Density is a fundamental physical property that describes how much mass is contained within a specific volume. It is often expressed in grams per cubic centimeter (\(\text{g/cm}^3\)), representing the compactness of a substance. The average density of a planet is calculated by dividing its total mass by its total volume. This concept helps scientists understand the internal composition and structure of celestial bodies. Surprisingly, while most planets are made of dense rock or highly compressed gases, one planet in our solar system possesses an overall density lower than water.
Saturn: The Planet Less Dense Than Water
The only planet in our solar system that has an average density lower than water is Saturn, the sixth planet from the Sun. Water has a known density of approximately \(1.0 \text{ g/cm}^3\). In contrast, Saturn’s mean density is remarkably low, measured at about \(0.687 \text{ g/cm}^3\). This means that, hypothetically, if an ocean large enough to hold the giant planet existed, Saturn would float on the surface.
The Gaseous Structure of Saturn
Saturn’s exceptionally low density is a direct result of its enormous size paired with its physical makeup. As a gas giant, the planet is overwhelmingly composed of the lightest elements available in the universe: hydrogen and helium. Hydrogen makes up the vast majority of the planet’s mass, which is distributed across an immense volume. This large volume dilutes the overall density, despite the planet’s considerable total mass.
The planet’s interior is organized into distinct layers that transition under increasing pressure. The outermost layer is a thick atmosphere of molecular hydrogen gas and helium, containing trace amounts of ices like ammonia and water. Moving inward, the intense pressure compresses the hydrogen into a liquid state, forming a deep layer of liquid molecular hydrogen. Deeper still, the pressure becomes so extreme that hydrogen enters a state known as liquid metallic hydrogen.
In this metallic state, hydrogen atoms lose their electrons, allowing the material to conduct electricity and heat like a metal. At the very center of the planet is a small, dense core composed of rock, silicates, and ices. This core is estimated to be between 9 and 22 times the mass of Earth, yet it is not massive enough to significantly raise the planet’s average density above that of water.
Comparing Saturn’s Density to Other Celestial Bodies
Saturn’s density provides a stark contrast when compared to the rocky terrestrial planets in the inner solar system. Earth, for example, is the densest planet, with a mean density of approximately \(5.514 \text{ g/cm}^3\). This high value reflects Earth’s composition of heavy elements like iron and silicates, which form its substantial core and mantle. The difference highlights the fundamental distinction between the small, rocky worlds and the large, gaseous outer planets.
Even among the other gas giants, Saturn remains an outlier. Jupiter, the solar system’s most massive planet, has a mean density of about \(1.326 \text{ g/cm}^3\), making it denser than water. Although Jupiter shares a similar composition of hydrogen and helium with Saturn, its significantly greater mass generates a much stronger gravitational force. This stronger gravity compresses Jupiter’s interior materials more tightly, packing more mass into a smaller relative volume and resulting in a higher overall density than its ringed sibling.
Uranus and Neptune, the ice giants, also have densities higher than water, measuring around \(1.27 \text{ g/cm}^3\) and \(1.64 \text{ g/cm}^3\), respectively. Their higher densities compared to Saturn are attributed to a greater proportion of “ices,” which include water, methane, and ammonia, as well as heavier elements, relative to their total hydrogen and helium content.