the available data volume. Current data volume estimates allow a complete coverage between 30°S and 30°N and about 150° in longitude
every 3 Earth days to track the dynamic evolution of meteorological features. A series of scans will be performed in close temporal proximity to each perijove passage. These observations will provide therefore the much needed continuous and regular time coverage not previously achieved neither by Juno-JIRAM, ground-based facilities, nor by JWST. The scans of Jupiter's disk according to these criteria satisfy most of the JUICE measurements requirements listed in JA.1c, JA.3a, JB.1a, JB.3b, JB.4a, JC.1a (Table 7).

- **Scans of Jupiter's auroras** (see Fig. 6 for planning statistics and Fig. 18 in Fletcher et al. (2023, this collection) for a conceptual representation and example coverage). These observations will be performed to study Jupiter's auroral emission over polar regions, by measuring emerging radiance in the $H_3^+$ bands ($\nu2$ and $2\nu2$, 3.105 – 3.798 µm and 2.003 – 2.199 µm), plus a few selected lines in the visible (H Balmer and Paschen series). They will consist of a series of four cubes acquired with a time interval of 50 minutes around the meridian passage of longitude 170 W, where the northern auroral oval reaches its southernmost extension (and therefore better visibility). The exposure time will be optimized for the weak $H_3^+$ emission and longer by a factor 5 than those adopted for ordinary disk scans. Physical parameters retrievable from spectra include the $H_3^+$ temperature and density (JA.4), and, for spectra acquired in daytime, stratospheric methane density and content of stratospheric hazes (JB.2). Repeated observations of the same area at close time intervals (50 min) also allows one to constrain the magnetospheric phenomena (among them, Alfvén waves) that drive the dynamic auroral morphology. Due to data volume limitation, we currently plan one series of auroral cubes per each orbit over one pole, in proximity of perijove passage. The scans of Jupiter's auroras according to these criteria are compliant with the JUICE measurement requirements JA.4a and JB.2b.

- **Scans of Jupiter's limb** (see Fig. 6 for planning statistics). These observations are performed to study Jupiter's atmospheric emissions at high altitudes (up to 1500 km above the 1-bar reference level), at several different latitudes. The spectral range transmitted to the ground covers the $H_3^+$ bands as well as in the CH$_4$ non-LTE emissions. In the initial orbits we plan to acquire a number of observations over the full spectral range, to search for previously undetected/unexpected high-altitude emissions. These observations will consist of a series of cubes acquired in a close time sequence around the passage of the S/C at phase angles (JUICE-Jupiter's center-Sun) of 90° (to allow the comparison of sub-solar and anti-solar regions) and 0° (to allow the comparison of dawn and dusk terminators). A series of observations will be acquired during the eclipse passages, to study the forward scattering of aerosol (e.g., on the nightside, with phases close to 180°). In order to minimize residual straylight due to the planet during daytime observations, MAJIS slit shall preferentially be placed tangent to Jupiter's limb during acquisition, and a cube will be acquired using the motion of MAJIS's own pointing mirror. Given the relatively large size of the MAJIS IFOV (typically 150 km) with respect to the atmosphere extension, two consecutive lines are required to be largely spatially overlapped (90% in area), to allow an *a posteriori* deconvolution of vertical radiance profile at each observed wavelength, for increased effective vertical resolution. Physical parameters retrievable from such observations include the vertical density profiles of $H_3^+$ and, for spectra acquired in daytime, of methane density and the opacity of stratospheric hazes. The requirement for slit orientation results in very complex operations for the S/C (especially from a power management perspective), thus limiting the number of cubes for each series to eight under the most favorable conditions. The scans of Jupiter's limb according to these criteria contribute to the fulfillment of the JUICE measurements requirements JA.5b and JC.2b.

- **Star occultations.** These observations are performed to study Jupiter's atmospheric transmissivity at high altitudes (up to 1500 km above the 1-bar reference surface), at several
different latitudes and over the entire MAJIS spectral range. These observations shall consist of a series of small cubes (6 lines and 32 spatially contiguous pixels out of the 400 of the entire slit) measuring the radiance of a bright background star as it falls below or rises from Jupiter's limb. The ratio of the radiance at a given vertical position above the limb to the radiance far away from Jupiter's disk provides a measure of the atmospheric transmissivity at that vertical position. While the spatial sampling of transmittance profiles is given by the frequency of the cube acquisitions, the effective vertical resolution of each transmittance measurement point is given by the actual size of the star's disk, and hence much smaller than MAJIS IFOV. Therefore, these observations will be performed preferentially away from perijove passages, to exploit smaller angular velocity of the star with respect to Jupiter and thus enabling a denser sampling of the profile. The 6 × 32 pixel cube size is the preferred size to account for possible initial pointing inaccuracies. The derived physical parameters will include the vertical density profiles of aerosols and optically active gases in stratosphere and upper troposphere (methane and possibly ammonia). MAJIS star occultations are compliant with the JUICE measurements requirements JA.2b and JA.3a; moreover, they contribute to the accomplishment of measurements requirements JA.5b and JC.2b.

**- Feature tracking.** These observations are conceptually similar to the standard Jupiter's disk scans (see above), but the S/C will be repointed at the start of each acquisition to ensure that the center of resulting cubes remains fixed at a given geographical location in the System III coordinate reference. The cubes are limited in latitude extension (typically 150 lines) to enable the acquisition of more cubes (up to eight) and at higher time frequency (minimum delay about 15 minutes). This mode is conceived for study of the local, short-term evolution of a feature of interest (e.g., GRS, a given 5-µm hot spot or vortex) in terms of composition or aerosol properties. Specific examples of these studies are the dayside/nightside comparison of possible thunderstorm formations crossing the terminator, to detect associated thunderstorm activity on the nightside, or the estimation of the decay time of $H_3^+$ low-latitude emissions across the evening terminators. Comparison of cubes at least two hours apart will also be valuable for tracking motion and retrieving wind speeds. In most cases, these observations will be scheduled along the pointings required by the JANUS camera during perijove segments. MAJIS feature tracking observations are compliant with the JUICE measurements requests in JA.1c (albeit at a lower degree of confidence) and JC.2b. Slightly modified versions (in terms of spectral coverage or pointing) will be used to achieve JUICE measurements requirements JA.5b and JC.1a.

Science observations of Jupiter are detailed later in the paper in Sect. 7.1 and Table 21.

## 2.4 Other Moons and Rings

### 2.4.1 Io

Io is the most volcanically active body in our Solar System. Its strong activity is mainly due to the Laplace resonance involving Io, Europa and Ganymede, which is responsible for the intense tidal heating of Io's interior (Peale et al. 1979). The global scale resurfacing induced by the strong volcanism constantly renews Io surface, which appears geologically young and free of impact craters (Lopes and Spencer 2007). The optical images returned by the NASA missions Voyager and Galileo revealed plenty of geological features, often associated with volcanic eruptions, such as paterae and lava flows, while most reliefs are interpreted as of tectonic origin (Carr et al. 1998; Schenk et al. 2001; Jaeger et al. 2003). Furthermore, volcanic eruptions can reach temperature values as high as 1400 K (McEwen et al. 1998a, b), suggesting Io may be characterized by silicate volcanism consistent with
Fe-bearing salts, silicates such as feldspars and pyroxenes at least in specific locations. This volcanic activity largely determines the surface composition, which is dominated by S-rich compounds/oxides and other minor species (e.g. NaCl, probably KCl, and atomic chlorine that may combine with SO₂ to form Cl₂SO₂ and ClSO₂) (e.g. Cruikshank 1980; Cruikshank et al. 1985; Fanale et al. 1979; Howell et al. 1984; Schmitt and Rodriguez 2003; Carlson et al. 2007). Io is considered containing little to no water, though small pockets of water ice or hydrated minerals have been tentatively identified (Douté et al. 2004; Lopes and Spencer 2007). Douté et al. (2004) analyzed Galileo/NIMS data with a spatial scale ranging between 3 and 46 km/pixel. Albeit the low NIMS spectral sampling due to radiation damage (Lopes et al. 2001; Douté et al. 2001), they observed an unusual absorption band at 3 µm localized in correspondence of a bright deposit northwest of Gish Bar Patera, and interpreted it as potentially linked with H₂O molecules forming ice crystals or being trapped in an unidentified matrix. Recent results from JIRAM revealed the existence of very weak spectral features centered at 2.97, 3.15 and 3.16 µm potentially associated with the presence of H₂O/SO₂ and H₂S/SO₂ mixtures (Mura et al. 2020b; Tosi et al. 2020). Although MAJIS will not achieve the spatial resolution values obtained by NIMS and JIRAM, the spatial scales of its data will be in many cases comparable with those of the data used in early works based on JIRAM (Mura et al. 2020b; Tosi et al. 2020). Furthermore, the higher spectral sampling will enhance de identification capability for weak and narrow absorption bands compared to previous observations, hence the detection of these compounds.

The widespread occurrence and temporal variability of the hot spots of Io has been studied by decades of ground-based observations (e.g., de Pater et al. 2004; de Kleer and de Pater 2016a,b; Cantrall et al. 2018; de Kleer et al. 2019), and remote sensing instruments onboard different S/C (Radebaugh et al. 2004; Rathbun et al. 2014; Lopes and Spencer 2007; Veeder et al. 2015; Mura et al. 2020b; Zambon et al. 2023). To date, several hot spot catalogs are available. Lopes and Spencer (2007) and Veeder et al. (2015) published the most complete catalog of hot spot based on Galileo data, while Io hot spots are constantly monitored by ground-based observations (e.g., Cantrall et al. 2018; de Kleer et al. 2019; de Pater et al. 2004; de Kleer and de Pater 2016a,b).

JIRAM acquired infrared images and spectra of Io during both the nominal and extended mission phase, achieving a ground sampling of 9 km/pixel (potentially up to 1 km/pixel by the end of 2023). The surface composition as observed by JIRAM confirmed the widespread occurrence of SO₂ frost including multiple isotopologues, whose distribution is largely correlated with sulfuryl chloride (Cl₂SO₂), and suggested the potential presence of hydrogen sulfide H₂S (Tosi et al. 2020). Based on the JIRAM dataset, Zambon et al. (2023) published an updated catalog, identifying 242 hot spots including new ones in the polar regions, which could not be covered by previous flybys and Earth-based telescopic observations.

During its Jupiter tour, JUICE will have many opportunities to observe Io from a distance. While close flybys are precluded by the mission profile, MAJIS will have 18 opportunities to observe Io at a MAJIS spatial resolution better than 100 km (none of which in eclipse) with the present trajectory baseline (crema5.1), achieving a maximum spatial resolution of 57 km/pixel (Fig. 7). These mappings correspond to the high-level JUICE objective SA.1 defined in the SciRD, which consist of monitoring Io's activity at a wide range of longitudes and local times. While Juno/JIRAM's spatial resolution cannot be surpassed by the JUICE mission focused on Jupiter's icy moons, the main strength of MAJIS is its broad spectral range that allows compositional mapping of the various sulfur-bearing species as well as hot spot monitoring at wavelengths up to 5.56 µm, enabling the study of cooler features.

