University of Liège Scientists Reveal Similarities Between Auroras on Ganymede and Earth

New observations of Ganymede reveal a striking similarity between the auroras on the largest moon in the solar system and those on Earth. The international team of astrophysicists, led by researchers from the University of Liège, has produced new results indicating that, despite different conditions, the fundamental physical processes that generate auroras are common to different celestial bodies, and not just planets.

A team of astrophysicists from the Laboratory of Atmospheric and Planetary Physics (LPAP) has observed for the first time the fine details of the auroras on Ganymede, the only moon in the solar system to have its own intrinsic magnetic field, similar to that of Earth. The observation of auroras is a cornerstone of space weather analysis, as it provides a comprehensive view of the characteristics and effects of space particle precipitation into atmospheres.

For centuries, humanity has witnessed a diffuse and changing glow that occasionally illuminates the night sky with red, green, purple and blue lights - known as the "aurora". Auroras are typically observed at polar latitudes, although we have just passed the peak of the 11-year solar cycle, which is producing many instances of intense auroras at mid-latitudes. On our planet, auroras are ultimately caused by solar wind colliding with the Earth's magnetic field and generating a powerful electromagnetic interaction, which causes particles to precipitate from space into the Earth's atmosphere. There, these precipitates excite atmospheric oxygen and nitrogen, producing light visible to the naked eye.

However, auroras are also observed on other planets, such as Venus, Mars, Jupiter, Saturn and Uranus. Over the past decade, exploration of the Jovian system has made remarkable progress thanks to the Juno spacecraft, which arrived at Jupiter in 2016 and is about to complete its tenth year of operation. The mission's primary target is Jupiter, but on 7 July 2021, Juno flew by Ganymede. "Auroras are also observed on Ganymede and are caused by the precipitation of electrons in its thin oxygen atmosphere," explains Philippe Gusbin, whose master's thesis in Space Sciences served as the basis for this study. “Observations of Ganymede's auroras prior to Juno were limited by the spatial resolution of ground-based observations, and they could not resolve the small-scale structures typical of planetary auroras."

The LPAP is actively involved in analysing auroral observations collected by Juno's infrared and ultraviolet instruments. Juno's ultraviolet spectrograph (UVS) has achieved spatial resolutions of a few kilometres, allowing the team to discover that Ganymede's aurora is locally fragmented into a chain of patches. "Similar structures, known as 'beads', have been observed in the auroras of Earth and Jupiter, where they are linked to sub-storms and dawn storms, large-scale rearrangements of the magnetosphere that release enormous amounts of energy and produce intense auroral activity," explains Alessandro Moirano, post-doctoral researcher at LPAP. Ganymede interacts with Jupiter's space environment in a similar way to how Earth interacts with the solar wind; therefore, the discovery of auroral patches on Ganymede similar to those on Earth suggests that the fundamental physical process(es) could be generally induced in the coupling between any celestial body, its magnetosphere, and external forces. The analysis and the results are reported in a peer-reviewed letter, “Juno’s high spatial resolution ultraviolet observations of Ganymede’s auroral patches”, published by “Astronomy & Astrophysics.”

“Juno's close observations of Ganymede lasted less than 15 minutes, and the spacecraft will never fly over Ganymede again. Therefore, we do not know how common these patches are or how they evolve over time. Fortunately, ESA's Juice (Jupiter Icy Moons Explorer) mission is currently on its way to Jupiter, where it will arrive in 2031, and will carry out dedicated observations of Ganymede," says Bertrand Bonfond, FNRS research associate. The spacecraft is equipped with an ultraviolet spectrograph similar to Juno's: this will allow us to collect observations over longer periods, monitor the evolution of Ganymede's auroras and, hopefully, uncover new mysteries.

About Juno
A division of Caltech in Pasadena, California, JPL manages the Juno mission for the principal investigator, Scott Bolton, of the Southwest Research Institute in San Antonio. Juno is part of NASA’s New Frontiers Program, managed at NASA’s Marshall Space Flight Center in Huntsville, Alabama, for the agency’s Science Mission Directorate in Washington.

Learn More About Juno

This research was conducted by the LPAP team - in collaboration with the Southwest Research Institute (United States), the National Institute for Astrophysics (Italy), the University of Cologne (Germany), the University of Hong Kong (China) and the University of Aix-Marseille (France)

Scientific reference
Moirano, P. Gusbin, B. Bonfond, D. Grodent, S. Duling, B. Hubert, Z. H. Yao, V. Hue, B. Benmahi, T. K. Greathouse, G. R. Gladstone, R. S. Giles, M. H. Versteeg and J. A. Kammer, Juno’s high spatial resolution ultraviolet observations ofGanymede's auroral patches. Constraints on the magnetospheric source region, 2026, Astronomy & Astrophysics, doi : 10.1051/0004-6361/202558379