The Hellas Basin is the lowest formation on Mars -- parts ~ 9km below. Hellas has long been interpreted as an ancient impact structure, the largest on Mars, and probably the largest in the Solar System. All well and good. However, other large impact structures such as the Caloris Basin on Mercury and Mare Orientale on the Moon are distinctly different. For one thing, they are only a few km lower than the surrounding surface, apparently due to infill and rebound. How has Hellas managed to stay unfilled for so many billions of years? Why was there no substantial rebound?

Or am I missing something here?

Cheers, BFelix