MAJIS will investigate possible compositional variations across the dayside of Io, complementing the multispectral data obtained by JANUS with 10 times greater ground sampling. Furthermore, MAJIS will monitor the spatial and temporal variability of hot spots,
<figure><img src="image_17.png" /><figcaption>**Fig. 7** Variability of the illuminated disk fraction during the JUICE distant observations of Io for MAJIS spatial resolution lower than 100 km/pixel. The Io 18 distant observations are indicated with numerated vertical lines. Their color is correlated with the priority of each observation. In red, we display Io observations in conflict with those of the other Galilean satellites, in green the high priority observations, while in blue and dark blue the lower priority ones. Blue dots are indicative of lower illuminated disk fraction values, while red dots are representative of higher values</figcaption></figure>

which is key to constrain the tidal heating and the mechanisms by which heat escapes from the interior (e.g. Lopes et al. 1999; Tackley et al. 2001; Hamilton et al. 2013; Tackley 2013; Rathbun et al. 2014; Davies et al. 2015; de Kleer et al. 2019). An appropriate observation strategy has been defined to achieve these goals. The optimal condition to study Io's surface composition occurs when most of the dayside (e.g., >60%) is observed. In this case, a broadly regional coverage can be obtained in a few hours by acquiring data cubes at 1-hour intervals. Conversely, the best opportunities to observe the hot spots occur when most of the nightside is observed. A higher observation frequency (e.g. every 30 minutes) is then more appropriate considering the large variability of volcanic activity over time.

## 2.4.2 Minor Moons

The question of the formation and early evolution of the four small regular satellites revolving in the internal region of the Jupiter system between 1.8 and 3.1 R<sub>J</sub>, immersed in the dust ring system, is still controversial, with important potential outcomes. They could have formed by accretion from the circumjovian nebula and thus should be composed of high-density refractory materials (Pollack and Fanale 1982). However, the density of Amalthea seems lower than that of water (Anderson et al. 2005) while the moon exhibits a deep absorption band at 3 µm, diagnostic of hydrated minerals and possibly organic materials (Takato et al. 2004), challenging its formation process, the origin of its constitutive materials, and/or its own potential migration.

MAJIS will be able to acquire spatially resolved spectra (1 to 2 pixels) of Amalthea and Thebe, the two largest inner satellite, to evaluate similarities or differences with respect to the composition of the rings. MAJIS will also be able to derive phase curves useful for constraining the properties of the surface regolith of these objects. These observations will be performed as JANUS ride-along observations.

The irregular satellites that represent most of Jupiter's known natural satellites are more difficult to be observed by MAJIS because the currently planned trajectory of JUICE does not include one close flyby of any of these bodies. One of the interests to study the irregular
satellites is to assess the possible contamination of the surfaces of the Galilean satellites by these objects, introducing exogenous elements with potentially different compositional characteristics. A striking example in the Saturn system is the dark material mantling the leading hemisphere of Iapetus, which originates from the irregular satellite Phoebe (e.g., Tosi et al. 2010). Callisto is the outermost Galilean satellite and displays the lowest albedo among the four regular moons, which might be at least partly indicative of a similar contamination, albeit at a smaller magnitude. The nature of the “contaminants” entering the Galilean satellite system from outside depends on the composition of the irregular satellites, which is closely linked to the regions of formation of their parent bodies.

### 2.4.3 Rings

The Jupiter ring system of Jupiter. It has four main components (Esposito 2002; Ockert-Bell et al. 1999): 1) a relatively bright, very thin ($<$ ~30 km) “main ring”; this main ring is subdivided into three distinct, continuous and uniform rings located between 128,000 km and 129,000 km from the center of the planet. It is composed predominantly of “large” grains in the mm–km size range, and is probably the source of dust in the ring that is seen inward; 2) a thick inner torus of particles known as the “halo ring”; 3) a wide, thick and faint outer “Amalthea gossamer ring” out to 185,000 km (a distance corresponding to the semi-major axis of Amalthea); 4) another “gossamer ring” mostly within Thebe’s orbit.

After numerous observations by large ground-based telescopes, the HST and 7 space missions (from Pioneer dust particle to recently Juno), the physical properties and the composition of rings are still poorly known. This is especially true for the compositional properties as the spectral observations are very sparse with a limited spectral range and spectral resolution (Wong et al. 2006). A compilation of the observations under backscattered illumination indicates that the main ring exhibits a very red slope from 0.4 to 3 µm (Wong et al. 2006; De Pater et al. 2018). A 3.8 µm band (at $1-\sigma$) has been reported which is consistent with an absorption feature seen in the spectrum of Amalthea. Images at different wavelengths and angles further reveal that the relative contribution of each component is variable.

The ultimate objective is to study the rings as a trace of the original properties of the disk material, of its primordial evolution (migration then turbulence/subsequent filling), in comparison with those of the satellites (small and large). JUICE will infer the physical and chemical properties of Jupiter’s rings, by imaging the ring system in 3D and in a wide range of phase angles, and by multi-wavelength mapping of the ring particles’ composition and photometric behavior from the UV to the IR (identified as requirement SB.1 in the SciRD). Spectra of the rings and variations in intensity with phase angle by MAJIS shall improve the knowledge of the composition and size distribution of the ring particles, both of which are tied to the source of the rings. The observation of the rings system with MAJIS is however very challenging due to their very low optical depth leading to normalized reflectance in the range of $10^{-5}$ to $10^{-7}$. Several MAJIS measurement types have been defined, and their relevance to the mission objectives is presented in Table 8. The observational strategy for rings aims at optimizing the SNR. These observations will be performed with a long integration time using the read-out mode with the lowest read-out-noise (100 kHz as shown in Table 11) and at a relatively far distance ($>$8.10⁵ km at closest approach) for mitigating spike rates from high energy electrons (see Sect. 5). This limits the minimal resolution of the images to 120 km without spatial binning (spatial and spectral binning may be considered for improving the SNR). Observations will be performed close to the equatorial plane ($<$ 0.5° away) so as to increase the column density of ring particles, preferentially in eclipse so as to avoid background contributions from Jupiter with the long integration times.
<table><caption>**Table 8** Relationship between high-level JUICE science objective and MAJIS measurements dedicated to the study of the ring system</caption><thead><tr><th>JUICE Investigations</th><th>MAJIS measurement requirement</th><th>MAJIS ring campaign</th><th>MAJIS data products</th></tr></thead><tbody><tr><td rowspan="4">SB.1 Study the physical properties of the inner small moons and rings</td><td rowspan="4">SB.1b.</td><td>Perform a radial (and azimuthal) mosaic of the jovian rings at high phase and low elevation (~&lt; 0.5°) using the MAJIS scan mirror (with its direction along X<sub>SC</sub> maintained in horizontal direction) during eclipse</td><td>3 overlapping cubes (swath of ~50,000 km) covering the radial distance of ~90,000-230,000 km for one ansa</td></tr><tr><td>Perform main ring observation at low phase (&lt;10°) and low elevation (&lt;0.2°)</td><td>1 to 3 cubes for each extremity (&gt;100,000 km) of the main rings</td></tr><tr><td>Monitor phase curves of main rings from high phase to low phase angles with a sampling of ~10°. Coordinated observations with JANUS would provide a phase function in the JANUS-MAJIS wavelength range</td><td>1 to 3 cubes per ansa depending on the duration of the eclipse and JANUS position in the case of ride-along with JANUS</td></tr><tr><td>Perform star occultation across the rings requiring inertial pointing of the S/C and the star maintained in the slit</td><td>A series of cubes (6 × 32)</td></tr></tbody></table>

A few tens of opportunities have been identified during the Jovian tour phase. Some will nevertheless not be feasible due to operational constraints (duration of the eclipse) and potential pointing conflicts with other disciplines or downlink windows.

# 3 Instrument Requirements and Design Overview

## 3.1 Requirements and as-Built Performances

MAJIS' high-level requirements are tabulated in Table 9 and compared with as-built performances. The most basic requirements (e.g. spatial and spectral sampling and wavelength range) are high level requirements of the JUICE project as defined by the SciRD; others are internal requirements to MAJIS. Both are derived from various previous imaging spectroscopy investigations of the planetary surfaces and the MAJIS objectives Sect. 2).

## 3.2 System Overview

The scientific and technical trade-off based on the requirements in Table 9 led to a hyperspectral imager covering the overall spectral range (0.5–5.55 µm) with two channels covering each more than one octave: the VISNIR channel (0.5–2.35 µm) and the IR channel (2.25–5.55 µm) with a small overlap around 2.3 µm. The two spectral channels operate simultaneously, sharing a single slit illuminated by a three-mirror anastigmatic telescope equipped with a pointing/scanning mirror (Sect. 4.1). A dichroic beam splitter placed after the entrance slit separates the VISNIR and IR beams towards the grating spectrometer
<table><caption>**Table 9** MAJIS driving requirements and as-built performances at nominal temperature (OH ~ 130 K). Comparison when possible with NIMS (Carlson et al. 1992) and JIRAM (Adriani et al. 2017a) are presented when possible and relevant</caption><thead><tr><th>Requirement</th><th>Required value</th><th>Achieved</th><th>Comment/justification</th><th>Comparison with NIMS and JIRAM</th></tr></thead><tbody><tr><td>Spectral range (VISNIR)</td><td>0.5-2.35 µm</td><td>0.49-2.35 µm</td><td>After launch</td><td>0.7+ µm 2+ µm</td></tr><tr><td>Spectral range (IR)</td><td>2.25-5.54 µm</td><td>2.27-5.56 µm</td><td>After launch</td><td>Up to 5.2 µm Up to 5 µm</td></tr><tr><td>Nominal spectral sampling (VISNIR)</td><td>≤ 3.65 nm ± 0.25</td><td>3.5-3.8 nm</td><td>Nominal case corresponding to 508 spectels (spectral element)</td><td>12.5/25 nm 10 nm</td></tr><tr><td>Nominal spectral sampling (IR)</td><td>≤ 6.50 nm ± 0.25</td><td>5.9-6.9 nm</td><td>Nominal case corresponding to 508 spectels</td><td>25 nm (10 nm for λ &lt;1 µm) 10 nm</td></tr><tr><td>Spectral FWHM (VISNIR)</td><td>≤ 5.5 nm</td><td>2.9-4.6 nm</td><td>Depends on wavelengths. See Sect. 6.2</td><td>12.5 nm</td></tr><tr><td>Spectral FWHM (IR)</td><td>≤ 10 nm</td><td>5.5-7 nm</td><td>Depends on wavelengths. See Sect. 6.2</td><td>12.5 nm</td></tr><tr><td>Swath width</td><td>3.4°</td><td>3.4°</td><td>400 number of pixels across FOV</td><td>3.5°</td></tr><tr><td>Spatial sampling</td><td>150 µrad</td><td>150 µrad</td><td>e.g. 75 m/pix during the GCO500 orbit</td><td>500 µrad 240 µrad</td></tr><tr><td>Lifetime</td><td>>14 y</td><td>>14 y</td><td>Estimate based on heritage and degradation tests at sub-system levels</td><td></td></tr><tr><td>Optical header temperature</td><td>&lt;140 K</td><td>~126 K attainable</td><td>Operational temperature depending on trajectory</td><td></td></tr><tr><td>VISNIR detector temperature</td><td>&lt;140 K</td><td>127 K attainable</td><td>Operational temperature depending on trajectory</td><td></td></tr><tr><td>IR detector temperature</td><td>&lt;90 K</td><td>88 K attainable</td><td>Operational temperature depending on trajectory and operations</td><td></td></tr><tr><td>Pointing</td><td>±4° along track</td><td>±4° along track</td><td>Using the scan mirror. Larger angle exploration is possible for ICU pointing (8.5°), 1 step is equal to 0.003139° (optical angle) so that a motion of 3 steps corresponds to 1.0957 IFOV</td><td></td></tr></tbody></table>

