Would Saturn float? The truth about its density
Saturn's average density is 0.687 g/cm³, about 70% that of water, which makes it the only planet in the solar system less dense than water. That is the origin of the famous claim that Saturn "would float in a bathtub big enough to hold it". The arithmetic is right, but the picture is misleading: Saturn is a ball of gas and fluid with a dense core, and in any real ocean it would mix, deform and collapse rather than bob like a cork.
Contents
Why Saturn is less dense than water
Density is mass divided by volume. Saturn is 95 times as massive as Earth but occupies about 764 times the volume (see size and mass), so each cubic meter holds on average only 687 kg of material.
Three factors produce this low figure:
- Composition. Saturn is mostly hydrogen and helium, the two lightest elements. Rock and ice make up only a modest fraction of its mass, concentrated deep inside.
- Not enough mass to squeeze itself. Jupiter has 3.3 times Saturn's mass and is made of nearly the same ingredients, yet it is only slightly larger. Its stronger gravity compresses the hydrogen much more, giving it almost twice Saturn's density (1.33 g/cm³). Saturn sits in a middle ground: big enough to be puffy, not massive enough to compress itself as hard.
- Internal heat. Saturn still radiates heat from its interior (see temperature), which keeps its gas slightly expanded.
Its fast spin adds a visible signature: Saturn is the most flattened planet, with an equatorial radius of 60,268 km against 54,364 km at the poles, partly because such low-density material bulges easily.
The density is not uniform
0.687 g/cm³ is an average over the whole planet. The real distribution is very different:
| Region | Approximate density |
|---|---|
| Upper atmosphere at the 1-bar level | about 0.0002 g/cm³ (roughly a sixth of Earth's air at sea level) |
| Molecular hydrogen envelope | rises gradually with depth |
| Metallic hydrogen layer | roughly 0.5–1 g/cm³ and more |
| Deep diffuse core of rock, ice and hydrogen | several g/cm³ |
Ring seismology from Cassini data suggests Saturn's core is not a compact ball but a "fuzzy" region where heavy elements are mixed with hydrogen over a large fraction of the radius (see interior). So the planet has a heavy center and an extremely light outer envelope; only the average lands below water.
Would Saturn really float in a bathtub?
The bathtub image works as a memory aid, not as physics. Here is what goes wrong if you take it literally:
- There is no surface to hold together. Saturn has no solid crust. Its outer layers are gas that would simply mix with the water, or boil it, on contact.
- Gravity changes everything. An "ocean" big enough to hold Saturn would weigh more than Saturn itself. Such a mass of water would collapse under its own gravity into a planet-sized body, and the two objects would attract each other and merge rather than one floating on the other.
- Pressure inverts the comparison. Water deep inside such a body would be compressed far above 1 g/cm³, while Saturn's heavy core is denser than water to begin with. A dense core sinks.
- Buoyancy needs a gravitational field from outside. Archimedes' principle assumes a uniform gravity, like Earth's in a pool. At planetary scales, the bodies themselves are the source of gravity.
The honest version is: Saturn's average density is lower than that of liquid water at Earth's surface. That is a meaningful and surprising fact; floating is a figure of speech.
Saturn compared with other worlds
| Body | Mean density (g/cm³) |
|---|---|
| Earth | 5.51 (densest planet) |
| Mercury | 5.43 |
| Venus | 5.24 |
| Mars | 3.93 |
| Jupiter | 1.33 |
| Uranus | 1.27 |
| Neptune | 1.64 |
| Saturn | 0.687 |
| Titan | 1.88 |
| Sun | 1.41 |
Saturn's largest moon, Titan, is almost three times denser than the planet it orbits, because it is half rock and half ice. Even the Sun, a ball of hot plasma, is twice as dense on average as Saturn. Several exoplanets known as "super-puffs" go much further, with densities below 0.1 g/cm³.
How the density was measured
Density requires mass and volume. Saturn's mass is derived from the orbits of its moons and, more precisely, from the trajectories of the spacecraft that flew past or orbited it, from Pioneer 11 to Cassini. Its volume comes from its shape, measured by radio occultations as spacecraft passed behind the planet. Cassini refined both, which is why modern values are known to several significant figures. For the effect of that mass on a visitor, see gravity on Saturn.
FAQ
What is Saturn's density?
Saturn's mean density is 0.687 g/cm³, or 687 kg per cubic meter. That is about 70% of the density of liquid water and about one-eighth of Earth's.
Would Saturn float on water?
Only in the sense that its average density is lower than water's. In reality Saturn has no surface, its gas would mix with the water, and any body of water large enough would collapse under its own gravity. Its dense core would not float either.
Why is Saturn so light for its size?
It is made mostly of hydrogen and helium, and it lacks the mass to compress them as strongly as Jupiter does. Its internal heat also keeps the gas slightly expanded.
Which planet is the least dense?
Saturn, at 0.687 g/cm³. The next least dense planet is Uranus at about 1.27 g/cm³, then Jupiter at 1.33 g/cm³.
Sources
- NASA Goddard: Saturn Fact Sheet
- NASA Goddard: Planetary Fact Sheet
- NASA Science: Saturn facts
- Mankovich, C. and Fuller, J. (2021), "A diffuse core in Saturn revealed by ring seismology", Nature Astronomy