Science Objectives of the Ganymede Laser Altimeter (GALA) for the JUICE Mission

Laser altimetry is a powerful tool for addressing the major objectives of planetary physics and geodesy, and have been applied in planetary explorations of the Moon, Mars, Mercury, and the asteroids Eros, and Itokawa. The JUpiter Icy Moons Explorer (JUICE), led by European Space Agency (ESA), has started development to explore the emergence of habitable worlds around gas giants. The Ganymede Laser Altimeter (GALA) will be the first laser altimeter for icy bodies, and will measure the shape and topography of the large icy moons of Jupiter, (globally for Ganymede, and using flyby ground-tracks for Europa and Callisto). Such information is crucial for understanding the formation of surface features and can tremendously improve our understanding of the icy tectonics. In addition, the GALA will infer the presence or absence of a subsurface ocean by measuring the tidal and rotational responses. Furthermore, it also improves the accuracy of gravity field measurements reflecting the interior structure, collaborating with the radio science experiment. In addition to range measurements, the signal strength and the waveform of the laser pulses reflected from the moon's surface contain information about surface reflectance at the laser wavelength and small scale roughness. Therefore we can infer the degrees of chemical and physical alterations, e.g., erosion, space weathering, compaction and deposition of exogenous materials, through GALA measurements without being affected by illumination conditions. JUICE spacecraft carries ten science payloads including GALA. They work closely together in a synergistic way with GALA being one of the key instruments for understanding the evolution of the icy satellites Ganymede, Europa, and Callisto.Comment: 10 pages, 6 figures, accepted for publication in Trans. JSASS Aerospace Tech. Japa

The Ganymede Laser Altimeter (GALA) for the Jupiter Icy Moons Explorer (JUICE): Mission, science, and instrumentation of its receiver modules

The Jupiter Icy Moons Explorer (JUICE) is a science mission led by the European Space Agency, being developed for launch in 2023. The Ganymede Laser Altimeter (GALA) is an instrument onboard JUICE, whose main scientific goals are to understand ice tectonics based on topographic data, the subsurface structure by measuring tidal response, and small-scale roughness and albedo of the surface. In addition, from the perspective of astrobiology, it is imperative to study the subsurface ocean scientifically. The development of GALA has proceeded through an international collaboration between Germany (the lead), Japan, Switzerland, and Spain. Within this framework, the Japanese team (GALA-J) is responsible for developing three receiver modules: the Backend Optics (BEO), the Focal Plane Assembly (FPA), and the Analog Electronics Module (AEM). Like the German team, GALA-J also developed software to simulate the performance of the entire GALA system (performance model). In July 2020, the Proto-Flight Models of BEO, FPA, and AEM were delivered from Japan to Germany. This paper presents an overview of JUICE/GALA and its scientific objectives and describes the instrumentation, mainly focusing on Japan’s contribution

Science Objectives of the Ganymede Laser Altimeter (GALA) for the JUICE Mission

Abstract Laser altimetry is a powerful tool for addressing the major objectives of planetary physics and geodesy. Through measurements of distances between a spacecraft and the surface of the planetary bodies, it can be used to determine the global shape and radius: global, regional, and local topography: tidal deformation: and rotational states including physical librations. Laser altimeters have been applied in planetary explorations of the Moon, Mars, Mercury, and the asteroids Eros, and Itokawa. The JUpiter Icy Moons Explorer (JUICE), led by European Space Agency (ESA), has started development to explore the emergence of habitable worlds around gas giants. The Ganymede Laser Altimeter (GALA) will be the first laser altimeter for icy bodies, and will measure the shape and topography of the large icy moons of Jupiter, (globally for Ganymede, and using flyby ground-tracks for Europa and Callisto). Such information is crucial for understanding the formation of surface features and can tremendously improve our understanding of the icy tectonics. In addition, the GALA will infer the presence or absence of a subsurface ocean by measuring the tidal and rotational responses. Furthermore, it also improves the accuracy of gravity field measurements reflecting the interior structure, collaborating with the radio science experiment. In addition to range measurements, the signal strength and the waveform of the laser pulses reflected from the moon's surface contain information about surface reflectance at the laser wavelength and small scale roughness. Therefore we can infer the degrees of chemical and physical alterations, e.g., erosion, space weathering, compaction and deposition of exogenous materials, through GALA measurements without being affected by illumination conditions. JUICE spacecraft carries ten science payloads including GALA. They work closely together in a synergistic way with GALA being one of the key instruments for understanding the evolution of the icy satellites Ganymede, Europa, and Callisto

Performance Model Simulation of Ganymede Laser Altimeter (GALA) for the JUICE Mission

GALA (GAnymede Laser Altimeter) is one of the payload instruments of the JUICE (JUpiter ICy moons Explorer) project to be launched in 2022 to the Jovian icy moons Ganymede, Europa, and Callisto. GALA is developed through an international collaboration between Germany, Japan, Switzerland, and Spain. With the performance model of GALA, we have sought to create the interface conditions that satisfy the science requirements on the probability of false detection (PFD) and the range accuracy. The science requirements on GALA performance can be summarized as involving the following four criteria: [1] for Europa fly-by, PFD is less than 0.2 from an altitude of 1300 km or lower, [2] under the worst observation condition for albedo and surface slope of GCO500 (Ganymede Circular Orbit whose height is 500 km), the accuracy of ranging is less than 10 m and PFD is less than 0.2, [3] under the nominal observation condition of GCO500, the accuracy of ranging is less than 2 m and PFD is less than 0.1, and [4] under the best observation condition of GCO500, the accuracy of ranging is less than 1 m and PFD is less than 0.1. For the assessment, however, we had used literature data as the characteristics of the laser detector of GALA, avalanche photodiode (APD), which should be degraded due to the severe radiation environment around Jupiter. Then we carried out a more realistic model simulation of GALA by incorporating these degradation effects of APD. Characteristics of APD, such as gain, quantum efficiency, excess noise index, surface dark current, and bulk dark current, were re-evaluated through radiation tests using the data of dark and photo current of the APD irradiated with 2-MeV-electron and 50-MeV-proton beams, which are the radiation conditions assumed for JUICEGALA around Jupiter. These degraded characteristics of APD by radiation were introduced to our performance model of GALA. As a result, our performance simulation of GALA showed again that the science requirements are satisfied even after taking into account the degraded characteristics of APD. The remaining matter is the effect of noise or digitization in the Analog Electronics Module (AEM), which must be taken into account for the final specifications of GALA