<figure><img src="image_18.png" /></figure>
<table><caption>**Table 9 (Continued)**</caption><thead><tr><th>Requirement</th><th>Required value</th><th>Achieved</th><th>Comment/justification</th><th>Comparison with NIMS and JIRAM</th></tr></thead><tbody><tr><td>Scan mechanism compensation</td><td>Up to 5.1 km/s</td><td>Compliant</td><td>GCO200 science case</td><td></td></tr><tr><td>Spectral smile (VISNIR)</td><td>≤ 3.65 nm</td><td>≤ 2.1 nm</td><td>2D smile</td><td></td></tr><tr><td>Keystone (VISNIR)</td><td>≤ 1 IFOV</td><td>≤ 1 IFOV</td><td></td><td></td></tr><tr><td>Spectral smile (IR)</td><td>≤ 6.50 nm ± 0.60 nm</td><td>≤ 3.1 nm</td><td>Limited spectral data</td><td></td></tr><tr><td>Keystone (IR)</td><td>≤ 1 IFOV</td><td>≤ 1 IFOV</td><td></td><td></td></tr><tr><td>Expected out-of-field strahlight</td><td>&lt;2%</td><td>Out-of-field strahlight below 1300 nm</td><td>Partly mitigated by subtraction of the background (shutter closed signal)</td><td></td></tr><tr><td>Expected in-field strahlight</td><td>&lt;10<sup>-4</sup></td><td>Not compliant for VISNIR channel Compliant for IR</td><td>See Sects. 6.4 and 6.5</td><td></td></tr><tr><td>Total Ionizing Dose (TID):</td><td></td><td></td><td>Passive despiking (shielding + spot shielding)</td><td></td></tr><tr><td>- Optical elements</td><td>&lt;100 to 500 krad</td><td>&lt;100 to 500 krad</td><td></td><td></td></tr><tr><td>- Electronic components</td><td>&lt;50 krad</td><td></td><td></td><td></td></tr><tr><td>- Detectors</td><td>&lt;80 krad</td><td>&lt;80 krad</td><td></td><td></td></tr><tr><td>Spikes resulting from high energy electron impact</td><td>At most 1% of impacted pixels after filtering</td><td>Compliant</td><td>Active on-board despiking; see Sect. 5.3.1</td><td></td></tr><tr><td>SNR (icy satellites)</td><td>&gt;100 for λ &lt; 2.8 µm<br/>&gt;30 in 3.2-3.6 µm<br/>&gt;20 for λ &gt; 3.6 µm</td><td></td><td>Compliant, except for the 0.6-0.7 µm range that may be impacted by a spectrally structured strahlight component</td><td>See Sects. 6.4 and 6.5</td></tr><tr><td>SNR (Jupiter)</td><td>&gt;100 for almost all wavelengths, methane absorption excluded<br/>&gt;10 for aurora</td><td></td><td>Compliant, except for the 0.6-0.7 µm range that may be impacted by a spectrally structured strahlight component</td><td>See Sects. 6.4 and 6.5</td></tr></tbody></table>

<figure><img src="image_19.png" /></figure>
relevant to each channel. Each pixel along the slit is thus spectrally dispersed over the entire spectral range. On the imaging side, the MAJIS FOV imaged on the entire slit covers a “line” of 3.4° long and ~31 arcsec large (58.9 mrad × 150 µrad). Thanks to the scanning mechanism MAJIS has a pointing/scanning/motion compensation capability in the across slit direction up to ±4° with respect to boresight (Sect. 4.1.2).

Two 1024 × 1024 pixels (18 µm × 18 µm pixel pitch) HgCdTe detectors (model H1RG by Teledyne) are used as focal planes of the two channels (Sect. 4.1.3). After binning 2 × 2, it provides a useful 2D spatial and spectral area of 400 × 508 pixels with a pitch of 36 × 36 µm². The spatial sampling and spectral sampling can be adjusted independently by on-board binning by x2 or x4 depending on the observational conditions. The two focal planes are equipped with linear variable optical filters (LVFs) to reject the grating’s high orders. The spectral samplings of the VISNIR and IR channels are < 4 nm/band and < 7 nm/band, respectively, over 508 spectral elements.

As customary for an imaging spectrometer, each frame registers the image of the slit (400 MAJIS pixels) dispersed by the two gratings in the spectral direction (nominally 508 bands per channel, up to 640 if spectral oversampling is applied as described in Sect. 5.4). Since the two channels operate simultaneously, the instrument is able to collect the full spectral range (0.5-5.55 µm) on each pixel of the slit after complex on-board data processing including various corrections, spectral and spatial capabilities, despiking... (see Sect. 5). Moreover, a shutter mechanism enables dark current measurements and thermal background frame measurements before and/or after a scientific frame sequence for science data correction.

Each of the two VISNIR and the IR focal plane arrays (FPA) is connected to their dedicated Focal Plane Electronics (FPE) ASIC (Application Specific Integrated Circuit) module (referred to as SIDECAR) housed inside the Optical Head (OH). Each FPE ensures the basic read-out functionalities (bias generation, accurate programmable DAC, read-out clocks, ADC, signal conditioning...) of the video signals coming from its associated FPA (Sect. 4.1.3).

To achieve the scientific performance of the MAJIS instrument over the whole spectral range, the optical head and the detectors of the two spectral channels are cooled to cryogenic temperatures (~130 K for VISNIR detector and OH structure, and ~90 K for the IR detector, nominal case) by means of passive thermal dissipation through two dedicated external radiators (Sect. 4.4). The entire OH structure is kept thermally isolated from the JUICE interface by isostatic mounting, thermal washers and multilayer blankets.

MAJIS has the flexibility to operate either in pushbroom or in scanning mode. In pushbroom mode, once the scanning mirror is fixed at a pre-defined pointing position, the acquisition of a single frame constitutes the (x,λ) hyperspectral dataset of a single line, imaged at the surface. When sequences of acquisitions are performed while the S/C moves along the scene, a tri-dimensional (x,y,λ) hyperspectral image cube is built. The optimized spatial resolution is obtained when the acquisition time of frame coincides with the dwell time, defined as the time interval during which the sub-spacecraft point at the surface moves by one IFOV. An acquisition time which is larger or smaller than the dwell time results in undersampling or oversampling, respectively. MAJIS can use its scanner for motion compensation, the dwell time being adjusted to the acquisition time by the rotation of the scanner. This is critical at low distances from the target such as the GCO-500 orbit or at closest approach during flybys (see Sect. 7.1): the dwell time can be very short, down to a few 10 ms, when a typical acquisition requires 500 ms per frame. Since the orientation of the S/C may require either starting at +2° or at -2°, the scan mechanism must provide:

- Start at +2°, negative angular velocity, increasing (before closest approach) or decreasing (after closest approach)
Start at –2°, positive angular velocity, increasing (before closest approach) or decreasing (after closest approach). The scanning mode can also be used to acquire hyper spectral cubes and mosaics of Jupiter and Galilean satellites during transfer, elliptical or high-altitude circular orbits without requesting a spacecraft slew.

The Main Electronics Box (ME Box) located inside the S/C vault handles control/command functionnalities and powers the OH subsystems by means of dedicated electronic modules. It also ensures the communication and power distribution with/from the JUICE On-Board Computer (Sect. 4.2). The DPU-CPCU (Digital Processing Unit/Command and Process Control Unit) ensures the telecommand/telemetry (TC/TM), Housekeeping (HK) and science functions by means of the Auxiliary (AUX) board and the Compression Units (CU). It also performs the power On/Off switching of the different boards. The communication between the S/C On-Board Computer (OBC) and the DPU is based on the embedded SpW interface inside the DPU-CPCU module. All the auxiliary units located inside the OH (scanning mirror motor and resolver, shutter, internal calibration unit and thermal sensors) are powered, commanded and monitored through the AUX board. Cold redundancy is implemented for the DPU-CPCU module, with a nominal and redundant unit. Two Proximity Electronic modules (PE-VISNIR and PE-IR) inside the ME Box, acquire the scientific digitized data from each FPE and perform preprocessing tasks (dark subtraction, despiking, binning, editing) before transmitting the scientific and the housekeeping data to the corresponding Compression Unit (CU) inside the Digital Processing Unit (ME/DPU-CU) of the ME Box.

The OH is equipped with an Internal Calibration Unit (ICU) whose purpose is to provide reference signals necessary to perform in-flight calibration sequences (Sect. 4.1.5). The ICU uses two light sources, a Quartz Tungsten Halogen (QTH) lamp and an IR emitter, and a diffusive target illuminating the full FOV of the telescope. Each source is powered with a stabilized current to provide a high reproducibility of the flux. The QTH is optimized for the VISNIR spectral range and the IR emitter for the IR spectral range. The two sources are equipped with a transmission filter exhibiting spectral absorptions that can be used as reference monitoring the instrumental spectral response along the mission. The ICU signal is acquired by commanding the scan mirror position towards the telescope baffle (beyond the edge of the FOV) where the ICU is housed.

As a result of Jupiter's magnetic field, which is 20 times stronger than Earth's, the energy distribution of electrons and ions extends to much higher energies than that of the radiation belts of the Earth. Radiation sensitive parts (optical and electronic components as well as detectors) are specifically shielded and mechanically adapted to reduce the TID to a dose that meets the specifications. Mitigating despiking procedures managed by on-board software also provide high flexibility so as to cope with the large and target-dependent impact rates of high energy electrons. The goal is to have less than 1% of MAJIS data elements corrupted by spikes due to high-energy electron impacts.

