  Furrows are a concentric system of tectonic troughs, and are the oldest
recognizable surface feature on Ganymede. We analyzed the distribution of
furrows utilizing Voyager and Galileo images and found that furrows over
Ganymede's surface are part of a global concentric circular structure. If this
multi-ring structure is impact origin, this is the largest impact structure
identified so far in the solar system. Deviations of the shapes of the furrows
from the concentricity are small everywhere, which implies that the relative
location of the blocks of the dark terrains over the entire surface of Ganymede
has not changed appreciably even during formation of the bright terrains. The
estimate of the impactor size is difficult, but an 150km-radius impactor is
consistent with the observed properties of furrows. The furrow-forming impact
should have significant effects on the satellite's geological and internal
evolution, which are expected to be confirmed by future explorations of
Jupiter's icy moons, such as the JUICE (Jupiter Icy moon Explorer) or Europa
Clipper mission.
