  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.