The MAJIS instrument implements survival and anticontamination heater lines (with dedicated monitoring sensors) inside the OH that are directly controlled by the JUICE DPU during the non-operating modes of the instrument.

Figure 8 and Table 10 present the functional block diagram of the MAJIS instrument, including the different modules (both nominal and redundant when relevant) summarized in this section, as well as the electrical interconnection links. The as-flown hardware is shown in Fig. 9.
<figure><img src="image_20.png" /><figcaption>**Fig. 8** Block diagram including Optical Head (OH), Main Electronics Box (ME) and data/power links. Note: redundancies are not represented (i.e DPU, heaters and sensors lines ...). The definitions of the units identified by numbers are shown in Table 10</figcaption></figure>

# 4 Instrument Description

This section describes the major sub-systems of the instrument. Some of the performances reported here were validated by measurements before integration and additional validation at instrument level.

## 4.1 Optical Head Unit

### 4.1.1 Optics Overview

The key drivers for the MAJIS optical design were the wide spectral range, the characteristics of the focal plane units, the adverse radiation environment, the necessity to maintain the mass as low as possible and the cryogenic operative temperatures. All MAJIS optical subsystems have been designed and optimized to work in an operative temperature range from 100 K to 140 K and all (optical and mechanical) materials were chosen to reduce as much as possible the deformations due to different thermal behavior between materials with different Coefficient of Thermal Expansion (CTE).

Figure 10 shows the conceptual scheme of the MAJIS optical head before illuminating the two focal plane units. The optical layout of the OH is disposed onto two planes: the telescope with the baffle and the internal calibration unit are located on the interior of the optical bench (Fig. 11A), while the spectrometers (constituted of the collimator, the dichroic, a grating and a focusing lens for each channel) are mounted on the top of the optical bench (Fig. 11B), in order to be as close as possible to the radiators. Such optical layout was chosen
<table><caption>**Table 10** Definition of the subunits shown in Fig. 8. See acronyms in text</caption><thead><tr><th></th><th>Unit/sub-system</th><th></th><th>Harness or Flex link</th></tr></thead><tbody><tr><td>1</td><td>Reference mirror</td><td>a</td><td>ME/AUX to scanning mirror mechanism, resolver power and TM link</td></tr><tr><td>2</td><td>Entrance baffle</td><td></td><td></td></tr><tr><td>3</td><td>ICU: Black Body (IR) and VISNIR lamps</td><td></td><td></td></tr><tr><td>4,5</td><td>Pointing/scanning mirror, motor and resolver</td><td></td><td></td></tr><tr><td>6</td><td>Three Mirror anastigmatic Telescope</td><td>b</td><td>ME/AUX to ICU power link</td></tr><tr><td>7</td><td>Entrance slit and shutter</td><td></td><td></td></tr><tr><td>8</td><td>Shutter mechanism and actuator</td><td>c</td><td>ME/AUX to shutter mechanism power &amp; TM link</td></tr><tr><td>9</td><td>Beam separator (dichroic window)</td><td></td><td></td></tr><tr><td>10</td><td>Spectrometers: (a) VISNIR channel, (b) IR channel</td><td>d</td><td>FPA to FPE to ME/PE control links (VISNIR &amp; IR)</td></tr><tr><td>11</td><td>FPA: (a) VISNIR channel, (b) IR channel</td><td></td><td></td></tr><tr><td>12</td><td>FPE: (a) VISNIR channel, (b) IR channel</td><td>e</td><td>FPE to ME/PE control links (VISNIR &amp; IR)</td></tr><tr><td>13</td><td>OH radiator</td><td></td><td></td></tr><tr><td>14,15</td><td>Thermal straps to OH radiator</td><td>f</td><td>ME/PCDU to ME/PE (VISNIR &amp; IR) power lines</td></tr><tr><td>16</td><td>IR channel detector radiator</td><td></td><td></td></tr><tr><td>17</td><td>IR channel detector to radiator thermal strap</td><td>g</td><td>ME/PE to ME/DPU-CU SpW link (VISNIR, Nominal and Redundant)</td></tr><tr><td>18</td><td>Anticontamination Heaters and sensors lines</td><td>h</td><td>ME/PE to ME/DPU-CU SpW link (IR, Nominal and Redundant)</td></tr><tr><td>19</td><td>Survival Heaters and sensors lines</td><td></td><td></td></tr><tr><td>20</td><td>ME/PE IR Board</td><td>i</td><td>SpW link between ME/CPCU and ME/AUX (Nominal and Redundant)</td></tr><tr><td>21</td><td>ME/PE VISNIR Board</td><td>j</td><td>ME/PCDU to ME/AUX power line</td></tr><tr><td>22</td><td>ME/DPU Boards (Nominal and Redundant)</td><td>k</td><td>ME/PCDU to ME/DPU power line</td></tr><tr><td>23</td><td>ME/DPU-CU: (a) VISNIR channel, (b) IR channel</td><td>l</td><td>JUICE to ME power lines (Nominal and Redundant)</td></tr><tr><td>24</td><td>ME/DPU-CPCU</td><td>m</td><td>ME to JUICE bus data/commanding SpW links (Nominal and Redundant)</td></tr><tr><td>25</td><td>ME/AUX Board</td><td>n</td><td>Anticontamination, survival heaters/thermistors power and TM lines to JUICE bus</td></tr><tr><td>26</td><td>ME/PCDU Boards (Nominal and Redundant)</td><td></td><td></td></tr><tr><td>27</td><td>ME Box inside the JUICE Vault</td><td></td><td></td></tr></tbody></table>

to minimize the size of the optical head. It also makes possible the mounting of the optical elements on both faces of the optical bench.

The MAJIS focal length is 240 mm, with a different f-number between the two directions because the entrance pupil has a non-circular shape to reduce the layout dimension in the direction perpendicular to the optical bench. The equivalent pupil is defined with a special aperture on the M2 mirror, resulting in an equivalent diameter of 75 mm and an equivalent system f-number of 3.2.
<figure><img src="image_21.png" /><figcaption>**Fig. 9** (A) Photograph of the optical head (dimensions: 912 × 765 × 356 mm³, 40.9 kg). The entrance of the telescope was protected by a red cover that was removed just before launch. The two white radiators are visible on the top of the optical bench. (B) Photograph of the VISNIR focal plane unit (dimensions: 74 × 111 × 102 mm³, 0.675 kg). (C) Photograph of the main electronics box (dimensions: 320 × 249 × 161 mm³, 8.2 kg). Several schematics of these units are presented in Sect. 4. Copyright IAS/LDO</figcaption></figure>

<figure><img src="image_22.png" /><figcaption>**Fig. 10** Functional block diagram of the MAJIS optical design before illuminating the two focal plane units</figcaption></figure>
<figure><img src="image_23.png" /><figcaption>**Fig. 11** MAJIS optical layout 3D overview. The telescope (A) is on the lower side while the spectrometer (B) is on the top. The light is driven towards the upper side of the optical bench by two folding mirrors, where the slit is placed. Approximative vertical dimension of the optical head is indicated in C</figcaption></figure>

The same detector array dimension is implemented for both channels: an array of 400 × 508 pixels (400 in the spatial direction, 508 in the spectral direction) with a 36 µm pitch. The resulting IFOV is 150 µrad, while the MAJIS FOV along slit is ±1.7°.

**Telescope** The MAJIS telescope is shared between the two channels. The telescope is a Three Mirrors Anastigmatic (TMA) telescope constituted by two off-axis (M1 and M3) and one on-axis (M2) conic mirrors. MAJIS telescope results to be telecentric in object space, with the aperture stop placed in correspondence of the M2 mirror, defining the MAJIS pupil shape. Along the optical path there are also four folding mirrors in order to maintain the optical path within the tight allocated volume. FM2 and FM3 guide the light toward the spectrometer side of the optical bench, while the first folding mirror is actually part of the scan unit. This kind of configuration guarantees an aberration correction without the use of any dioptric corrector (that would be not efficient for the large spectral range of MAJIS) and a good optical quality inside the whole FOV.

In order to avoid any defocusing due to the large operative temperature range and the excursion between the room temperature (at which the telescope was aligned) and the cold op-
<figure><img src="image_24.png" /><figcaption>**Fig. 12** Sketch of the shutter mechanism (shield controlled by the motor) in closed position</figcaption></figure>

operative temperature, the selected material for all the telescope mirrors (and their mountings) is Aluminum RSA 6061, that has the same CTE as the optical bench. The only exception is the scan unit, with a rotating flat mirror in beryllium. The scan mirror allows scanning the line of sight in a direction perpendicular to the slit (operative scan angle ±4°), in order to have the possibility to image a given target while the S/C moves, and also to increase the dwell time on a moving target (Sects. 3.2 and 4.1.2). The maximum excursion foreseen by the scan unit is ±19°, which allows the necessary rotation to direct the beam in the direction of the Internal Calibration Unit (ICU) at an angle of 8.5° from the boresight.

**Spectrometer** The entrance slit (14.4 mm × 36 µm) is located on top of the optical bench. It defines the instrument FOV along the long axis and the IFOV across the short axis. Since the spectrometers work at unitary magnification the slit width matches the image pixel size (36 µm). After the slit the light is collimated by a Schmidt-off axis collimator with a specular-correcting plate placed in correspondence of its pupil. Collimator focal length is 105 mm, with a f/3 aperture.

A shutter mechanism is implemented to block the incoming optical beam and to allow measuring the dark current during science operations. The shutter is placed close to the spectrometers' entrance slit (optical field stop) to minimize area and mass of the movable element. The shutter mechanism is a bi-stable device. The rotation of the motor drives the shield in front of the entrance slit of the collimator. The positioning of the shutter requires little angular movement of the small shield to open or close the slit aperture. The shutter is mechanically integrated with the collimator as it is mounted on the same support of the entrance slit, as shown in Fig. 12.

The separation between the two channels is done by a dichroic filter inserted after the collimator primary mirror (M5) in order to adjust a different correcting plate for each channel in terms of aspheric coefficients but also in terms of tilt. This solution guarantees the necessary correction for the spherical aberration and the coma. The field curvature is corrected with an additional small spherical mirror (M4) as close as possible to the focal plane. As for the telescope mirrors, all collimator mirrors (including correcting plates) are made of Aluminum RSA 6061, while the dichroic coating is deposited on a substrate of CLEARTRAN®. The choice to split the spectral range into these two domains has been made on the basis of the scientific objectives and the optimization of radiometric performances. The spectral characteristics of the planetary bodies to be observed (in particular Ganymede) have been carefully
examined when defining the wavelength (~2.3 µm) of the spectral boundary between the two channels. The spectral range between 2.25 and 2.35 µm is optically covered by both channels.

