The reason why I mentioned slamming Europa into Mars is not only that Mars needs more water and atmosphere but also to add the heat of the collision and the mass of Europa's core. I suspect that the reason why the Earth has such a hot core and good magnetic field lies with the Mars-sized body that slammed into the early Earth creating the Moon. While a lot of the light material flew off to form the Moon, a lot of the heavier material sank into the Earth's core, not to mention the energy released by the collision. Basically, it liquified and reformed the Earth with a heavier core and probably more water and atmosphere.