@tony873004: You are right. What I said is true for massless particles, but I guess Theia was far from massless...

@aurora: Yes, but even when the moon (mass = 1.23e-02 Earth masses) was pure mantle material, and the impactor had a core / mantle mass ratio of ~1:1 (as the Earth has, in a very rough approximation), then the mass of the core of the Earth is only increased by 1.23% compared to what the situation would be if the moon had the same core / mantle mass ratio as the Earth. Instead of a core / mantle mass ratio of ~1:1, the Earth would have a ratio of ~1:1.01. That's what I meant when I said it doesn't make any difference.