Earth is significantly denser than Jupiter. While Jupiter is the largest and most massive planet in the Solar System, its size does not translate to a high average density. Earth’s average density is approximately \(5.51 \text{ g/cm}^3\), the highest of any planet orbiting the Sun. In contrast, Jupiter has an average density of only about \(1.33 \text{ g/cm}^3\), making it only slightly denser than water. This means a handful of Earth material would be over four times heavier than the same volume of Jupiter material. The vast difference is due to the fundamental composition and internal structure of the two planetary types.
Understanding Density in Planetary Science
Density is a physical property defined as the amount of mass contained within a specific volume. It measures how tightly matter is packed together inside an object. When comparing planets, density is calculated by dividing the planet’s total mass by its total volume.
This average density figure is a diagnostic tool for planetary scientists, offering insights into a planet’s bulk composition. A high average density suggests a body is made of heavy, compressed materials like rock and metal. Conversely, a low average density points to a composition dominated by lighter elements, such as hydrogen and helium gas.
To illustrate this concept, consider the difference between a bowling ball and a beach ball of the same size. The bowling ball has a much higher density because its mass is concentrated into a solid, heavy form, whereas the beach ball contains mostly air. Applying this logic allows researchers to infer the unseen internal structures of distant worlds.
The High Density of Earth’s Rocky Interior
Earth’s high average density results directly from its formation and planetary differentiation. Early in its history, the planet was largely molten, allowing heavier elements to sink toward the center under gravity. This process created a distinct layering of materials based on density.
The result is a dense, metallic core composed primarily of iron and nickel, making up roughly 31% of the planet’s total mass. This central reservoir of heavy elements is the largest contributor to Earth’s high density. Surrounding the core is the mantle, consisting of silicate rock material, and the outermost layer is the less dense crust.
Immense pressure deep within the interior also drives up Earth’s density. The crushing weight of the overlying layers compresses materials, particularly in the inner core, to densities up to \(13.0 \text{ g/cm}^3\). Even silicate rocks in the lower mantle are compressed to a density higher than surface basalt.
Why Jupiter’s Gaseous Nature Leads to Lower Density
Jupiter’s low average density, despite its enormous mass, is a consequence of its composition as a gas giant. The planet is composed overwhelmingly of the two lightest elements, hydrogen and helium, in proportions similar to the Sun. Because these elements occupy a vast amount of space, Jupiter has an immense volume that dilutes its overall mass when calculating average density.
Moving inward from the cloud tops, Jupiter’s gaseous outer layers transition into more exotic states of matter due to increasing pressure and temperature. The hydrogen gas compresses into a liquid state, forming an enormous layer of liquid molecular hydrogen. This deep layer constitutes a massive ocean of hydrogen, which remains much less dense than Earth’s solid rock and metal.
Deeper still, the pressure becomes so extreme that electrons are stripped away from the hydrogen atoms. This creates liquid metallic hydrogen, which conducts electricity like a metal and is the source of Jupiter’s powerful magnetic field. Even though this metallic hydrogen is incredibly compressed, the light nature of the hydrogen atoms ensures Jupiter’s average density remains low.