Over four centuries after Galileo Galilei first observed Ganymede in 1610, scientists have pieced together a visual record that tells a story of geological evolution marked by impacts, tectonics, and surface reshaping.

In a significant milestone for planetary science, a team of researchers led by Geoffrey Collins from Wheaton College has produced the first comprehensive global geologic map of Ganymede, the largest moon in the solar system. This map, a collaborative effort using images from NASA’s Voyager and Galileo missions, offers an unprecedented look at the icy world orbiting Jupiter, revealing its complex and varied surface.

Ganymede stands out not only for its size—it is larger than the planet Mercury—but also for its complex geological makeup. The surface of Ganymede is characterized by stark contrasts between its two main terrain types. The darker regions are heavily cratered and date back to ancient times, while the lighter areas, crisscrossed with grooves and ridges, are somewhat younger but still ancient by Earth’s standards.

The map is more than just a pretty picture; it is a critical tool that helps scientists decode the moon’s dynamic history. The details captured include everything from ancient, cratered regions to younger terrains marked by grooves and ridges. These features allow researchers to outline Ganymede’s geologic periods, providing insights into how this celestial body has changed over billions of years.

“This map illustrates the incredible variety of geological features on Ganymede and helps to make order from the apparent chaos of its complex surface,” said Robert Pappalardo of NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “This map is helping planetary scientists to decipher the evolution of this icy world and will aid in upcoming spacecraft observations.”

The detailed imagery from the Galileo spacecraft, in particular, provided evidence against the hypothesis that cryovolcanism—volcanoes that erupt water and ice—played a significant role in shaping Ganymede’s landscape. Instead, it appears that tectonic forces were more dominant in sculpting the moon’s surface.

“The highly detailed, colorful map confirmed a number of outstanding scientific hypotheses regarding Ganymede’s geologic history, and also disproved others,” said Baerbel Lucchitta, scientist emeritus at the U.S. Geological Survey in Flagstaff, Ariz., who has been involved with geologic mapping of Ganymede since 1980. “For example, the more detailed Galileo images showed that cryovolcanism, or the creation of volcanoes that erupt water and ice, is very rare on Ganymede.”

In addition to the broader geological context, detailed studies have also focused on Ganymede’s impact features. Recent research has mapped 19 large impact structures on Ganymede and its neighboring moon Callisto, showcasing a variety of crater types, including traditional impact craters and more unusual features like penepalimpsests and palimpsests — large, flat impact features on icy moons, formed by impacts that penetrated the ice shell, revealing older, underlying materials and often characterized by concentric ridges.

These impact features are not uniform; their morphology varies widely based on their size and the conditions under which they formed. For instance, some craters exhibit central pits and raised rings, while others have flat, smooth floors. The differences arise from the varying conditions during impact, such as the size of the impactor, the speed at which it struck, and the thermal state of the moon’s icy crust at the time.

In about a decade, the Jupiter Icy Moons Explorer (Juice) will revolutionize our understanding of Ganymede and its fellow Jovian moons. Juice will provide unprecedented close-up observations of its surface, subsurface, and magnetic environment, building on the foundation laid by the new global geologic map. By studying Ganymede’s unique geology and ice-covered landscape, Juice will help unravel the moon’s evolutionary history, probe for signs of subsurface oceans, and explore its potential for habitability. This mission represents not just a journey to a distant moon, but a quest to answer fundamental questions about the origins and evolution of our solar system, offering the tantalizing possibility of discovering the conditions for life beyond Earth.