GALA on JUICE Part 1 - All set for launch to Jupiter
IIn April 2023, ESA's JUpiter ICy moons Explorer (JUICE) spacecraft will launch to the Jupiter system. The DLR Institute of Planetary Research is playing a key role in the mission with the Ganymede Laser Altimeter (GALA) and the JANUS camera. The launch preparations are now fully under way. With the successful completion of GALA's tests on the spacecraft, the team can look back on an intensive project phase and is now eagerly awaiting the launch.
What is our goal?
Jupiter's Galilean moons, Io, Europa, Ganymede and Callisto, orbit Jupiter in slightly elliptical orbits lasting between 42 hours (Io) and just under 17 days (Callisto). They are comparatively close to the giant planet, which means that enormous tidal forces are exerted on the moons by Jupiter during their orbits. These forces are particularly strong on the inner moons Io and Europa, but they are also clearly detectable further out on Ganymede and Callisto. These forces lead to a periodic deformation of the surface, which can be detected with suitable instruments. This is exactly what GALA will do for Ganymede, the largest moon in the Solar System.
So, what is the benefit of measuring this tidal deformation? To explain this, we have to go a little further. The moons Europa, Ganymede and Callisto contain a lot of ice, which is the water ice we are all familiar with. In fact, water is a very common constituent of bodies in the outer Solar System – beyond the asteroid belt between Mars and Jupiter. Temperatures were very low in the Jupiter system when the moons formed approximately 4.5 billion years ago, so water could be 'incorporated' into the moons as a solid constituent during their formation, a process referred to as accretion, without evaporating beforehand. In fact, the large icy moons Ganymede and Callisto contain about 50 percent ice in addition to their rock content; most of this is found in the outer layers.
Model calculations of the internal structure and thermal evolution predict that there are global liquid oceans beneath the outer ice layers. Each of these oceans on Europa, Ganymede and Callisto would contain more water than all the oceans on Earth combined, according to typical model calculations.
There is also evidence from previous observations that suggests the existence of the oceans. They could represent areas in the outer Solar System that are favourable for the development of life. Although this requires more than just water, in particular the right chemistry, an essential prerequisite would have been in place for a long time. The task now is to detect these oceans and determine their physical properties.
Tidal measurement enters the picture
The periodic deformation of the surface is much more pronounced (up to seven metres for Ganymede) in the case of an ocean beneath the ice surface. Without an ocean, the tidal amplitude would only be approximately 10 centimetres – and thus hardly detectable. The aim is to use GALA to detect this periodic deformation of the surface of several metres and, in interaction with other instruments, to determine properties such as the density and extent of the ocean and the overlying ice layer.
In addition to detecting the ocean on Ganymede, another main goal of GALA is to determine the surface topography both globally and locally. This is a common task for laser altimetry and an important basis for interpreting the geological and geophysical processes at the surface, which have produced unique surface formations on Ganymede.
Ganymede has undergone very intense phases of development in its past, characterised by large energy transfers and processes that were most likely also driven by tidal effects. GALA will 'scan' the surface using approximately 600 million individual laser shots and thus – especially in combination with image data from the JANUS camera (which the DLR Institute of Planetary Research also played a major role in developing) – create a highly accurate 3D model of the surface.
The GALA instrument, like all the other nine experiments, has been integrated into the JUICE spacecraft at Europe's Spaceport in Kourou, French Guiana, for some time. We are eagerly awaiting the launch, as I am sure all teams are, with the launch window open from 5 to 30 April 2023. An Ariane 5 rocket will launch JUICE on its interplanetary mission and eight-year journey to Jupiter. This will require three Earth flybys and even a flyby of Venus to obtain the necessary velocity to reach Jupiter. As Principal Investigator (PI), I am responsible for the development, operation and scientific data evaluation of GALA. This can only succeed through excellent teamwork.
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