With the same conceptual layout in both channels, the collimated light is then reflected by a flat ruled grating that disperses the spectrum, and finally it crosses a completely dioptric objective. Both gratings are made on a NG5 substrate and they are optimized to maximize the efficiency of the first order: higher orders are rejected thanks to a linear variable order rejection filter placed in front of each detector deposited on a sapphire transparent substrate. In particular, the LVF is a band-pass filter for the IR channel (so to suppress also part of the thermic background) and a high-pass filter for the VISNIR channel (Sect. 4.1.3). The pupil anamorphism introduced by the gratings is compensated by a wedge on top of the objectives. The gratings smile and distortions have been minimized, introducing a suitable amount of radial distortion and lateral color in the objectives. The two plane gratings have different groove densities (85.9 grooves/mm for the VISNIR, 49 grooves/mm for the IR) to yield the required dispersions of the two channels (requirement of 3.66 nm/band for the VISNIR and 6.51 nm/band for the IR).

Both objectives have the same focal length (that of the collimator, f-number = 3), so that both spectrometers have 1x magnification. The VISNIR objective is composed by one wedge and seven lenses, two of them (L2 and L3) have one aspherical surface. The selected materials are CaF₂, LaK9G15, F2G12 and CLEARTRAN®. In addition, it was necessary to introduce a folding mirror (FM4) after the first two lenses in order to accommodate in a better way the spectrometer (since the size and the shape of the optical bench was constrained by the accommodation on the S/C). The whole VISNIR objective (wedge and mirror included) is mounted inside a-barrel in Titanium, so the folding mirror is in BK7 (since its CTE is closer that of Titanium than to Aluminum RSA 6061). The IR objective is composed of four lenses in Silicon and one in Germanium (all of them spherical). As for the VISNIR channel, on the optical path just before the lenses there is a wedge (in Silicon) in order to limit the pupil anamorphism introduced by the grating.

## 4.1.2 Scanning Mechanism

The scan mirror unit has the following functionalities:

- It makes possible to build up a 2-D image with similar spatial resolution on both directions (along and across slit, the latter being the scan direction) to limit the oversampling in pushbroom mode whenever the S/C relative speed is too low (large slant distance);

- It implements a motion compensation mode capable to increase the dwell time through a continuous motion of the mirror tuned to match the S/C drift when the relative speed is too high (short slant distance and/or high relative speed).

- It allows to reach the position of the ICUs to perform in-flight radiometric calibrations.

The most demanding modes are linked to observations close to the icy satellites, which require motion compensation: flybys near closest approach, first and last GEO orbits and GCO500 (near circular orbit). In such cases, the dwell time is much shorter than the 500 ms to 600 ms sampling time which has been defined as nominal for high spatial resolution observations in a tradeoff between coverage and SNR (Sect. 6.5). As an example, on GCO500, the sub-S/C velocity is 1.5 km/s with nadir pointing, hence the dwell time is only 50 ms at full resolution (75 m/pixel) and 100 ms with spatial binning by 2 (150 m/pixel). The close flybys of Ganymede, Europa and Callisto lead to even shorter dwell times, down to 11.5 ms for the Callisto flybys of the high inclination phase (200 km altitude, 5.1 km/s approach velocity) even with binning by 2 (pixel size: 60 m). Such a short integration time is not possible with the detection chain (Sect. 5.4.1).
<figure><img src="image_25.png" /><figcaption>**Fig. 13** Scan unit external view. The mechanical rotation of the mirror is obtained by two cantilevered flexural pivots. The rotating part of each pivot is fitted centered on concentric seats realized in the mirror's half axes bonded with adhesive</figcaption></figure>

The scan unit is made of an aluminum structure housing the scan mirror and the related mechanism. Figure 13 offers an external view of the unit. The scan unit houses a flat mirror made of beryllium (see below) and its related mechanism. The mechanical rotation of the mirror is obtained by two cantilevered flexural pivots driven by a DC motor coupled to a phase shift resolver. The unit ensures the following functional requirements (Table 9):

- Operative mechanical scan angle: ± 2° from boresight (equivalent to an optical angle of ± 4°)

- Calibration mechanical scan angle: + 8.5° from boresight (equivalent to an optical angle of + 17°)

The robustness of the design is based on the use of frictionless bearing to support the scan axis. This choice allows to overcome potential problems related to lubrication lack and lifetime reduction that are the major drawbacks when roller bearings are used. In order to prevent risk of damages due to the inertial loads during the launch phase the mirror's movement along the axial and the radial directions is avoided by a "simple" locking device that is reliably unlocked at low temperature (<203K) by virtue of the different thermal contraction between the shaft and the housing. To this purpose, two conical bushes are mounted on the mechanism rotor so that the gap between these bushes and the corresponding conical seats allocated on the supports is set to zero at ambient temperature without introducing preload. This locking/unlocking concept ensure the locking of the mechanism at ambient temperature and in particular during the launch phase when the vibrations in combination with pivot rotation could lead to an overstress in the elastic blades of the pivot. The pivot can rotate only when the temperature falls below -20 °C and therefore once in orbit, when the vibration and the consequent radial load are no longer present. In this way the pivot integrity is ensured as well as its nominal performance regarding angular deflection and lifetime.

The choice of beryllium offers several advantages such as excellent optical performance at low temperature, low density, high stiffness and low CTE. Due to the difference between the CTEs of the housing and of the beryllium, an axial gap indeed arises at the operating (low) temperature between the conical bushes and each conical seat. The created gap is sufficient to disengage the scan axis making it free to perform the required scanning function, while locking the system at room temperature.

The resolver motor provides the scanning mirror position. A full rotation of the resolver motor corresponds to a rotation by 360/28 = 12.857° of the scanning mirror. There are 8192
<figure><img src="image_26.png" /><figcaption>**Fig. 14** Focal plane units of the IR and VISNIR channels. The main components are indicated. See text for details about the functionalities of these components</figcaption></figure>

steps for a full rotation, leading to steps of 0.0015695° (~26.5 µrad) for the position of the scanning mirror. All observations requiring an exploration of the scan mechanism will cover the full range available without vignetting from +2° to -2° (or -2° to +2°) which corresponds to 2548 steps (0.0015695° each). Additional parameters that control the scan mechanism are the velocity and acceleration. As an operational example, on GCO500, or at closest approach during a flyby, a constant angular velocity is adequate as the altitude is constant or nearly constant. However, for close flybys, there is a very strong dependence of the dwell time on the true anomaly. For observations before the closest approach, the magnitude of angular speed needs to increase during the observation. For observations after the closest approach, it needs to decrease.

### 4.1.3 Focal Plane Units

The FPU's of the VISNIR (0.49-2.35 µm) and IR (2.27-5.56 µm) channels of MAJIS share a common basic design using similar families of four components: 1) Focal Plane Array (FPA); 2) LVF, 3) Focal Plane Electronics (FPE), 4) Focal Plane Flex (FPF). These components were tailored to the needs and science performance-driven constraints specific to each spectral range. A breakdown of the FPU's architecture is provided in Fig. 14. Specifically, the VISNIR focal plane uses a visible/near-infrared focal plane array detector and one filter which are designed to cut-out the high order flux from the spectrometer grating, as well as from in-and out-of-path straylight from the fore-optics. The IR focal plane uses a IR focal plane array detector, a set of filters which provide the same functionalities as those of the VISNIR channel but additionally filter out the parasitic flux induced by thermal radiation of the spectrometer and telescope. The IR focal plane array and filter are also baffled in a cryogenic housing to further reduce thermal background that is roughly 3-4 orders of magnitude stronger than the scientific signal of interest. The IR FPA package is thermally isolated from the instrument structure and coupled to a specific radiator (Sect. 4.4).

The focal plane electronics module is also provided by Teledyne Imaging Solutions (Beletic et al. 2008), and mounted in a customized mechanical package which operates
at temperatures >120 K. The FPE serves both to power, drive, read-out and digitize the ROIC (read-out integrated circuit) signal from the FPA. Both FPUs have been characterized at their nominal operational temperatures during several campaigns at sub-system level (FPA, filter) and integrated level (Table 15) providing the key parameters such detector Bias and total CDS (difference between a pair of read-reset and a reset-read frames) read-out noise (RON), dark current and Dark Signal Non Uniformity (DSNU), quantum efficiency (QE), linearity and full well capacity (FWC), pixel response non uniformity (PRNU), persistence, conversion gain and pixel operability. Below we present the main characteristics and functionalities of these sub-systems, where their performances are summarized. Additional details can be found in a set of papers (Carter et al. 2002a,b; Haffoud et al. 2022; Langevin et al. 2022).

The focal plane arrays are HgCdTe detectors grown by molecular beam epitaxy and then substrate-removed. They are of the H1RG product line manufactured by Teledyne Imaging Sensors (TIS) of California (USA). These are “P/N” detector arrays hybridized with a source follower per detector type ROIC. The matrix size is 1024 × 1024 pixels with native pitch of 18 µm, of which 1016 × 1016 pixels are sensitive to light. The remaining four lines/columns are called “reference” pixels, used to characterize the detector electronic state and are routinely read out by the onboard pipeline during the calibration check (Sect. 7.2). The Cd fraction of the HgCdTe detectors is tuned to provide customized gap energies which in turn sets the cut-off wavelength ($\lambda_{co}$) tailored to each channel. The VISNIR detector is sensitive to light from the visible to $\lambda_{co} \sim 2.5$ µm, providing high quantum efficiency. The IR detector has a $\lambda_{co}$ of $\sim 5.6$ µm (temperature dependent and varying by $\sim 1\%$ over the FPA) and has also high quantum efficiency. The substrate of both detectors is removed to increase the quantum efficiency in the lower spectral ranges, and to eliminate any potential substrate glow effect from the Jupiter radiation environment. Anti-reflection coatings are also applied by the manufacturer. One of the key parameters is the pixel dark current. Both FPAs are equipped with a Cernox sensor mounted on the FPA package to monitor their temperature. The nominal functioning temperature of the VISNIR FPA is $\sim 130$ K and 90 K for the IR FPA. The choice of detector cut-on and cut-off wavelengths coupled with the choice of beam splitter in the optical head, allows for a spectral bridging zone where both channels are sensitive to light in the 2.27-2.35 µm spectral range.

