The Jovian InfraRed Auroral Mapper (JIRAM) on board the NASA Juno spacecraft is a dual‐band imager and spectrometer in the 2–5 μm range with 9‐nm spectral sampling, primarily designed to study the Jovian atmosphere and aurorae. In addition to these goals, JIRAM is used to obtain images and spectra of the Galilean satellites, every time the spacecraft attitude is favorable. Here we present JIRAM images and spectra of Ganymede obtained during the first 4 years of the mission. In particular, on 26 December 2019, during a relatively close passage of Juno with the moon, a dedicated reorientation of the spacecraft was performed to achieve optimized observations of Ganymede by Juno's remote sensing instruments, including JIRAM. In the outbound phase of the flyby, observing the northern polar regions of Ganymede at a distance of roughly 100,000 km, JIRAM collected infrared images and spectra of the surface at a spatial resolution as high as 23 km per pixel, covering high northern latitudes that were scarcely mapped previously. A photometric model of Ganymede reflectance is produced, which diverges from the Lambert model. The spatial distribution of the obtained spectra complements the available coverage of the surface, with particular regard to the 2.0‐µm water ice absorption band and, to a lesser extent, to the 4.26‐µm spectral feature diagnostic of CO₂ trapped in water ice. The water ice distribution is compatible with sputtered‐induced water ice grain enrichment at high latitude (>45°). Several minor species (hydrated salts, trapped H₂, CO₂, and acids) are also identified in the measured spectra.

中文翻译：

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朱诺木星红外极光成像仪对木卫三的红外观测

NASA Juno航天器上的Jovian红外极光绘图仪（JIRAM）是2–5μm范围内的双波段成像仪和光谱仪，具有9 nm的光谱采样，主要用于研究Jovian大气和极光。除了这些目标之外，每次航天器姿态有利时，JIRAM都将用于获取伽利略卫星的图像和光谱。在这里，我们展示了在任务的最初4年中获得的Ganymede的JIRAM图像和光谱。特别是在2019年12月26日，在朱诺与月球相对较近的飞行过程中，对航天器进行了专门的重新定向，以通过朱诺的遥感仪器（包括JIRAM）实现对木卫三的优化观测。在飞越的出港阶段，在大约100,000 km的距离观察木卫三的北极地区，JIRAM以高达每像素23 km的空间分辨率收集了红外图像和表面光谱，覆盖了以前很少绘制的北高纬度地区。产生了木卫三反射率的光度模型，该模型与兰伯特模型不同。所获得光谱的空间分布补充了表面的可用覆盖范围，特别是对于2.0 µm的水冰吸收带，并在较小程度上针对了4.26 µm的CO光谱特征诊断₂被困在水冰中。水冰的分布与高纬度（> 45°）上的溅射诱导水冰粒富集兼容。在测量的光谱中还鉴定出几种次要物种（水合盐，捕获的H ₂，CO ₂和酸）。