Ganymede-Jupiter's largest moon-is the only known moon in the Solar System to generate its own internal magnetic field (Gurnett et al., 1996;Kivelson et al., 1996) and therefore its space environment is of high scientific interest. In part, what makes Ganymede so interesting is that its magnetic field forms a mini-magnetosphere, with field lines connected to both hemispheres, that is, "closed," embedded deep within (semi-major axis = 14.97 Jovian radii or R J ) Jupiter's magnetosphere where the Alfvénic Mach number (M A ) is < 1. Classified as a sub-Alfvénic (and thus also sub-fast-magnetosonic by definition) interaction (e.g., Saur, 2021), no bow shock develops around the moon. Additionally, another aspect that makes Ganymede particularly interesting is the phenomenon of magnetic reconnection. Unlike Earth, and other planetary magnetospheric environments, which are embedded in dynamic solar wind conditions, the upstream conditions near Ganymede are relatively steady compared to plasma convection through Ganymede's magnetospheric system (Kivelson et al., 1998), therefore, making it possible to probe the nature of magnetic reconnection under relatively steady driving conditions (Ebert

Jupiter's moon Ganymede is the largest moon in the solar system and the only moon with its own intrinsic, permanent magnetic field that extends far beyond its surface to form a magnetosphere (Gurnett et al., 1996;Kivelson et al., 1996Kivelson et al., , 2002. The first missions to investigate Ganymede in detail were the Voyager flybys of Jupiter in 1979, and the Galileo mission, which orbited Jupiter from 1995 to 2003 and executed a series of six close Ganymede flybys (summarized by Volwerk et al., 2022). Key findings of these missions, combined with Earth-based observations from facilities such as Hubble and ALMA, are as follows.Ganymede has a fully differentiated interior including a metallic core (Anderson et al., 1996; Hussmann et al., 2022). A subsurface liquid salt-water ocean forms a global layer, the top of which can be no more than 330 km below the icy surface (Kivelson et al., 1999;Saur et al., 2015Saur et al., , 2018. Ganymede is slightly larger and denser than, and forms a class with, the two other large solar system moons Callisto and Saturn's Titan. However, Ganymede's complex geologic history, which includes tectonic and/or volcanic production of grooved terrain well after the moon's formation, cutting through ancient heavily cratered terrain, shows a very different formation and evolution history than the other large moons (Schenk et al., 2022).Ganymede's neutral molecular oxygen exosphere (Hall et al., 1998), sourced from sputtering, radiolysis, and sublimation of surface ice, is ionized by photo-ionization and electron impact on open field lines. O, H, and H 2 O have also been detected (

In 1966, while examining some of the earliest images of the lunar surface, researchers identified several dark features, which were postulated to be cave entrances (Heacock et al., 1966). Later that year, Halliday (1966) further mused over the existence of lunar caves and briefly discussed their potential importance for future human

[1] In this paper, we use morphological and numerical methods to test the hypothesis that seasonally formed fracture patterns in the Martian polar regions result from the brittle failure of seasonal CO 2 slab ice. The observations by the High Resolution Imaging Science Experiment (HiRISE) of polar regions of Mars show very narrow dark elongated linear patterns that are observed during some periods of time in spring, disappear in summer and re-appear again in the following spring. They are repeatedly formed in the same areas but they do not repeat the exact pattern from year to year. This leads to the conclusion that they are cracks formed in the seasonal ice layer. Some of models of seasonal surface processes rely on the existence of a transparent form of CO 2 ice, so-called slab ice. For the creation of the observed cracks the ice is required to be a continuous media, not an agglomeration of relatively separate particles like a firn. The best explanation for our observations is a slab ice with relatively high transparency in the visible wavelength range. This transparency allows a solid state green-house effect to act underneath the ice sheet raising the pressure by sublimation from below. The trapped gas creates overpressure and the ice sheet breaks at some point creating the observed cracks. We show that the times when the cracks appear are in agreement with the model calculation, providing one more piece of evidence that CO 2 slab ice covers polar areas in spring.

The Juno Jupiter orbiter's camera, Juno camera (JunoCam), acquires wide field of view color images. It was designed to image Jupiter's polar regions from Juno's highly elliptical orbit (Hansen et al., 2017). Late in Juno's primary mission, there was a close encounter with Ganymede. The extended mission will have close flybys of Europa (2022) and of Io (2023 and 2024). Here we report initial results from the Ganymede encounter on perijove 34 (PJ34). A mosaic of two JunoCam images of the PJ34 Ganymede sequence is shown in the upper image in Figure 1. PJ34 Ganymede ImagesJuno flew by Ganymede west to east, with a minimum altitude of 1,046 km. After crossing the terminator, Juno-Cam acquired four images in color (red-green-blue), separated by 60 s (Figure 1, lower). Spacecraft rotation scans 70 RGB JunoCam "framelets" for each image. The altitude above Ganymede increased from 1,256 km (first image) to 3,372 km (fourth), increasing the nadir scale from 0.84 km/pixel to 2.27 km/pixel. The incidence angles range from 90° (first image) to 0° (fourth).These four images cover from south of 10°S to north of 60°N and from ∼40°W to ∼40°E, including eastern Perrine Regio and Sicyon Sulcus in the north and Phrygia Sulcus and Barnard Regio to the south. The first two images provide particularly good coverage of the 92 km diameter, youthful crater, Tros. The subjovian point is in the southern part of the second and third images (Figure 1). Further details of Juno's Ganymede encounter are included in Hansen et al. (2022). Methods Comparison of JunoCam With Voyager 1 CoverageTo compare the JunoCam PJ34 images with previous Voyager coverage of Ganymede, we reprojected both data sets into mosaics in a common simple cylindrical projection at a scale of 1 km per pixel. This sampling is a reasonable representation of the intrinsic scale of the JunoCam images, and matches the scale of the Voyager 1