Double "Concentric" Basin on Mercury (Natural Colors; credits: Lunexit)

Caption NASA, da "NASA - Picture of the Day" del giorno 7 Ottobre 2009:"What created the internal second ring of this Double Ringed Basin on Mercury?
No one is sure.
The unusual feature spans approx. 160 Km and was imaged during the robotic MESSENGER Spacecraft's swing past our Solar System's innermost planet. Double and Multiple Ringed Basins, although rare, have also been imaged in years past on Mars, Venus, Earth and Earth's Moon.
Mercury itself has several doubles, including huge Caloris Basin, Rembrandt Basin and enigmatic Raditladi Basin. Most large circular features on planets and moons are caused initially by a forceful impact by a single asteroid or comet fragment. Since it is unlikely that a second impact would occur right in the center of the first, large double rings are usually attributed to a subsequent volcanic lava flow inside the impact crater.
Possibly, though, a second ring could be caused by the melting and flowing of material upon impact. One clue to the origin of the above-imaged double ring is that the basin center appears much smoother than the region between the rings. MESSENGER has now completed its last flyby of Mercury but will return and attempt to enter orbit in March 2011".