Worlds made of ice
How cameras and lasers map Jupiter’s moons
JUICE is prepared for launch aboard an Ariane 5 rocket, which will send it on its journey towards the Jupiter system © ESA
Staying power: Ganna Portyankina and Hauke Hußmann investigate Jupiter’s moons from DLR in Adlershof © WISTA Management GmbH
Far out in the solar system, some 700 million kilometres from Earth, the gas giant Jupiter reigns over an entire army of moons. These are worlds of ice, scarred by kilometre-long cracks and chasms, pockmarked with craters, and streaked with bright lines that look like gaping scars on the surface. The largest moons bear names from ancient Greek mythology—Ganymede, Europa, and Callisto. Beneath their massive icy shells lie oceans that likely contain more water than all five of Earth’s oceans combined.
Exploring these alien worlds is the mission of JUICE, a space probe from the European Space Agency (ESA). JUICE stands for Jupiter Icy Moons Explorer. Roughly the size of a small garden shed, it has been travelling through space since April 2023 and is scheduled to arrive at its destination in 2031. Onboard is a small treasure trove of scientific instruments designed to unlock the secrets of these icy moons.
“The camera is one of the most important instruments on any mission—if not the most important,” says Ganna Portyankina from the Planetary Geology Department at the German Aerospace Center (DLR) in Adlershof. “It is the eye of the mission.” For JUICE, that eye is called JANUS, and Portyankina serves as its co-leader. However, referring to it as just a camera doesn’t do the the instrument any justice. JANUS is a precise ensemble of lenses, filters, ultra-sensitive sensors, and finely tuned mechanics—engineered to operate flawlessly in the far reaches of the solar system. Once JUICE reaches its destination, JANUS will map the surfaces of Ganymede, Europa, and Callisto in unprecedented detail.
To do so, the camera system features twelve different filters. Besides seeing in black and white, they provide detailed information about colour and material composition. “We want to understand how these moons are built and how they have changed over time,” explains the physicist, who has been involved with studying icy celestial surfaces since her doctoral thesis. For her, the deep craters, bright plains, and enigmatic streaks—spanning entire continents like tectonic seams—are more than just stunning images. They reveal the past activity of the ice crusts—and maybe even their present dynamics. Any brightly coloured ejection from a crater, any jagged edge, or subtly displaced block of frozen water tells a story about what is happening beneath the surface: Are there warmer layers rising? Is old material breaking apart? Or are new ice sheets pushing towards the surface?
While JANUS captures photographic images of the moons in the light of the distant Sun, a second instrument relies on laser beams: the GALA altimeter, which was also developed by DLR in Adlershof. GALA sends laser pulses toward the surface thirty time per second, which bounce back and are then re-captured by the instrument. A precision clock measures the probe’s distance from the ground by timing the signal.
In this way, millions of such measurements are used to create a three-dimensional map of valleys, hills, cracks, and deformed ice layers. “We want to determine the topography of these moons as accurately as possible,” says Hauke Hußmann, who is the scientific director of the experiment. For him, this is not merely a question of technical elegance: The shape of the surface reveals the internal structure of the moon.
He is particularly interested in tidal deformations on Ganymede. Just as our Moon affects Earth’s oceans as it passes by, Jupiter tugs at its moons. Since water responds far more readily to such forces than solid matter, the displacement of a global subsurface ocean can reach up to eight metres; a rigid ice crust would deform by only a few centimetres. “If the measurements from GALA show metre-scale shifts, it would provide further confirmation of a massive ocean beneath the ice,” says Hußmann. Technically, GALA is operating at the limits of what is possible in the Jupiter system. The extreme radiation environment demands robust lasers and highly sensitive detectors capable of reliably capturing even the faintest return signals. At the same time, energy efficiency is paramount. The sunlight reaching the probe’s solar panels is twenty-five times weaker than near Earth.
Together, JANUS and GALA are creating an image of these far-away worlds that will surpass anything we have seen. They might reveal just how lively Jupiter’s icy moons truly are.
Kai Dürfeld for Adlershof Journal