GALA is one of the 10 instruments on board JUICE that will measure the topography of Ganymede and tidal changes in its shape (tidal amplitude) with high precision. The instrument was developed and built under the leadership of the DLR Institute of Planetary Research in cooperation with German industry and institutes from Japan, Switzerland and Spain. Within JUICE, laser altimetry will be used for the first time to study the icy moons. GALA is expected to provide fundamental new scientific knowledge about Jupiter’s icy satellites Ganymede, Europa, and Callisto.
GALA's scientific goals can be summarized as follows:
The scientific significance of each of these will be briefly described below in their respective contexts and in relation to the goals of JUICE.
Accurate measurement of the surface of Ganymede and its rotational state are fundamental to establishing geodetic reference system. This is essential for the measurements and context of all other instruments. Here, laser altimetry in combination with camera data makes crucial contributions. In particular, an accurate reference system is a prerequisite for the measurement of periodic deformation due to tides and the measurement of the rotational state (e.g. physical librations). Both measurements can provide information on the existence of a global water ocean at depth. The global shape of Ganymede also allows conclusions to be drawn about its internal state and thus its long-term evolution.
To determine the gravitational field of Ganymede, which is measured by Doppler tracking with JUICE 3GM Radio Science Experiment, knowledge of the topography is required. By comparing the gravity field and the topography, it can be determined to what extent, for example, height distributions in the ice layer are isostatically compensated. In addition, investigations of cross-over points of altimeter tracks may contribute to a better determination of the position of the spacecraft in orbit and thus to an improved knowledge of the gravitational field. Global, regional, and local topographic elevation models, are also indispensable data sets for the geologic interpretation of the satellites’ surface records. Here, in particular, the combination of laser altimeter, camera, spectrometer and radar, which can penetrate several kilometers into the ice layer, will provide new insights into the evolution of Ganymede, Europa and Callisto.
The deformation of the surfaces of the moons by tidal forces caused by Jupiter depends strongly on the presence of an inner ocean. The amplitudes of tidal deformation are much larger when the outer ice layer is not firmly attached to the interior but is nearly free to move on a global water layer. The expected amplitudes in the case of an existing ocean are up to 7 m for Ganymede. This is well above the precision of GALA, which is on the order of about 10 cm. The tidal amplitudes for the case where no ocean is present are on the order of 10 cm for Ganymede. This significant difference provides indirect evidence of an ocean. In combination with other measurements, especially gravity field and magnetic field measurements, properties such as the depth of the ocean can be inferred.
The geological processes on the Galilean moons are extremely diverse. The surfaces of Ganymede and Europa are characterized by complex processes based on tectonic activity but also on cryovolcanism. The presence of liquid water may have aided these processes. What role these processes played in the evolution of Ganymede is still unclear. Important information about the formation processes can be obtained from laser altimeter elevation profiles, e.g., cryovolcanically shaped surfaces are smoother than tectonically shaped surfaces and are often deeper than the surrounding area. From elevation profiles especially of extensional structures, conclusions can be drawn about the thermal state at the time of formation. Likewise, the morphology of impact craters gives important clues about the thermal evolution of Ganymede (relaxation processes).
Callisto's surface is mainly characterized by erosion processes (bombardment by micrometeorites, radiation and incident particles) in addition to impact craters. By comparing the three very different icy surfaces of Europa, Ganymede and Callisto, important conclusions can be drawn within JUICE about the evolution of the individual moons and the evolution of the Jupiter system as a whole.
The analysis of the profile of the reflected laser pulses provides information about surface properties such as slope and roughness. This can be used to additionally characterize the different geological structures and units. Of particular interest are correlations of roughness or albedo with geological structures.
GALA consists of two electronic units and an optical part containing the laser and the receiver telescope. During the measurements, laser pulses are sent to the surface of Ganymede 30 to 50 times per second from an altitude of 500 kilometers in the near infrared. A highly sensitive detector records the reflected pulses. Because GALA can measure the light travel time to within less than a nanosecond, the range of the spacecraft to the surface can be determined very precisely, allowing Ganymede's surface to be scanned with optical precision. The data will be used to create a global elevation model.
Left: GALA Electronics Unit, right: GALA Laser Electronics Unit
JUICE is an ESA mission selected in 2012 as the first L-class mission (L stands for 'large') in the Cosmic Vision program. Its goal is to study Jupiter, its moons, and its magnetosphere. In particular, Jupiter's moon Ganymede will be studied in detail. JUICE is thus the first mission ever to enter orbit around a moon of another planet. Launch is planned for April 2023 with an ARIANE 5 ECA launch vehicle from the European spaceport Kourou.
Flight time to reach the Jupiter system will be about 7.4 years. Initially, flybys of Earth and Venus will be used to minimize flight time and save propellant. These so-called 'gravity assists' to the planets Earth and Venus will give the spacecraft additional acceleration before it embarks on the long journey to the outer solar system. Jupiter, at a distance of 5.2 astronomical units, is about five times as far from the Sun as Earth. Thus, delays due to light travel time between ground stations and the spacecraft in transmitting telecommands and data range from 33 to 52 minutes, depending on the position of the two planets. After Jupiter orbit insertion in 2031 , the spacecraft is in an elongated elliptical orbit. The eccentricity of the orbit is reduced by steering maneuvers and flybys of the Galilean moons. The numerous flybys are also of scientific interest, especially two planned flybys at Europa. After about 3 years in Jupiter orbit, during which the Jupiter’s atmosphere and magnetosphere will be studied in detail, the spacecraft will enter a polar elliptical orbit around Ganymede. The closest distance (pericenter) to Ganymede will be 500 km (measured from Ganymede's surface); the farthest distance (apocenter) will be at an altitude of 10,000 km. This orbit is ideal for studying Ganymede's magnetosphere and its interaction with Jupiter's magnetosphere. Due to gravitational perturbations of Jupiter on the spacecraft, the orbit will evolve naturally, i.e. without additional consumption of propellant, into a circular orbit at an altitude of 5000 km. This mission phase is ideal for mapping Ganymede globally through the camera and spectrometer. Again, due to perturbations of Jupiter, an elliptical 500 x 10,000 km orbit is established. Passing through the pericenter, a steering maneuver is then required to bring the spacecraft to a circular orbit at 500 km altitude. This orbit will be used to characterize the ice layer using laser altimetry and sub-surface radar, which can pick up structures of the ice layer at depths of several km. Gravitational field measurements, characterization of Ganymede's exosphere, magnetic field measurements, and targeted observations of geologic units and structures at the highest resolution by camera and spectrometer will also take place during this phase. The nominal mission will end after 132 days in 500-km orbit. If resources allow to extend the mission, a further lowering of the orbit to about 200 km altitude would be conceivable.
Onboard the spacecraft are 10 instruments selected by ESA in 2013. In addition, VLBI (Very Large Baseline Interferometry) observations from Earth will be part of JUICE.
GALA on Twitter: https://twitter.com/GALA_JUICE
GALA on Mastodon: https://wisskomm.social/@GALA_JUICE
GALA webpage in german: DLR - Institut für Planetenforschung - Das Ganymed Laser Altimeter (GALA)
ESA JUICE mission: https://www.esa.int/Science_Exploration/Space_Science/Juice