Ganymede’s Surface Scarred by Colossal Asteroid Impact

In a groundbreaking study, scientists have revealed that Ganymede, the largest moon in our solar system, was struck by an asteroid far larger than previously thought. This colossal impact, estimated to have occurred around four billion years ago, was so immense that it caused a shift in Ganymede’s rotational axis. The marks of this ancient collision are still visible on the moon’s surface today.

Ganymede stands out as a celestial giant. With a diameter of approximately 3,273 miles (5,268 kilometers), it is larger than Mercury and only slightly smaller than Mars. As the largest moon and the ninth-largest object in the solar system, Ganymede surpasses the eight planets in size. It is unique not only for its size but also because it is the only moon known to possess both an atmosphere and a magnetic field. Among Jupiter’s four large Galilean moons—Europa, Callisto, Io, and Ganymede—this moon’s surface is especially distinctive. It is characterized by extensive furrows that create concentric circles around a particular area. These features have intrigued scientists for years, leading to the hypothesis that they were formed by an ancient asteroid impact.

Initial theories suggested that Ganymede’s grooved and pockmarked surface might have been caused by an asteroid up to 90 miles (150 kilometers) in diameter. However, new calculations have dramatically revised this estimate. According to Hirate Naoyuki, a planetologist at Kobe University and the lead author of a paper recently published in Scientific Reports, the asteroid responsible for Ganymede’s furrows was likely about 186 miles (300 kilometers) in diameter. This new estimate reveals that the impactor was approximately 20 times larger than the asteroid that created the Chicxulub crater on Earth. The Chicxulub event, which occurred 66 million years ago, led to the extinction of the dinosaurs and caused a global impact winter lasting several decades.

Hirate, an expert in simulating cosmic impacts, explains that the colossal asteroid that struck Ganymede not only created the observable surface features but also significantly altered the moon’s rotational axis. This shift has left a lasting imprint that scientists continue to study. The implications of this finding extend beyond Ganymede. The newly estimated size of the impactor provides insight into the scale of collisions in the early solar system. It underscores the significant role such impacts may have played in shaping the surfaces and evolutionary paths of planetary bodies.

Ganymede’s intriguing characteristics extend beyond its surface features. Observations by NASA’s Hubble Space Telescope have detected a thin oxygen atmosphere around the moon, which is believed to originate from its icy surface. In addition to the oxygen atmosphere, a 2021 study using Hubble data revealed the presence of water vapor in Ganymede’s atmosphere. These findings suggest that Ganymede harbors an underground ocean beneath its icy crust, adding another layer of complexity to its already remarkable profile.

NASA’s Juno spacecraft provided crucial data during its brief flyby of Ganymede in 2021. The spacecraft captured images of the moon’s surface, including its pockmarked and grooved terrain, which offer valuable clues about the impact event that occurred billions of years ago. Preliminary analysis from Juno indicates the presence of mineral salts and organic compounds on Ganymede’s surface, further enriching our understanding of this distant moon.

Looking ahead, the European Space Agency’s JUpiter ICy moons Explorer (JUICE) spacecraft is en route to Jupiter. Scheduled to enter orbit around Ganymede in 2034, JUICE will embark on a six-month mission to study the moon in detail. This will mark the first dedicated mission to Ganymede, providing unprecedented insights into its composition, atmosphere, and the legacy of the ancient impact.

The new evidence of a massive asteroid impact on Ganymede offers a dramatic revision of our understanding of the moon’s history. By significantly increasing the estimated size of the impactor, scientists have highlighted the scale and significance of cosmic collisions in shaping celestial bodies. As ongoing and future missions continue to explore Ganymede, they will undoubtedly uncover more about this enigmatic moon’s past and its role in the broader context of our solar system’s formation.