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Two of the basic properties of Solar System objects are mass and density. Mass is a measure of the amount of matter in a particle or object. The standard unit of mass in the International System (SI) is the kilogram (kg). This is usually determined by measuring the object's gravitational influence on other objects, e.g. natural or artificial satellites.

Once the volume of an object is known, its bulk density can be calculated. In this book, density is usually expressed in grams per cubic centimeter (g/cm³). As a guide, the density of water is 1.0 g/cm³. Objects which have a density lower than water are able to float (assuming enough water is available!).

If a planet has a high density, it means that it is largely made of dense, rocky, or metallic materials. Objects often have low densities because they contain a lot of gases or ices, but few rocky materials. This is why all of the giant planets in the Solar System have low densities, despite their huge size.

The planet with the lowest density (0.7 g/cm³) is Saturn. The reason that Saturn has such a low density is that it is mainly composed of gas, particularly hydrogen and helium. There is only a small rocky core at its center.

Other objects, including many small satellites and asteroids, have low densities because they are piles of loosely consolidated rubble or highly porous, i.e. they contain numerous empty spaces.

The densities of planets are also a reflection of their size and the layering of their interiors. Earth has the highest density of all the planets in the Solar System because it is made of dense, rocky materials. At the surface, crustal rocks have densities between 2.5 and 3.5 g/cm³. However, Earth's average density is much higher (5.5 g/cm³).

This is partly because the denser elements, such as iron and nickel, have sunk to the center of the planet, while the less dense materials have risen to the surface. Many planets were internally differentiated in this way early in their lives.

The centers of the planets are also more compressed by the weight of the overlying material. In the case of Earth, for example, the normal, uncompressed density of its rocks is about 4.4 g/cm³, but the central core is compressed to greater than normal density by the overlying layers.

More massive planets should experience greater compression at their centers, and hence higher average densities, if they are made of the same rocky and metallic materials as Earth. The opposite should apply to smaller planets. However, the smallest of the rocky planets, Mercury, actually has an average density of 5.4 g/cm³, only slightly lower than Earth's.

Mercury's density rises to a remarkable 5.3 g/cm³ after it has been corrected for the effects of internal compression – much higher than Earth's. The only way to explain this is to assume that the little planet has a huge core of iron and nickel that takes up almost half of its interior (see Chapter 5).

Any theory must also account for the fact that Jupiter and Saturn are huge hydrogen–helium planets, whereas Uranus and Neptune are notably smaller and contain sizeable amounts of elements that form ices: oxygen, carbon, and nitrogen. If the latter pair began as icy nuclei, they must have grown quite slowly in the more rarefied conditions of the presolar nebula beyond about 15 AU. By the time they were massive enough to draw in large amounts of gas, the nebula had dissipated, and the supply was cut off.