This paper describes the Ultraviolet Spectrograph (JUICE-UVS) that will fly on ESA’s Jupiter Icy Moons Explorer (JUICE) mission. The instrument was built at the Southwest Research Institute and contributed by NASA. It will collect ultraviolet light between 50 and 204 nanometres to study the thin atmospheres, aurora, surfaces, and local space around Jupiter and its large icy moons.

JUICE-UVS is an imaging spectrograph, which means it records both where light comes from and its colors (wavelengths) at the same time. An optical grating spreads extreme-ultraviolet (EUV) and far-ultraviolet (FUV) light across a detector so the instrument produces a spectrum for every point along a long slit on the sky. The slit spans 7.5 degrees and is made of contiguous sections 7.3°×0.1° and 0.2°×0.2°. The instrument is small—about the size of a briefcase (35.4 × 41.1 × 15.8 cm)—and was delivered to ESA in February 2020.

The paper explains the science plans, how the instrument will be used, and the data that will be produced. JUICE-UVS will make many kinds of observations: push‑broom nadir imaging (looking straight down), disk scans across moons or Jupiter, limb stares (watching the edge of a body to see faint atmospheres), stellar and solar occultations (measuring how a moon’s atmosphere dims a star or the Sun as it passes in front), and transit observations of moons moving across Jupiter. These techniques let the team map gases, search for plumes, and study auroral emissions and plasma around the moons and Jupiter.

The instrument will focus on specific ultraviolet emission lines that reveal composition and processes. Atomic oxygen emits near 130.4 nm and 135.6 nm, and hydrogen emits at 121.6 nm. The ratio of the two oxygen lines helps diagnose how the emissions are made: for example, dissociation of molecular oxygen (O2) gives a 135.6/130.4 ratio near 2.3, while electron impact on atomic oxygen or water gives ratios below 1. Past ultraviolet observations show Europa and Ganymede have atmospheres dominated by O2 with water vapor stronger on sunlit areas, and Callisto may have a denser, more collisional atmosphere. JUICE-UVS will make full-disk and nightside maps, limb spectra, and occultation measurements to better quantify these species and to search for localized denser regions that might indicate plume activity.