The Ganymede Laser Altimeter (GALA) on the Jupiter Icy moons Explorer (JUICE) Mission

• DOI: 10.1007/s11214-025-01149-7
• Publication Date: 2025-03-18T15:04:06Z
• Authors: Hauke Hussmann; Kay Lingenauber; Alexander Stark; Keigo Enya; Nicolas Thomas; Luisa M. Lara; Christian Althaus; Hiroshi Araki; Thomas Behnke; Jan Binger; Doris Breuer; Stefano Casotto; Jose M. Castro; Gaël Choblet; Ulrich Christensen; Willem Coppoolse; Henri Eisenmenger; Sylvio Ferraz-Mello; Masayuki Fujii; Naofumi Fujishiro; Giovanni Gallina; Klaus Gwinner; Ernst Hauber; Ulrich Heer; Reiner Henkelmann; Miguel Herranz; Christian Hüttig; Satoru Iwamura; Jaime Jiménez; Jun Kimura; Okiharu Kirino; Masanori Kobayashi; Kei Kurita; Valéry Lainey; Thomas Leikert; Alexander Lichopoj; Horst-Georg Lötzke; Fabian Lüdicke; Ignacio Martínez-...; Harald Michaelis; Hiroyuki Nakagawa; Noriyuki Namiki; Gaku Nishiyama; Hirotomo Noda; Jürgen Oberst; Shoko Oshigami; Antoine Pommerol; Markus Rech; Thomas Roatsch; Rafael Rodrigo; Adrián Rodríguez; Kerstin Rösner; Kazuyuki Touhara; Yoshifumi Saito; Sho Sasaki; Yuki Sato; Frédéric Schmidt; Ulrich Schreiber; Stefan Schulze-Wa...; Frank Sohl; Tilman Spohn; Gregor Steinbrügge; Katrin Stephan; Kazuo Tanimoto; Pascal Thabaut; Simone del Togno; Bert Vermeersen; Henry Wegert; Kai Weidlich; Belinda Wendler; Kai Wickhusen; Mark Wieczorek; Konrad Willner; Friederike Wolff; Takeshi Yokozawa; Marie Yseboodt

Abstract The Ganymede Laser Altimeter (GALA) on the Jupiter Icy Moons Explorer (JUICE) mission, is in charge of a comprehensive geodetic mapping of Europa, Ganymede, and Callisto on the basis of Laser range measurements. While multiple topographic profiles will be obtained for Europa and Callisto during flybys, GALA will provide a high-resolution global shape model of Ganymede while in orbit around this moon based on at least 600 million range measurements from altitudes of 500 km and 200 km above the surface. By measuring the diurnal tidal deformation of Ganymede, which crucially depends on the decoupling of the outer ice shell from the deeper interior by a liquid water ocean, GALA will obtain evidence for (or against) a subsurface ocean on Ganymede and will provide constraints on the ice shell thickness above the ocean. In combination with other instruments, it will characterize the morphology of surface units on Ganymede, Europa, and Callisto providing not only topography but also measurements of surface roughness on the scale of the laser footprint, i.e. at a scale of about 50 m from 500 km altitude, and albedo values at the laser wavelength of 1064 nm. GALA is a single-beam laser altimeter, operating at a nominal frequency of 30 Hz, with a capability of reaching up to 48 Hz. It uses a Nd:YAG laser to generate pulses with pulse lengths of 5.5 ± 2.5 ns. The return pulse is detected by an Avalanche Photo Diode (APD) with 100 MHz bandwidth and the signal is digitized at a sampling rate of 200 MHz providing range measurements with a sub-sample resolution of 0.1 m. Research institutes and industrial partners from Germany, Japan, Switzerland and Spain collaborated to build the instrument. JUICE, conducted under responsibility of the European Space Agency (ESA), was successfully launched in April 2023 and is scheduled for arrival at the Jupiter system in July 2031. The nominal science mission including multiple close flybys at Europa, Ganymede, and Callisto, as well as the final Ganymede orbit phase will last from 2031 to 2035. In May 2023 GALA has completed its Near-Earth Commissioning, showing full functionality of all units. Here we summarize the scientific objectives, instrument design and implementation, performance, and operational aspects of GALA.