Each H1RG is connected to its FPE module, the compact and low-noise SIDECAR ASIC that is in its own cryogenic package. The FPE generates the FPA biases to operate and reset the detector, it configures the read-out modes (e.g. frame windowing, pixel binning), it provides low-noise ADC, and communicates both-ways with the MAJIS Main Electronics. It is also equipped with a Cernox temperature sensor and has survival heater lines with independent heating and temperature monitoring (2 redundant heater lines and 3 PT1000 sensors) controlled by the S/C to ensure minimum safe temperature for the packaged SIDECAR module. These lines were automatically activated after the JUICE launch to preclude any contamination and can be used in the future as decontamination lines in case of the check-outs during the cruise or the nominal scientific phase reveal contamination by organic and/or water ice. As described in Sect. 5, the FPE can set the detector to work at two read-out speeds, one clocked at 100 kHz with a 16 bits ADC and one clocked at 1 MHz with a 12 bits ADC. The 1 MHz read-out speed allows for shorter integration times at the expense of lower performance for weak signals (a $\sim 2.5$-fold increase in the read-out noise) but with the benefit of a $\sim 15\%$ increase in full-well depth (hence a higher saturation limit).

The use of HxRG-type detectors for this planetary science mission was challenging. Such detectors are commonly used in ground-based astronomy application, where the integration time is on long timescales, and the requirement for sensitivity to weak signals is stringent.
The JUICE mission profile induces challenges for such detectors because 1) the dwell time can be short (100 s ms or less in some cases, Sect. 7); 2) the pixel de-spiking strategy also limits the maximum integration time to a fraction of a second in multiple phases of the mission; and 3) some scientific sources sought after have a high fluence which can easily saturate the H1RG detectors as they are featured for low well depth while other sources have expected signals on the order of 100 s of photons per integration, thus posing a unique challenge on the noise performance, 4) the IR detector further requires low dark current. Using the low-noise bundle of an H1RG FPA (P/N technology) coupled to the SIDECAR FPE allows for the detection of such weak scientific sources, but poses the challenges of a fast read-out and avoiding saturation. These are tackled thanks to highly customized clocking and read-out strategies (Langevin et al. 2022, Sect. 5).

The VISNIR FPA has a linearly variable high-pass filter set placed ~1 mm above it. The monolithic filter was manufactured by Safran REOSC and was deposited on a 1 mm thick sapphire substrate with additional anti-reflection coatings. The filter high-pass transmission is above 90% and on average the out-of-band rejection is better than OD3. Tolerance on alignment is relaxed compared to the narrow band-pass IR channel filter (see below). A rejection mirror is part of the assembly to suppress the zero-order light reflected by the grating (Fig. 14). The performance of the VISNIR channel is further described in Haffoud et al. (2022).

The IR focal plane channel has an infrared filter that is also mounted directly ~1 mm above the FPA. The filter was manufactured by Viavi Solutions of California, USA. It consists of two epoxy-bonded segments of optical coatings deposited on a 1 mm-thick sapphire substrate, with additional anti-reflection coatings. One segment is a broad band pass filter (BPF) which covers the 2.27-2.85 µm spectral range of the channel, acts primary as an order-sorting filter. It has a high throughput > 90% and out-of-band rejection performance (OD>3). The second segment is a linearly variable narrow band-pass filter whose band-pass central wavelength position varies along the dispersion direction of the FPA and covers the channel range ~2.85-5.56 µm. The two filters serve both as an order sorting filter and as a filter of the polychromatic thermal radiation flux from the instrument. The two filters also contribute to the rejection of spatially and spectrally structured straylight from the instrument. The bonding area between segments induces a narrow spectral gap in the IR channel affecting a few wavelengths around 2.85-2.95 µm. The LVF bandpass width was actually chosen as a compromise between alignment tolerance and out-of-band rejection performance. The narrow band pass of this segment is expressed as a fraction of the band-pass central wavelength, and varies from 4.3% to 3.6% from the focal plane channel range of 2.27 to 5.56 µm. The out-of-band rejection is better than OD3. It exhibits an intrinsic spectral smile perpendicular to the dispersion direction which has a negligible impact on performance. The spectrometer grating provides dispersed flux onto the FPA which varies along the same direction as the LVF spectral direction (the direction along which the LVF bandpass central wavelength varies). Therefore, the LVF was precisely aligned (to within 100 µm) in this direction to match its bandpass centers with the spectrometer's spectral point spread function. The resulting effective LVF throughput was found to be >80% up to 4.5 µm, and then decreasing down to 30% from ~4.5 to 5.56 µm. The performance of the IR channel is further described in Carter et al. (2002b).

Each set of FPAs and filters is housed in a structure which is cooled to the nominal operating temperature of each channel. The housings serve several functions: protect the FPAs from contamination (by particles or chemical compounds); provide a thermal shield from the exterior environment, especially for the IR channel, whose focal plane is significantly colder than the surrounding spectrometer and focal plane electronics; allow power/data connections to the FPE module; and provide some shielding from the Jupiter's high-radiation
<table><caption>**Table 11** Summary of MAJIS detectors performances at 18- µm pixel pitch derived from the characterization campaign (Sect. 6) Additional parameters are presented in Haffoud et al. (2022) for the VISNIR FPA and Carter et al. (2002b) for the IR FPA. The full well capacities and dark currents are four times larger and the read-out noises are two times larger after nominal binning (2 × 2)</caption><thead><tr><th>Parameters</th><th>Value</th><th>Comments</th></tr></thead><tbody><tr><td>Read-out Noise (VISNIR)</td><td>19 e<sup>-</sup> rms @ 100 kHz<br>34 e<sup>-</sup> rms @ 1 MHz</td><td></td></tr><tr><td>Read-out Noise (IR)</td><td>13 e<sup>-</sup> rms @ 100 kHz<br>30 e<sup>-</sup> rms @ 1 MHz</td><td>Slight dependence with detector outputs, temperature-dependence negligible</td></tr><tr><td>Dark current (VISNIR)</td><td>&lt;15 e<sup>-</sup>/s</td><td>@132 K</td></tr><tr><td>Dark current (IR)</td><td>Avg. &lt;700 e<sup>-</sup>/s</td><td>@90 K<br>Temperature-dependence as following:<br>Log(Dark(TFPA)) = 0.112xTFPA-7.346</td></tr><tr><td>Operating temperature (VISNIR)</td><td>120-160 K</td><td>Characterized in the range of 125-144 K</td></tr><tr><td>Operating temperature (IR)</td><td>68-110 K</td><td>Characterized in the range of 75-105 K</td></tr><tr><td>Full well capacity (VISNIR)</td><td>95.3 ke<sup>-</sup> for 100 kHz<br>101.1 ke<sup>-</sup> for 1 MHz</td><td>@125 K, FWC decreases with temperature</td></tr><tr><td>Full well capacity (IR)</td><td>43 ke<sup>-</sup> for 100 kHz<br>56 ke<sup>-</sup> for 1 MHz</td><td>@90 K, FWC decreases with temperature</td></tr><tr><td>Power (FPE)</td><td>250 mW for 100 kHz<br>350 mW for 1 MHz</td><td></td></tr></tbody></table>

environment, which induces electron spikes in the data and can degrade detector and filter transmission performance over time (Langevin et al. 2020). In terms of optical constraints, the housings maintain the optimal alignment at operating temperature between the filters and FPAs along the dispersion direction. This is achieved by adopting a high stiffness / low mass housing structure mounted in the spectrometer's optical bench. They also reject zero-order straylight from the grating, and the IR housing further baffles thermal radiation.

The analog data interface between the FPA and FPE is made through a custom-designed flex cable. Then the internal optical head harness connects the FPE to the Proximity Electronics (PE). Due to noise requirements, the harness of each channel is separated by EMC classes resulting in a Y-shaped harness going from each of the FPE to the connector panel of the OH. The IR FPA is cooled by the cold radiator using a copper thermal strap (Sect. 4.4).

The performance of the VISNIR channel is very weakly dependent on the range of operating temperatures expected during the mission phases. Conversely, the performance of the IR channel varies significantly with operational temperature due to the IR FPA response with temperature (Table 11). It has a cutoff wavelength that varies at the ~1% level with operational tested temperatures ranging from 75-105K (covering the operational temperatures expected during the science phase as presented in Sect. 4.4.2 and Table 13); Its well depth also varies by up to ~40% over the same range. The worst case well depth is ~28 ke⁻ (IR FPA at 105K and 100 kHz read-out clock); the best case is ~60 ke⁻ (FPA at 75K and 1 MHz read-out clock). The most detrimental effects however are the dark current and detector cosmetics. The dark current varies from <10 e⁻/sec to >30000 e⁻/sec over that range. Low dark current values result in a read-out noise limited regime for weak signals, while high dark current levels result in shot-noise limited regime. Thus, high dark levels will adversely impact the SNR for weak signals while further reducing the dynamic range available for
<figure><img src="image_27.png" /><figcaption>**Fig. 15** Optical head external view with the two (“warm” and “cold”) radiators (Sect. 4.4.1) (here transparent)</figcaption></figure>

scientifically relevant signals. The number of operative pixels also decreases by several per-
cent at high temperature, especially above 100K. At this temperature, operability remains
above 95% for short integration times (~ 100 ms), but long integration times (~ 800 ms, as
required for 100 kHz readout) lead to an operability < 90%, which is not acceptable. The
expansion of non-operable areas with temperature occurs preferentially around “hot” pixels
and close to the physical edges of the FPA, so that the spectral range can be reduced by up
to 3 l spectels at both ends of the spectral range (near 2.27 and 5.56 µm) for an operating
temperature ≥100K (Langevin et al. submitted).

**4.1.4 Mechanical Design**

MAJIS OH mechanical configuration is conceived to provide support, proper positioning and stability to the numerous optical subsystems of the instrument, while being compliant with its accommodation on the JUICE S/C. The optical elements are designed to be mechanically mounted and aligned on the optical bench that provides support to all optical, thermal and electrical subsystems.

The external view of the OH assembly is shown in Fig. 15 where some of the main subsystems are visible:

- the main support structure which also constitutes the optical bench of the instrument and is connected to the JUICE panel by means of three bipods,

- the bipods realized in carbon fiber composite with terminal ends in titanium,

- the input baffle equipped in its interior with the calibration unit (Sect. 4.1.5),

- the spectrometer cover (visible below the radiators on Fig. 15), which encloses the upper side of the optical bench offering protection and shielding to the optical elements of the spectrometer channels. The assembly of the cover with the optical bench is also named “Cold Box” as it encloses the optical parts to be cooled during the operation,
<figure><img src="image_28.png" /><figcaption>Fig. 16 (A) View of the optical bench and the spectrometer elements inside the cold box. (B) View of the optical bench and the telescope elements inside the carter</figcaption></figure>

- the carter telescope which encloses the lower side of the optical bench offering protection to the elements of the telescope and the scanning mirror,

- the cooling system that uses a dual stage passive radiators system placed in such a way that during the mission should be oriented towards the cold space (+$X_{SC}$ axis).

