On Mon, 10 Dec 2012 20:17:20 +0100, Szczepan Bialek wrote:

The density of the Earth is 5.52

The Mars has 3.95 and The Moon 3.34

Is the density the diameter dependent?

No, density is mass per unit volume.

The Earth, Moon and Mars are simmilar and are close to each other.

The differences are rather big. So should be a lot of theories to
explain this phenomenon. Are such theories?

Donno what exists out there but the obvious one goes back to the
formation of these objects. The smaller ones have a lower gravitational
field so stuff being attracted won't hit as hard as stuff attracted toa
larger object. The objects themselves will also have less tendancy to
collaspe onto themselves. Thus the smaller objects will tend to be less
dense.

--
Cheers
Dave.