The composition of Venus and Mercury is similar, they're both terrestrial planets made of rock and metal. Mercury is more dense than Venus and thought to consist of 60-70% metal, with the rest rock. As mentioned above, Mercury lacks an atmosphere, while Venus has the thickest atmosphere of all the terrestrial planets
Further to this the rotation is very slow: a sidereal day on Venus lasts 243 Earth days. This is even longer than a Venusian year which is only 224.7 Earth days.