The adoption of a passive cooling architecture determined the choice of placing the spectrometer on the upper side of the optical bench. This accommodation allows to minimize the distance between the IR FPA and the radiator, thereby optimizing the efficiency of the cooling system.

The main elements of the spectrometer enclosed in the cold box are visible in Fig. 16A, where the radiators and the spectrometer cover have been removed. The lower side of the optical bench is depicted in Fig. 16B that shows the TMA telescope mirrors, the baffle and the scanning unit enclosed within the carter. The baffle is divided in two parts: the internal section, which is contained inside the optical bench, and the external part that protrudes from the instrument arriving to reconnect to the SLI/MLI fixed in front of the radiator. Both parts are realized in black anodized aluminum. The baffle has been designed considering several constraints such as to observe the maximum FOV along the $X_{S/C}$ direction (± 1.7°), to allow the maximum motion compensation angle along the YS/C direction to be reached by the scanning mirror (± 4°), to allow and optimize the housing of the ICU in one of its lateral sides so as to have a minimum mechanical scanning angle of + 8.5° to point towards it and to have a maximum length to avoid vignetting from the first folding mirror of the TMA telescope (FM1). The baffle is also designed to shield the Sun light within an angle > 22° along the X direction and >33° in the Y axis on the side of ICU lamp and >16° in the opposite side.

The optical bench, whose dimensions are 605 × 520 mm, supports all the optical elements of the system and keeps them properly positioned. In order to minimize its mass and improve the structure dimensional stability, the optical bench is made of stabilized aluminum (Al 6061) black anodized (for thermal purpose). Although not necessary for structural reasons, specific areas of the optical bench have been thickened in order to provide shielding against radiation.

The main structure is supported by three bipods connecting the instrument to the JUICE panel. This bipod arrangement ensures that the direction of minimal flexural stiffness of each bipod is directed toward a common point located in the proximity of the center of mass of the instrument. The thermal elongation of the optical bench is thus compensated by the bipod transversal flexibility minimizing the induced stress in the optical bench. The bipods are essential elements for the structural and thermal performance of the instrument, hence their design has been defined after a trade-off among different solutions. This analysis
<figure><img src="image_29.png" /><figcaption>Fig. 17 (left panel) CFRP/titanium bipod configuration. (right panel) Detail of the bipod leg construction</figcaption></figure>

<figure><img src="image_30.png" /><figcaption>Fig. 18 (A) Calibration unit elements inside the baffle. (B) Section of the baffle with sources light beams</figcaption></figure>

considered the two opposite requirements of high mechanical stiffness and resistance vs a
high thermal insulation. A compromise solution that couples carbon fiber honeycomb legs
with titanium terminals has been recognized as optimal, as sketched in Fig. 17.

**4.1.5 Internal Calibration Unit**

The ICU is housed in the telescope's entrance baffle and is designed to perform calibration
tests during the entire operative life of the instrument allowing to evaluate the potential time
evolution of instrumental characteristics. Specifically, it will enable to 1) identify defective
and out of linearity pixels, 2) check the spectral calibration, as well as 3) the flat-field ra-
diometry and 4) the radiometric and linearity response. Two different calibrated sources, one
optimized for the VISNIR channel (incandescent lamp) and one for the IR channel (infrared
emitter) are used to illuminate a diffusive element. Each source is equipped with lenses
to properly shape the beam. In Fig. 18A is reported a cross-section of the baffle where the
lamps and the diffusive element of the ICU are visible, while a lateral view of the calibration
unit simulates the light beams coming from the two radiation sources (Fig. 18B). By rotating
the scanning mirror by 8.5° towards the ICU optical axis angle it is possible to match the
telescope and ICU pupil so as to illuminate the full spectrometer's slit by the ICU beam.

A transmission filter (Didymium for the VISNIR channel and Polystyrene for the IR channel) is placed at the exit of each illuminator to introduce reference absorption features
<figure><img src="image_31.png" /><figcaption>**Fig. 19** Simplified ME block diagram. The ME box is in orange, the OH box in blue. See text for acronyms. Radiator #1 (resp. #2) is the warm (resp. cold) radiator (Sect. 4.4.1)</figcaption></figure>

on the ICU light beam. These features can be used to compare the instrumental spectral response with that acquired with the same filters during on-ground laboratory calibration. A comparison of the ICU measurements acquired during the calibration campaign (Sect. 6.1) with laboratory spectra of the ICU source is shown in Sect. 6.5.

## 4.2 Main Electronics Unit

### 4.2.1 General Description

The electrical architecture is based on a single electronics box, called the ME unit composed of several boards (Fig. 19). The ME unit design is based upon the modular stacking electronics system developed for the SIMBIO-SYS (Cremonese et al. 2020) and JIRAM instruments (Adriani et al. 2017a). It features all the control/command functionalities and powers off the different OH subsystems thanks to several dedicated electronic modules. Specifically, it ensures the communication with/from the JUICE OBC (On-Board Computer), provides regulated secondary power to MAJIS from the unregulated 28-32 V power from JUICE, processes commands from JUICE, controls the MAJIS subsystems, and acquires and formats MAJIS science and housekeeping data then sent to the S/C solid-state mass memory. Below are listed the different elements of the ME (Fig. 19):

- Two Proximity Electronics (PE) boards, each one embedding a field-programmable-gate-array (FPGA) that ensures several functionalities for a given channel (VISNIR PE and IR PE). They receive commands from the DPU by means of SpaceWire (SpW) and drive the FPE (Sect. 4.2.2) to acquire science data, perform some preprocessing and eventually send a despiked PE-frame to the DPU via SpW.

- The Auxiliary Board (AUX board) is dedicated to control and to command the auxiliary sub-units inside the optical head: scan unit, shutter and internal calibration sources (Sect. 4.2.2). The AUX board is in interface with the DPU by means of SpW.

- Two Digital Processing unit (DPU), one nominal DPU-N and one cold redundancy DPU-R respectively (Sect. 4.2.2). Each DPU is divided into two elements: 1) a Compression Unit (CU) per channel (VISNIR and IR) for the data transmitted by the relevant PE. Each CU includes a LEON3 processor, a SpW IF and a memory buffer; 2) a Command and Process Control Unit (CPCU) based on a LEON3 processor that manages the SpW interface to the
S/C: reception and execution of telecommands, powering on and off of specific PE and AUX units, synchronization with the On-Board Time (OBT), formatting and transmission of telemetry packets (science data and housekeeping) to the S/C. The redundant DPU is identical to the nominal one.

- The PCDU is in charge of the Power Conversion and Distribution to all the sub units of the ME box (Sect. 4.2.2). As the DPU, the PCDU is in cold redundancy. It embeds Electro-Magnetic Interference (EMI) input filters for common mode and differential mode, Voltage and Current protection at primary interface, four independent DC/DC converters (one for each board, namely DPU, AUX, IR PE, VISNIR PE), galvanically insulated I/F to switch On/Off each channel (AUX, IR, VISNIR) by the operating DPU (Nominal or Redundant), EMI output filters on the secondary voltages, O-Ring functions to provide secondary voltages to each non-redounded channel (PE IR, PE VISNIR, AUX).

- The internal SpW and Power flexes that links the different boards together.

- The high-speed data links and power/telecommand lines that connect the ME to the JUICE bus.

### 4.2.2 Main Electronics Functional Configuration

Except for the front-end electronics of the detectors, the electronic boards are in a dedicated self-standing box. Their detailed functionalities are detailed in the following subsections.

**PCDU Board** The PCDU is equipped with power line input filters, DC/DC converters and output EMI filters to provide the needed secondary voltages to other ME modules (digital 3.3 V and analog 10.5 V for the DPU; digital 3.3 V and analog 6 V for the IR and VISNIR PE's, analog 3.8 V and analog 5.8 V for the FPE's; digital 3.3 V, analog ±12 V, analog 5 V and analog 8 V for the AUX). The PCDU module manages the current thanks to EMI input filters for both common mode and differential mode. The voltage and current protection at primary interface use reverse polarity protection, inrush limiter, under-voltage lock-out and over-voltage protection. The two fully independent converter boards (Nominal and Redundant) have each four independent DC/DC converters based on the half-bridge technology for the DPU (Main or Redundant), AUX Electronics, IR PE and VISNIR PE. A galvanically insulated interface permits to switch ON or OFF independently the IR PE, VISNIR PE and AUX electronics DC/DC converters by the operating DPU (nominal or redundant). EMI output filters are present on the secondary voltages. Additionally, ORing function with diodes and MOSFET transistors supply the secondary voltages to the non-redounded modules IR PE, VISNIR PE and AUX Electronics regardless of which primary power line is switched ON and thus which DC/DC converter is running. The PCDU also conditions the analog housekeeping that are digitalized on the DPU board (primary currents for each DC/DC converter and one PCDU temperature sensor).

**DPU Board** The DPU is the centerpiece of the ME. Its three main modules are a) the CPCU that ensures the Control/Command (TC/TM) and the ME Housekeeping functions with an embedded SpW interface for the TC/TM link with the JUICE OBC and b) two CUs for the science data processing of the two channels with a SpW interface for the communication with the corresponding PE. The DPU has multiple functionalities. It controls the two detectors via high level commands to be sent to the related PE through the SpW communication link, acquires the Housekeeping (HK) data and processed frames (CDS and line corrected, despiked, dark subtracted,...) from PE, performs some additional processing on these frames (spatial and spectral binning, spectral editing, PE-frame binning) leading to the
generation of the CU-frames, performs scientific data compression, produces and formats the scientific data for the browse mode, manages the synchronization with the OnBoard Time (OBT), controls the auxiliary units of the optical head via high level commands to be sent to the dedicated AUX board, controls the power up and down of AUX, IR PE, VISNIR PE boards via control signals sent to the PCDU, acquires and digitalizes analog housekeeping of both PCDU and DPU, receives command and send science/housekeeping data through the SpW communication link from/to the JUICE.

The main functions ensured by the CPCU consist of TC reception, decoding and execution from the JUICE OBC, telemetry (HK TM and Native Science Data Packets) encoding, formatting, and transmission to the JUICE Solid State Mass Memory (SSMM) and the instrument observation mode decoding and monitoring. The CPCU handles TC and TM interfaces with the S/C. It is responsible for switching on the two sets of units involved in the acquisition (one for the VISNIR channel, one for the IR channel) and the AUX board which controls OH functionalities (scanner, shutter, ICU light sources). It is controlled by a GR-712 Sparc processor (running at 50 MHz), which was proposed as a “common DPU” by the JUICE project to the instrument teams.

