Most comets are surprisingly tenuous, possessing an average density that is incredibly low, often less than that of water ice or even packed snow. The measured average density of a comet nucleus is typically around 0.6 grams per cubic centimeter. This is far lower than the 1.0 g/cm³ density of liquid water or the 0.9 g/cm³ density of water ice. This low value results directly from the materials that form these celestial bodies and the extremely porous way those materials are structured. Understanding this low density provides a window into the conditions of the solar system’s earliest days.
The Icy and Dusty Ingredients
The low-density nature of comets begins with their fundamental composition, often described by the “dirty snowball” model proposed by astronomer Fred Whipple. The bulk of a comet’s mass consists of volatile ices mixed with silicate dust, rocky grains, and complex organic molecules. These ices include frozen water, carbon monoxide, carbon dioxide, methane, and ammonia.
These volatile compounds are inherently much lower in mass than the rock and metal that dominate the composition of denser bodies like asteroids. Water ice is less dense than the typical rock material found in the inner solar system. Embedded within this icy matrix are dust and silicate materials, including dark, carbon-rich organic compounds that make the comet surface one of the least reflective objects in the solar system. This mixture results in a relatively low mass compared to the denser materials of other solar system objects.
The Highly Porous Structure
While the light ingredients contribute to low mass, the highly porous internal structure is the main reason for the extremely low density measurements. Comets are not solid, uniform blocks of material but are instead loosely aggregated collections of ice and dust, sometimes referred to as a “fluffy snowball.” The internal structure is dominated by voids, gaps, and loosely connected clumps of material, giving it a structure similar to a sponge or a snow cone.
For instance, comet 67P/Churyumov–Gerasimenko was measured to have a density of about 0.5 g/cm³, indicating a porosity of over 50%. This means more than half of the comet’s total volume is empty space. This high porosity significantly increases the volume without adding any mass.
The fragile nature of this structure is evident in the fact that a comet’s own weak gravity is not strong enough to compact the material into a solid sphere. Studies of cometary dust show that even the individual grains are porous aggregates of smaller, submicron-sized subunits. The lack of dense packing within the nucleus is a direct result of how the comet formed, where particles gently clumped together rather than being compressed under intense pressure.
Formed in the Deep Freeze
The final piece of the low-density puzzle is the environment where comets formed, far from the Sun in the extreme outer solar system. Comets are thought to originate primarily from the Kuiper Belt and the distant, spherical Oort Cloud. These regions are known for their incredibly low temperatures, which prevented the volatile materials from vaporizing or melting.
The formation process in this deep-freeze environment involved the slow, gentle accretion of ice and dust grains. Without the heat and pressure present closer to the Sun, the materials did not undergo the high-speed collisions or gravitational compression needed to compact them into a dense, solid body. This gentle clumping resulted in the highly porous, loosely aggregated structure observed today.
Furthermore, when a comet eventually travels into the inner solar system, the slight solar heating causes the ices to turn directly into gas, a process called sublimation. This outgassing creates jets that blast away material and can further hollow out the interior, contributing to the overall low density observed. This combination of low-mass ingredients, high porosity from gentle accretion, and subsequent hollowing by sublimation explains why the average density of a comet is so remarkably low.