That Must’ve Hurt: Ganymede Covered by Giant Crater

Researchers from Kobe University and the National Institute of Technology, Oshima College have investigated the orientation and distribution of the ancient tectonic troughs on Jupiter’s moon Ganymede through a detailed reanalysis of image data from probe missions. They discovered that these troughs are distributed concentrically across almost the entire surface of Ganymede, indicating that these troughs may be part of one giant crater covering Ganymede. If so, this is the largest impact structure identified in the Solar System so far.

Figure 1: Jupiter (background, left) and its moon Ganymede (foreground, right) visualized using the Four-Dimensional Digital Universe viewer “Mitaka.” The furrows found on the older, Dark Terrain, areas of Ganymede’s surface could actually be parts of a single, giant, multiring impact crater. (Credit: Tsunehiko Kato, 4D2U Project, NAOJ)
Download: [PNG (5.58 MB)]

Many furrows, or trough formations, have been observed on the surface of Ganymede, one of the Jovian moons. This research group comprehensively reanalyzed image data of Ganymede obtained by NASA’s Voyager 1, Voyager 2, and Galileo spacecrafts. The results revealed that almost all of these furrows appear to be arranged in concentric rings centered around a single point, indicating that this global multiring structure may be the remains of a giant crater. The radial extent of the multiring structures measured along Ganymede’s surface is 7800 km. For comparison, the mean circumference of Ganymede is only 16,530 km. If correct, this is the largest crater yet identified in the Solar System. The previous record holder with a 1900 km radius is on Calisto, another Jovian moon.

Figure 2: Images of Ganymede’s surface taken by Voyager 2 (left) and Galileo (right). The Dark Terrain and Bright Terrain areas can be recognized, with concurrent furrows present in the Dark Terrains. (Image credit: NASA)
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Figure 3: (Top) Azimuthal equidistant map centered at 20° south 180° west showing Ganymede’s Dark Terrain and furrows (indicated by yellow lines). (Bottom) Azimuthal equidistant map of Ganymede’s surface centered at 20° north and 0° west. This shows the opposite hemisphere of Ganymede from the top image. The white areas indicate Bright Terrain. (Image credit: NASA)
Download: [PNG (1.22 MB)]

Based on a computer simulation conducted using “PC Cluster” at the National Astronomical Observatory of Japan (NAOJ), the team speculates that Ganymede’s giant crater could have resulted from the impact of an asteroid with a radius of 150 km traveling 20 km/s.

Lead researcher Naoyuki Hirata comments, “The European Space Agency’s JUICE (Jupiter Icy Moon Explorer) mission, scheduled to launch in 2022 and arrive in 2029, will examine Jupiter and its moons, including Ganymede, with instruments such as the GAnymede Laser Altimeter (GALA) which NAOJ is helping to develop and imaging spectrographs. GALA is being developed mainly by the German Aerospace Center in collaboration with institutes in Switzerland, Spain, and Japan, including JAXA, Chiba Institute of Technology, Osaka University, and NAOJ. We hope that JUICE will confirm the results of this study and further advance our understanding of the formation and evolution of Jupiter’s moons.”

Figure 4 & Movie: Impact simulation of an asteroid with a 150 km radius colliding into Ganymede at 20 km/s. A 1000 km radius of Ganymede's surface is melted by the violent impact, and the melted material spreads out around it. In the center of the multiring crater found on Ganymede in this study, there is an area where furrows are not conspicuous. This is thought to be the part of Ganymede’s surface that was melted by the collision that produced the furrows. Its size is in good agreement with this simulation. This simulation was conducted by Ryu Suetsugu at the National Institute of Technology, Oshima College. The sharp vertical accumulation of material along the vertical axis at 12,000 seconds is likely a numerical artifact caused by the boundary conditions in the simulation, but the research team confirmed that this does not affect the main results of this study. (Credit: Hirata et al., 2020)
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These research results were published online as Hirata, N., Suetsugu, R., Ohtsuki, K.,“A global system of furrows on Ganymede indicative of their creation in a single impact event” by Icarus on July 15, 2020.

(August 13, 2020 press release.)

[Research Paper]

Title: "A global system of furrows on Ganymede indicative of their creation in a single impact event"
Journal: Icarus
Author: Naoyuki Hirata, Ryo Suetsugu, Keiji Ohtsuki
DOI: 10.1016/j.icarus.2020.113941

[Computers Used in This Research]

The numerical simulations performed in this study were calculated using the “PC Cluster” operated at the Center for Computational Astrophysics (CfCA), National Astronomical Observatory of Japan (NAOJ). This cluster is mainly used for small-scale, non-parallel numerical simulations which require a long calculation time with a large number of initial conditions. Currently, the cluster is composed of 1344 CPU cores in total. (Photo Credit: NAOJ)

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