The two CUs are dedicated to the IR and VISNIR channels data processing on each DPU board (nominal and redundant). The main tasks of each compression unit are to handle the data flow from its IFE (Instrument Font-End) and to generate compressed data packets thanks to a GR712 LEON3 processor running at 60 MHz (see Sect. 5.4.3). The 60 MHz clock frequency was selected for the CU to maintain adequate work load margins with the largest CU frame format (640 × 400) and shortest CU-frame repetition rate (500 ms) when data compression is performed, which is nominal for science observations (see Sect. 5.4).

**PE IR and PE VISNIR Boards** The 2 PE modules, one for each observation channel IR and VISNIR, perform the following functions: EMI filtering of secondary power supply voltages to the FPA, coming from PCDU; reception of the commands from DPU through the SpW communication link, management of the power up and down of the FPE and FPA, control of the FPE and FPA according to the commands received from DPU, acquisition of the digitized frames from the FPE, data preprocessing, handlings and acquisition of the analog housekeeping (voltages and temperatures of FPA, FPE and PE), sending of scientific and housekeeping data to the DPU via the SpW link.

**AUX Board** The architecture of the AUX module is based on an FPGA having the control and supervision of all main functions of the unit EMI filtering on secondary power supply voltages coming from the PCDU, communication with DPU via SpW link, scan mirror mechanism control and monitoring, Shutter control and monitoring, calibration sources biasing and monitoring, temperature acquisition and monitoring, sending of housekeeping to the DPU via SpW link such as scanner position, shutter state, and OH temperatures.

### 4.2.3 MAJIS ME Redundancies

Cold redundancy is implemented for the PCDU, the DPU and the SpW link in order to improve the safety and reliability of the MAJIS instrument. Either the nominal or the redundant set of units is selected by TC (no cross-redundancy capability). For this purpose, the primary and redundant 28 V power lines from JUICE are kept fully independent and isolated from each other. Specifically, the JUICE primary (resp. redundant) 28 V is connected to the nominal (resp. redundant) converter board through its dedicated common mode EMI filter and then to each nominal (resp. redundant) DC/DC converter through dedicated differential
<table><caption>**Table 12** Data rate between the different modules</caption><thead><tr><th></th><th></th><th></th></tr></thead><tbody><tr><td rowspan="2">Instrument level</td><td>JUICE → MAJIS ME/DPU/CPCU</td><td>100Mbps*</td></tr><tr><td>JUICE ← MAJIS ME/DPU/CPCU</td><td>100Mbps</td></tr><tr><td rowspan="6">ME level</td><td>DPU (IR CU) → IR PE</td><td>10Mbps</td></tr><tr><td>DPU (IR CU) ← IR PE</td><td>40Mbps</td></tr><tr><td>DPU (VISNIR CU) → VISNIR PE</td><td>10Mbps</td></tr><tr><td>DPU (VISNIR CU) ← VISNIR PE</td><td>40Mbps</td></tr><tr><td>DPU → AUX board</td><td>10Mbps</td></tr><tr><td>AUX board → DPU</td><td>24Mbps</td></tr><tr><td rowspan="4">DPU level</td><td>CPCU → IR CU</td><td>50Mbps</td></tr><tr><td>CPCU ← IR CU</td><td>60Mbps</td></tr><tr><td>CPCU → VISNIR CU</td><td>50Mbps</td></tr><tr><td>CPCU ← VISNIR CU</td><td>60Mbps</td></tr></tbody></table>

<figure><img src="image_32.png" /><figcaption>**Fig. 20** Scheme showing the different electronics boards within their housing. See text for acronyms</figcaption></figure>

mode EMI filters. The ME can receive TC and send TM using a primary or redundant SpW link. The nominal or redundant PCDU delivers the secondary voltages to sub-systems that have no redundancy (VIS/NIR PE board, IR PE board and AUX board).

### 4.2.4 SpaceWire

The MAJIS ME uses SpW links at different levels to connect the units and subunit together. The data rate of each direction of each link is an optimized ratio between bandwidth and power consumption (Table 12).

### 4.2.5 Mechanical Accommodation

The mechanical layout of the ME is presented on Fig. 20. The different electronic boards of the ME are stacked within an aluminum alloy structure ensuring housing, heat dissipation, radiation hardness and EMC. The box is placed in the JUICE vault for radiation mitigation purpose. Additional spot shielding was required for some electronic components that require a radiation dose lower than 50 Krad. The box is composed of five belts (including mechanical frame and PCB), stacked together (top to bottom):

- PCDU belt with the two PCDU boards (nominal and redundant) and the ORing board

- AUX belt with AUX board

<footnote>*(data rate set by JUICE)</footnote>
<figure><img src="image_33.png" /><figcaption>Fig. 21 Detailed electrical interfaces between the MAJIS OH and ME units and JUICE</figcaption></figure>

- DPU belt with the two DPU boards (nominal and redundant)

- PE IR belt with the IR PE board

- PE VISNIR belt with the IR PE board

In addition, there are 11 internal flexible harnesses that connect the PCB between them for power and SpW, one JUICE bonding stud IF and two DPU JTAG interfaces for firmware updates (1 debug connector for each DPU).

## 4.3 Harnesses

MAJIS is constituted of two separated units (OH and ME) that are electrically connected with each other (Fig. 21). Specifically, two harnesses dedicated to the HK and the power connect the AUX board to the feed and control the auxiliary units inside the OH. Two harnesses for the digital signal and low-level analog power lines go from the VISNIR PE (resp. IR PE) to the VISNIR FPE (resp. IR FPE) inside the OH. The ME also communicates with the S/C and returns the acquired data through external harnesses. The monitoring of the survival and anticontamination heaters and thermal sensors (Sects. 3.2 and 4.4) that are located near the sensitive electronics and detectors inside the OH is also performed and controlled by specific harnesses. These thermal harnesses would be a very efficient path for generating conducted noise from the platform through conducted emissions of the JUICE PCDU as well as impact the thermal behavior of the cryogenic FPU's. To filter the conducted noises, torus has been inserted on the power and sensing lines inside the backshells of connectors plugs on the OH panel.
# 4.4 Thermal Architecture

## 4.4.1 Design

The thermal design is driven by the nominal operative temperature limits of the IR and VIS-NIR detectors. These design-based limits are respectively about 70 ÷ 90 K with ± 0.5 K of stability for the IR FPA, and 120 ÷ 140 K for the VISNIR one (Table 13). The cooler the better for the achievement of the scientific objectives, the higher values constitute a threshold beyond which the performances of the instrument become marginal w.r.t. some of these goals. To reach and maintain these operative temperatures, two radiators are necessary to dissipate thermal loads. The minimization of the thermal exchanges of the instrument structure was also critical. In this context, the bipods play a key role since they provide thermal connection between the instrument and the JUICE optical bench, while effective thermal insulation from the S/C is required through suitable external coating and the interposition of MLI.

The OH is externally mounted and the thermal control is performed independently from the S/C as required. The selected configuration was the use of two passive radiators for autonomous cooling of the optical bench and the detectors (Fig. 15). The so-called “warm” radiator (980 × 765 mm²) is thus in charge of cooling the optical head and the VISNIR detector, while the smaller “cold” radiator (690 × 600 mm²) is thermally conducted just to the IR FPA through a thermal strap and in charge of cooling it down to the required temperature indicated above, driven by the dark current (Sect. 4.1.3). The “warm” radiator that surrounds the cold one acts as a shielding element against external sources of heat. Both external surfaces of the radiators are covered by a white electrically conductive paint in order to increase the heat flux exchanged with the deep space (along the +X<sub>SC</sub> axis). The radiators are made up with an aluminum sandwich panel, with several anchor points to the structure of the instrument, so as to reach the required structural eigenfrequency and to mitigate the deformations due to the launch vibration. The “warm” radiator needs to be supported on the -Z<sub>SC</sub> side with cantilevered brackets connected to the vertical wall of the cover. Such brackets have also the function of conducting the heat from the cover towards the radiator ensuring in this way a high thermal conductance even for the outermost parts of the radiator. Since the cold radiator is devoted to the cooling of the IR detector, its operating temperature is much less than that of the cover, and its fastening hardware must guarantee high thermal insulation. To this purpose, fiberglass thermal insulating blades screwed to the supports have been adopted. The blades are oriented to obtain lesser stiffness in the direction in which the different thermal expansions occur between the radiator and the cover to minimize the thermo-elastic loads. The large size and mass of the cold radiator require an additional fixing point in the central position, in order to avoid excessive lateral vibration due to the blades bending. A rigid fiberglass frame connects the center of the radiator to the cover. It supports the lateral loads and limits the flexural deflection of the radiator.

The temperature differences that are present between the radiators and the other parts of the instrument and the environment require also a careful shielding to the radiated heat; as a consequence, the rear surface of both the radiators and all the other surfaces are protected by MLI. The scheme showing the MLI configuration is reported in Fig. 22.

The reduction of heat flows from the bipods is particularly complex due to the high temperature difference between the instrument and the JUICE panel. The structure of the instrument has to be maintained at about 130 K while the mounting interface can oscillate between -30 °C and + 20 °C leading to a maximum temperature difference of 163 K. In order to provide the requested thermal insulation, the use of solid titanium bipod was no longer
<figure><img src="image_34.png" /><figcaption>**Fig. 22** MLI schematic arrangement</figcaption></figure>

possible and a configuration based on thermal insulating Carbon-fiber-reinforced polymers (CFRP) composite connecting titanium head and feet has been adopted. The bipod legs are constituted by carbon fiber skins molded into a tubular shape by a foam insert. Through holes are machined in CFRP of the leg edges and two opposite titanium bushings are fitted and glued in the hole in suitable position to meet the corresponding hole machined on the titanium fitting wings. The junction between legs and fittings is made with the insertion of a titanium calibrated special pin. The pin is drilled and threaded in order to be locked by an M4 screw which presses the fitting wings against the bushings. This type of connection provides high strength and stiffness to the junction between the legs and the head and the feet of the bipod, while the transversal flexibility of the assembly is provided by the reduced thickness of the leg ends. The conductance of the bipods has been calculated by analysis, then measured with a very good agreement between the analysis and measurement.

### 4.4.2 Expected in-Flight Performances and in-Flight Monitoring

The in-flight performances are extrapolated, through our thermal model, from the results of the thermal tests performed on the S/C with the MAJIS dedicated unit (STM) (Table 15). A second test campaign, performed with the MAJIS PFM, in a different vacuum environment was in line with the first measurements and the thermal performances of the instrument that are summarized in Table 13.

The model predictions show that the instrument will stay within its Non-Op temperature range during the cruise, which is the predicted coldest part of the mission. It also ensures that it will stay in the Op temperature range when orbiting during the Jupiter tour and the Ganymede phase, for all scientific data acquisition periods. For the last mission phase GCO500 where the S/C is at its closest point to another celestial body, we expect that the temperatures of the detectors will be exceeding the target temperature (95.7 K instead of 90 K) which will slightly impact the performances.