Enceladus, Saturn’s small icy moon, is famous for its geysers, but its actual impact on the giant planet remained partly mysterious. Understanding this interaction is crucial to grasping how energy flows in a planet’s space environment. A study based on data from the Cassini mission has now revealed a fascinating discovery : Enceladus’ influence extends over a record distance of more than 500,000 km, more than 2,000 times its own radius. This result radically transforms our view of the moon.
To achieve this result, the scientific team used wave and particle data from the Cassini probe (NASA/ESA/ASI) accumulated over the 13 years of the mission. Using a multi-instrumental approach, the scientists were able to identify precise signatures of wave structures commonly known as “Alfvén wings” that propagate along field lines on either side of Enceladus. Like an electromagnetic wake, these wave structures form when Saturn’s magnetic field sweeps across Enceladus. Detailed analysis of the data revealed that these waves extend far downstream behind the moon in Saturn’s equatorial plane, but also to very high northern and southern latitudes.
The main finding shows that the interaction is not limited to the vicinity of the ice plumes, but forms a complex, structured system stretching over more than 500,000 km. This phenomenon can be explained by the multiple reflection of these Alfvén wings on Saturn’s ionosphere and the boundaries of the plasma torus surrounding Enceladus’ orbit. This is the first time such an extension has been observed, proving that this small moon acts as a giant Alfvén wave generator on a planetary scale. This work opens up new perspectives on the study of other systems, such as Jupiter’s moons or exoplanets, by showing that a small celestial body can influence its giant host over very long distances, on the order of the latter’s size.
Bibliography
Hadid, L. Z., Chust, T., Wahlund, J.‐E., Morooka, M. W., Roussos, E., Witasse, O., and al. (2026). Evidence of an extended Alfvén wing system at Enceladus : Cassini’s multi‐instrument observations. Journal of Geophysical Research : Space Physics, 131, e2025JA034657.
Collaborations
This is a study by the LPP, in collaboration with researchers from other laboratories, both national (IRAP, ISAE-SUPAERO, LATMOS, LAM, and LIRA/Paris Observatory) and international (IRFU/Sweden, MPS/Germany, CAS/Czech Republic, ESA, APL, UCLA, the University of Michigan, Boston University, and the University of Iowa in the United States, DIAS/Ireland, MSSL/UCL, and Imperial College in the United Kingdom).
List of French laboratories involved
- Laboratoire de Physique des Plasmas (LPP)
Tutelles : CNRS / École Polytechnique / Sorbonne Univ
- Institut de Recherche en Astrophysique et planétologie (IRAP - OMP)
Tutelles : CNRS / CNES / Université de Toulouse
- Laboratoire d’astrophysique de Marseille (LAM - OSU Pythéas)
Tutelles : CNRS / CNES / AMU
- Laboratoire d’Instrumentation et de Recherche en Astrophysique (LIRA - Observatoire de Paris - PSL)
Tutelles : Observatoire de Paris - PSL / CNRS / Sorbonne Univ / Univ Paris Cité
- Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS-IPSL)
Tutelles : IPSL / CNRS / Sorbonne Univ / UVSQ