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Studying the morphology of Jupiter’s icy moons and monitoring the planet’s atmosphere with JANUS

JANUS is the visible camera onboard the JUICE spacecraft. It will be used to study the morphology and surface processes of icy moons and perform mapping of the clouds of Jupiter. It will substantially enhance our existing image database of the Jupiter system, through its expanded spatial coverage, heightened spatial resolution, and extended time coverage.

The JUICE camera system JANUS (Jovis, Amorum ac Natorum Undique Scrutator) will fulfill the visible imaging scientific requirements of the JUICE mission. The main focus will be on the detailed investigation of the icy moons Europa, Callisto, and Ganymede through 9 months in orbit around Ganymede, 21 flybys of Callisto and 2 flybys of Europa, at distance as close as 300-400 km (Figure 2). These three satellites exhibit a large diversity of surface features, attesting of very different evolutions. Major targets of investigation include polar deposits, potential cryovolcanic areas (Figure 2), impact craters, tectonic structures, and bright and dark terrains. Other important targets for JANUS are the volcanically active moon, Io, moon’s exospheres, Jupiter’s atmosphere and ring system, and small and irregular moons, from distant observations (> 500,000 km).

Figure 1: Simulated footprints of JANUS observations of Europa during the first JUICE flyby (7E1 – July, 2 2032) (produced using Planetary coverage).

JANUS has been developed by a consortium of institutes in Italy, Germany, Spain and UK, all supported by the respective Space Agencies (ASI, DLR, Ministerio de Ciencia e Innovación, UKSA). Since the JANUS design has to cope with a wide range of targets, it is equipped with a filter wheel mechanism with 13 wide and narrow-band filters, allowing wavelength coverage in the 340 – 1080 nm range. Thus, JANUS will provide panchromatic and color images at various spatial resolutions and coverage, as well as regional DTMs by stereo-imagery. JANUS ground sampling ranges from 400 m/pixel to < 3 m/pixel (at closest approach (Figure 1)) for the three main icy moons, and from few to few tens of km/pixel for Jupiter and other targets in the Jovian system, such as Io, irregular moons, and Jupiter’s rings. JANUS observations of Jupiter’s atmosphere will range from full mapping to regional imaging at spatial resolutions down to 10 km/pixel. JANUS observations will fully cover Ganymede at least in 4 colours with a resolution of about 100-400 m/pixel, as well as high resolution target areas (below 50 m/pixel and down to 7.5 m/pixel).

Figure 2: An example of target (or region) of interest (RoI, dashed line, 69 km) on Musa Patera, Ganymede (Figure from Stephan et al. 2021). This scalloped depression is interpreted as a possible caldera-like source vent for icy volcanism. Footprints of different JUICE instruments (JANUS (green), MAJIS FOV built over time (magenta), MAJIS single slit pointed at boresight (yellow), UVS (blue), GALA (red) during a) high altitude (5000 km - GCO5000) and b) intermediate altitude (500 km - GCO500) are indicated.

Combination of JANUS data with other JUICE instruments such as the spectro-imager MAJIS, the altimeter GALA and the radar sounder RIME  in specific regions of interest (Figure 2) will further increase the outcome of the JUICE mission and greatly improve our understanding  how geological processes shape the evolution of these icy moon surface and control the exchange processes with the subsurface oceans.

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