Saturn is much less dense than Jupiter because Saturn has much less mass than Jupiter, but it is only slightly smaller in terms of volume. In fact, Saturn is only about a third as dense as Jupiter is.
Saturn is not only less dense than Jupiter; it is less dense than all of the other planets. In fact, the density of Saturn is approximately 0.687 grams per cubic centimeter. This is less dense than fresh water, meaning that Saturn would float on water. It is important to note that the density of a planet is not the same at all locations — some parts of Saturn are denser than others are, just like the density on Earth varies. Accordingly, when scientists describe the density of a planet, they are referring to the average density of the body.
Jupiter and Saturn are called gas giants because they are very different in composition relative to the terrestrial planets that orbit closer to the sun. These planets lack a solid surface and a clear distinction between the surface and the atmosphere. Uranus and Neptune are also gaseous planets, but they are sometimes called ice giants, as they comprised largely of methane, water and ammonia ices.