Venus

Venus has no natural satellites and consequently its mass has only been determined through the gravitational effect that the planet has on passing space probes. A mass equivalent to 82% that of Earth's, or 4.9x10²⁴ kg, has been found for Venus. The bulk density of Venus is 5240 kg/m³, slightly smaller than that of Earth's.

The similarity between the mass, radius, and bulk density of Venus and Earth suggests that the two planets have similar internal structure. Venus most probably has, therefore, a thin rocky crust, a large iron- and magnesium-silicate mantle, and an inner nickel-iron alloy core (~ 25% of the planet's mass).

One Venusian anomaly that defies present-day theory relates to the planet's magnetic field, or more correctly to the complete lack of any detectable magnetic field. It is believed that the earth's magnetic field is created by a dynamo effect that operates in its hot, liquid nickel-iron alloy core. If, as has been previously argued, Venus has an internal structure similar to that of Earth, why does it not have a similar magnetic field? The answer may lie with the slow Venusian rotation rate. One of the key ingredients of the dynamo theory is that the conducting, liquid core is rotating. Since Venus rotates much more slowly than Earth, by a multiplicative factor of 1/243, it may be that the dynamo effect cannot operate in the planet's core.