JIRAM is an imager/spectrometer on board the Juno spacecraft bound for a polar orbit around Jupiter. JIRAM is composed of IR imager and spectrometer channels. Its scientific goals are to explore the Jovian aurorae and the planet's atmospheric structure, dynamics and composition. This paper explains the characteristics and functionalities of the instrument and reports on the results of ground calibrations. It discusses the main subsystems to the extent needed to understand how the instrument is sequenced and used, the purpose of the calibrations necessary to determine instrument performance, the process for generating the commanding sequences, the main elements of the observational strategy, and the format of the scientific data that JIRAM will produce.

Microwave observations by the Juno spacecraft have shown that, contrary to expectations, the concentration of ammonia is still variable down to pressures of tens of bars in Jupiter. We show that during strong storms able to loft water ice into a region located at pressures between 1.1 and 1.5 bar and temperatures between 173 and 188 K, ammonia vapor can dissolve into water ice to form a low‐temperature liquid phase containing about one‐third ammonia and two‐third water. We estimate that, following the process creating hailstorms on Earth, this liquid phase enhances the growth of hail‐like particles that we call mushballs. We develop a simple model to estimate the growth of these mushballs, their fall into Jupiter's deep atmosphere, and their evaporation. We show that they evaporate deeper than the expected water cloud base level, between 5 and 27 bar depending on the assumed abundance of water ice lofted by thunderstorms and on the assumed ventilation coefficient governing heat transport between the atmosphere and the mushball. Because the ammonia is located mostly in the core of the mushballs, it tends to be delivered deeper than water, increasing the efficiency of the process. Further sinking of the condensates is expected due to cold temperature and ammonia‐ and water‐rich downdrafts formed by the evaporation of mushballs. This process can thus potentially account for the measurements of ammonia depletion in Jupiter's deep atmosphere.

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Jovian lightning flashes were characterized by a number of missions that visited Jupiter over the past several decades. Imagery from the Voyager 1 and Galileo spacecraft led to a flash rate estimate of ~4×10 -3 flashes/km 2 /yr on Jupiter. 1,2 The spatial extent of Voyager flashes was estimated to be ~30 km at half-width half-maximum intensity (HWHM), but the camera was unlikely to have detected the dim outer edges of the flashes given weak response to the brightest spectral line of Jovian lightning emission, the 656.3 nm H-alpha line of atomic hydrogen (known from lab experiments). 1,3-6 The spatial resolution of Galileo and New Horizons cameras allowed investigators to confirm twenty-two flashes with HWHM >42 km and estimate one between 37-45 km. 1,7,8,9 These flashes, with optical energies only comparable to terrestrial "superbolts" (2×10 8 -1.6×10 10 Joules), have historically been interpreted as tracers of moist convection originating near the 5 bar level of Jupiter's atmosphere (assuming photon scattering from points beneath the clouds). 1-3,7,8,10-12 All previous optical observations of Jovian lightning have been limited by camera sensitivity, proximity to Jupiter, and long exposures (~680 ms to 85 s) hence some measurements were likely superimposed flashes reported as one. 1,2,7,9,10,13 Here we report optical observations of lightning flashes by Juno's Stellar Reference Unit 14 with energies of ~10 5 -10 8 Joules, flash durations as short as 5.4 ms, and inter-flash separations of tens of milliseconds. The observations exposed Jovian flashes with typical terrestrial energies. The flash rate is ~6.1×10 -2 flashes/km 2 /yr, more than an order of magnitude greater than hitherto seen. Several flashes are of such small spatial extent they must originate above the 2 bar level, where there is no liquid water. 15,16 Juno's Stellar Reference Unit (SRU) captured images of Jovian lightning on the dark side of Jupiter from a unique perspective of as close as 53,000 km above the 1 bar level (30 km/pixel resolution). The SRU is a broadband (450 -1100 nm) imager designed to detect dim stars in support of spacecraft attitude determination. The camera's point spread function (PSF) spreads the optical signal of a point source over ~5 × 5 pixels, allowing unambiguous identification of small optical sources (see Extended Data Fig. 1). Fourteen lightning flashes (see Extended Data

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