The South Pole-Aitken (SPA) impact basin, located on the far side of the Moon, is about 8 km deep and appears to be in isostatic balance. Describe how you would use the principle of isostasy to compute the thickness of the crust surrounding the basin, under the assumption that the basin-forming impact excavated all the way to the mantle. For crustal and mantle densities of 2800 kg/m³ and 3400 kg/m³, respectively, compute the crustal thickness and discuss the reliability of this estimate.