Orientation and rotation model of the small inner moons of Jupiter

The orientation and rotation of synchronous satellites can be described using two different sets of angles: the Euler angles defined relative to a fixed reference plane (typically the Laplace) and the right ascension, declination, and prime meridian location angles with respect to the J2000 Inertial Celestial Reference Frame, following the convention adopted by the International Astronomical Union (IAU; Archinal et al. 2018).

Yseboodt and Baland (2026) recently derived an analytical transformation between these two angle systems, correct up to the second order in small parameters as orbital inclination and obliquity. Their method was successfully applied to the Galilean satellites (Io, Europa, Ganymede, and Callisto) to produce rotation models intended to supersede the currently adopted IAU models.

In this work, we extend the methodology of Yseboodt and Baland (2026) to Jupiter’s small inner moons: Metis, Adrastea, Amalthea, and Thebe. These satellites are small bodies, with diameters up to ~260 km, orbiting close to Jupiter and rotating synchronously with periods shorter than 0.7 day. Little is currently known about their physical and dynamical properties. In the present study, they are modelled as rigid solid bodies. We perform a frequency decomposition of their ephemerides in order to express the orientation of their orbit as quasi-periodic series. Applying the classical Cassini state model (e.g. Baland et al 2012) term by term then yields a quasi-periodic representation of the spin-axis orientation, initially expressed through Euler angles, then transformed into IAU angles.

Although no dedicated close flybys are currently planned, the JANUS camera system onboard Jupiter Icy Moons Explorer (JUICE) will contribute to constraining the orbital orientation of these inner moons and improving the mapping of their surfaces (Denk et al. 2025). In the context of mission preparation, an updated rotational model for the inner moons may already be valuable as a replacement for the current IAU standard solution. Following future observations, the proposed methodology could also be used to further refine the IAU rotational models.

[1]  M. Yseboodt and R.-M. Baland (2026). Transformation of orientation and rotation angles of synchronous satellites: Application to the Galilean moons. Icarus, 450, 116977.  https://doi.org/10.1016/j.icarus.2026.116977

[2]  B. Archinal et al. (2018). Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements: 2015. Celestial Mechanics and Dynamical Astronomy, 130, 22. https://doi.org/10.1007/s10569-017-9805-5

[3]  T. Denk et al. (2026). Io and the Minor Jovian Moons – Prospects for JUICE. Space Science Reviews, 222, 27. https://doi.org/10.1007/s11214-025-01263-6

[4]  R.-M. Baland, M. Yseboodt, and T. Van Hoolst (2012). Obliquity of the Galilean satellites: The influence of a global internal liquid layer. Icarus, 220, 435–448. http://dx.doi.org/10.1016/j.icarus.2012.05.020