If 水星是剝掉外殼的內核, then its density should be much bigger, like the Earth's core has much bigger density to balance earth's light crust (2.7-2.9). Earth's 5.5 density is due to the moon (density = 3.3) which removed light part of out crust of the earth, so earth has a bigger density (5.5). Generally the closer to the sun the bigger density.