Ganymede, Europa and Callisto are all believed to have oceans of liquid water under their icy surfaces. But why don’t those oceans freeze? Where is the heat coming from? The answer is tidal forces from Jupiter.
We all know about tides on Earth. Each day, the ocean level goes up and down with about 6 hours time difference between high tide and low tide. The cause is the gravitational pull from the Moon. It pulls the ocean water away from the Earth directly under the Moon, and it pulls the Earth away from the ocean water on the opposite side. The result is a stretching of the oceans along the direction to the Moon. The Figure will help you understand.
Jupiter has the same effect on its moons: it stretches them. As a moon rotates, the stretch travels along the surface and sort of massages sphere, warming it up from the inside. This is where the heat to keep the oceans liquid comes from. This is the theory, but we want to know more detail to confirm it. We need to study this stretching effect, and so JUICE will measure it as carefully as possible on the moon Ganymede.
The instrument will hit the surface of Ganymede with about 600 million individual laser shots to ‘scan’ its surface and measure the elevation of the stretched regions with great accuracy. To build an extremely accurate 3D picture of the surface of Ganymede, the GALA data will be combined with image data from the JANUS camera.
Comparable altimetry technology, generally called ranging, is used on Earth to measure the height of trees and of the water in oceans and lakes (see Earth application links below). Besides laser altimetry (LiDAR), other satellites use RADAR altimetry, where radio waves are used instead of lasers. Altimetry can also be used to detect small vertical shifts of terrains and even human-made infrastructure such as buildings and bridges, which may indicate danger of collapse. Some of our ESA BIC startups (e.g. Spotlite) have turned this technique into a successful business.