Which Moon Has The Lowest Density In The Solar System?

Saturn’s moon, Tethys, holds the title for the lowest density among the known moons in our solar system. With a density so low, Tethys is composed nearly entirely of water ice, which is less dense than rocky material.

Tethys is a major moon of Saturn and boasts a density of just 0.984 g/cm³. This suggests it is composed almost entirely of water ice. This stands in stark contrast to other moons of Saturn, like Titan and Rhea. It also contrasts other moons across our solar system, many of which have significantly higher densities due to their rock content or metal core. For instance, Earth’s moon has a density of 3.34 g/cm³, owing mostly to its rocky composition.

The concept of a moon’s density holds a great deal of significance in the field of planetary science, as it can offer insights into its composition, origin, and evolutionary history. In the case of Tethys, its low density speaks to a composition primarily of water ice. This provides scientists clues about its formation and any potential for subsurface oceans. Additionally, understanding a moon’s density can help comprehend the impact history of that celestial body, thereby thinning the veil on the intricate dynamics of our solar system.

Identification of the Moon with the Lowest Density

Among the diverse collection of moons in our solar system, Tethys—despite being only the 16th-largest—surprisingly comes out as the champion of low density. This mysterious moon has a mean radius of about 531 kilometers and a mass of around 6.2 x 10^20kg. What’s truly fascinating though, is that its average density measures a mere 0.984 grams per cubic centimeter.

The key to Tethys’ low density is found in its composition. Unlike many moons which have rock or even metallic cores, Tethys is believed to be composed almost entirely of water-ice. This relationship between composition and density provides an interesting look into the varied materials and physical properties of celestial bodies in our solar system. As a result, Tethys offers invaluable insights into the assortment of distinctive moons that orbit our planets.

Comparisons between Tethys and Other Moons

Contrasting Tethys with other significant moons in the solar system gives us a more comprehensive understanding of what makes this particular moon so unique in terms of density.

  • Ganymede, Jupiter’s largest moon and the biggest moon in the solar system, is considerably larger than Tethys, boasting a higher density of 1.936 g/cm³, due to its iron-rich core.
  • Callisto, another hefty moon of Jupiter, also eclipses Tethys in terms of size, but its density clocks in a bit less than Ganymede, at 1.834 g/cm³.
  • Io, with its volcanic landscapes, and Europa, suspected of harboring subsurface oceans, both have densities greater than 3 g/cm³, almost three times that of Tethys.
  • Moving to moons of Saturn, Enceladus and Mimas have densities of around 1.61 and 1.15 g/cm³ respectively, both higher than Tethys.
  • Likewise, irregularly shaped Hyperion and Titan, with its dense atmosphere, also have densities surpassing that of Tethys.
  • Uranus’ moon, Miranda, although smaller in size than Tethys, also has a higher density, indicating a rockier composition.

This comparison gives us an idea of why Tethys, with its primarily icy composition, finds itself in a league of its own in terms of low density among the moons in our solar system.

If you would like to find out more about the moons in our solar system check out our article on how many moons does each planet have?

The Significance of a Moon’s Density

Unraveling the puzzle of a moon’s density, such as that of Tethys, is a key focus in planetary science. A moon’s density gives us a doorway into understanding its internal structure and composition, which in turn helps us uncover the specifics of its formation and geological history.

  • Tethys, with its notably low density, suggests a predominantly water-ice composition, reflecting a quite different formation and evolution compared to higher density, rockier moons.
  • Moreover, understanding Tethys’ density and water-ice composition offers scientists tantalizing hints about potential subsurface oceans, a topic which draws significant interest in the search for extraterrestrial life. The idea being, where there’s water, there might be life!

Hence, analyzing the density of a moon like Tethys is way more than crunching numbers. It’s about delving into the mysteries of how these celestial bodies are built, and, perhaps, getting a bit closer to answering the eternally captivating question: are we alone in the universe?


To sum up, Tethys, the 16th-largest moon in the solar system, has a unique claim to fame as the moon with the lowest density, attributed largely to its composition predominantly of water-ice. When contrasted with other large moons like Ganymede, Callisto, Io, and Europa, Tethys stands apart for its exceptionally low density. Understanding the density of celestial bodies such as Tethys offers fascinating insights into their internal structure, composition, and geological history, and may even provide clues to life beyond Earth. As we continue to explore the farthest reaches of our solar system, every bit of information, even something as seemingly simple as the density of a moon, can lead to profound discoveries about our universe.