Atmospheric Variability and Influence of Micrometeorite Vaporization on Ganymede and Callisto’s Leading and Trailing Hemispheres

Ganymede and Callisto will be soon visited by ESA’s JUICE spacecraft along with complementary flybys by NASA’s Europa Clipper. Ganymede, with its intrinsic magnetic field, and Callisto, located outside the Laplace Resonance and peak plasma torus environment, exhibit complex exospheric environments that differ markedly from Europa’s. Callisto, due to its lack of an intrinsic magnetic field, is a natural laboratory for disentangling endogenic and exogenic contributions to the surface-bounded water exospheres on icy moons. Both moons exhibit pronounced leading/trailing hemispheric dichotomies in surface composition, albedo, and plasma irradiation, yet the relative weight of sublimation, sputtering, radiolysis, and outgassing in shaping their tenuous atmospheres is not yet fully understood. A major unknown, particularly for the darker terrains of Ganymede and Callisto – potentially linked to dust contamination or non-pure ice compositions – is the role of micrometeoroid impact vaporization in shaping their tenuous atmospheres. The JWST/NIRSpec detection of a patchy CO₂ exosphere at Ganymede [1], together with HST/STIS detections of H, O, and O₂ auroral emissions [2,3] and Juno/JADE constraints on the ionosphere [4], have reopened questions that pre-mission models focused on solar UV and Jovian plasma sources [5–9] cannot fully address. The observed variability of O₂ at Europa, also seen at Ganymede [10], further implies an active thermal release from the surface [11] balanced by plasma destruction. These coupled processes are further complicated by micrometeoroid gardening, making it difficult to disentangle their respective contributions.

 

To address this, building on the framework introduced by Robidel et al. [12], incorporating micrometeoroid impact vaporization into a 3D rotation-dependent Monte Carlo Exospheric Global Model (EGM) for Ganymede, we extend the analysis to Callisto with a focus on hemispheric asymmetry. The EGM treats each test particle's ballistic trajectory, photo- and electron-impact destruction, sticking, and re-ejection, allowing direct comparison of the steady-state column densities produced by sublimation, sputtering, radiolysis, and impact vaporization across both hemispheres of both moons.

Using the Bond albedo and surface temperatures of Ganymede’s trailing and leading hemispheres [10], we can quantitatively show that micrometeoroid vaporization provides a nearly symmetric source of neutrals. This process becomes the dominant non-thermal driver on Callisto’s leading hemisphere, where Jovian plasma sputtering is suppressed and darker surface regions enhance thermal contrast. Further simulations are needed to quantify the role of meteoroid impacts in supplying carbon-bearing material and contributing to CO₂ production at the surfaces and in the exospheres of Ganymede and Callisto.

 

Lastly, the trailing-to-leading column-density ratio provides a diagnostic observable to disentangle the relative contributions of source processes. These predictions are directly testable by JUICE (during Ganymede flybys and orbital phase, as well as Callisto flybys) and Europa Clipper, thereby linking Solar System exosphere studies to the broader question of how impacts affect the volatile budgets of irradiated icy moons.

 

References: [1] Bockelée-Morvan et al. 2024, A&A 690; [2] Hall et al. 1998, ApJ 499; [3] Feldman et al. 2000, ApJ 535; [4] Waite et al. 2024, JGR: Planets 129; [5] Marconi 2007, Icarus 190; [6] Turc et al. 2014, Icarus 229; [7] Plainaki et al. 2015; [8] Leblanc et al. 2017, Icarus 293; [9] Leblanc et al. 2023, Icarus 399; [10] Oza et al. 2026, Astrobiology; [11] Oza et al. 2019, PSS 167; [12] Robidel et al. 2024, EPSC17-129