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.

MAJIS is an imaging spectrometer operating in the visible and IR range which is part of the payload of the JUICE mission of ESA, dedicated to the study of the icy moons of Jupiter, Jupiter and the Jupiter environment, to be launched in April 2023. MAJIS implements two channels, one for the visible-near IR range (0.5 – 2.35 μm) and one for the IR range (2.25 – 5.52 μm). H1RG detectors from Teledyne where selected for these two channels. The specific constraints of MAJIS (very wide dynamic range, windowing, on-board de-spiking) required very significant adjustments to readout microcodes provided by Teledyne. The de-spiking approach implemented by MAJIS relies on on-board processing of series of consecutive sub-integrations5 . For reaching optimum performance, possible biases between successive CDS readouts of the same signal needed to be lower than the total noise. The first of the two detector readouts required for CDS had to be implemented very shortly after reset so as to benefit from the full well depth for dynamic range. This required coping with the reset anomaly which is specific to the HxRG family of detectors. The MAJIS team selected an approach based on drop frames and dark subtraction which minimized microcode adjustments so as to limit development and schedule risks. Detector characterization then instrument calibration showed that this approach was quite successful in terms of performances with no impact on the duty cycle for observations with short integration times.