JANUS instrument proposal & preliminary design

Objective of the proposal and its compliance with those indicated in the Sci-RD The detailed investigation of three of Jupiter‘s Galilean satellites (Ganymede, Europa, and Callisto), which are believed to harbour subsurface water oceans, is central to elucidating the conditions for habitability of icy worlds in planetary systems. The study of the Jupiter system and the possible existence of habitable environments offer the best opportunity for understanding the origins and formation of the gas giants and their satellite systems. The JUpiter ICy moons Explorer (JUICE) camera system JANUS (Jovis, Amorum ac Natorum Undique Scrutator) will determine the formation and characteristics of magmatic, tectonic, and impact features, relate them to surface forming processes, constrain global and regional surface ages, and investigate the processes of erosion and deposition. Global medium resolution imaging of Ganymede and important parts of the surface of Callisto better than 400 m/pixel (resolution limited by mission data volume) will provide context information. Selected targets will be investigated with high-resolution imaging with spatial resolution from 25 m/pixel down to 2.4 m/pixel. The camera system has 13 panchromatic, broad- and narrow-band filters in the 0.36 μm to 1.1 μm range, and provides stereo imaging capabilities. JANUS will also allow relating spectral, laser and radar measurements to geomorphology and thus will provide the overall geological context. The Europa flybys will enable high-resolution observations of selected high priority targets of young and potentially active areas. Monitoring of Jupiter’s atmosphere, Io’s volcanic activity, the ring system and small moons will complete the Jovian system exploration. JANUS is designed to meet all scientific requirements foreseen by JUICE Sci-RD for both NAC and WAC. Overall performance of the instrument required to fulfil the anticipated goals JANUS is the next logical step after the impressive successes of the imaging studies by Voyager, Galileo and Cassini of the Jovian system. JANUS will allow orders-of-magnitude steps ahead in terms of coverage and/or resolution and/or time evolution on many targets in Jupiter system. JANUS spatial resolution ranges from 400 m/pixel to 2.4 m/pixel for the three main Galilean satellites, and from few to few tens of km/pixel for Jupiter and the other targets in the Jovian system. Based on data rate estimates, JANUS observation will cover about a 100% of Ganymede in 4 colours. About 3% of the surface of Ganymede will be covered with resolutions better 24 m/pixel and 60 targeted areas will be observed with highest resolution of about 2.4 m/pixel. Stereo imaging at Ganymede is doable at resolutions between 5 to 10 m/pixel and around 25 m/pixel. Stereo images will be acquired both in nadir pointing and by tilting the spacecraft in a subsequent orbit. For Callisto, assuming only nadir pointing, JANUS will cover 100 % of the surface at ≤ 2 km/pixel, 41 % at ≤ 1 km/pixel, 14 % at ≤ 400 m/pixel, 6.5% at ≤ 200 m/pixel, and some targets at resolutions ~ 10 m/pixel, with 4 colour imaging at medium resolutions. 55% of Europa’s surface can be covered at ≤ 3 km/pixel, 42 % at ≤ 2 km/pixel, 25 % at ≤ 1 km/pixel, 11 % at ≤ 700 m/pixel, 3.2% at ≤ 400 m/pixel, 1.1% at ≤ 200 m/pixel and some targets at resolutions ~ 10 m/pixel. Low- to medium-resolution imaging (i.e. km to ~100 m-range) is obtained in four colours. Spatial resolutions at Io will gain 5 to 20 km/pixel with long monitoring time. The ring and satellites can be observed at maximum resolutions of 6 km/pixel. The resolution at Jupiter’s atmosphere will reach 7.1 km/pixel. The SNR > 100 can be maintained in almost all observational scenarios.