Comets are small, icy bodies that orbit the Sun, representing primitive material preserved since the Solar System formed 4.6 billion years ago. They are often described by the “dirty snowball” model, which envisions a core composed of frozen gases mixed with dust and rocky material. The chemical composition is a direct record of the molecular inventory present in the cold, outer regions of the solar nebula where they formed. This composition is broadly divided into the volatile ices of the nucleus and the non-volatile refractory materials embedded within.
The Nucleus: The Icy Core
The heart of a comet is its nucleus, a solid body composed of an icy conglomerate of frozen volatiles and dust. The dominant material is water ice (\(\text{H}_2\text{O}\)), which typically constitutes about 80% of the total volatile content. This abundance makes water the primary driver of cometary activity when the body approaches the Sun.
Beyond water, a suite of other simple, frozen molecules makes up the remaining volatile inventory. Abundant secondary ices include carbon monoxide (\(\text{CO}\)) and carbon dioxide (\(\text{CO}_2\)), which are usually present at levels between 1% and 20% relative to water. These carbon-based ices sublimate at lower temperatures than water, meaning they can drive cometary activity even when the comet is far from the Sun.
Other significant, though less abundant, ices include methane (\(\text{CH}_4\)), ammonia (\(\text{NH}_3\)), methanol (\(\text{CH}_3\text{OH}\)), and hydrogen sulfide (\(\text{H}_2\text{S}\)). The specific ratios of these volatile species vary significantly from comet to comet, suggesting a chemical diversity that reflects different formation conditions in the solar nebula. For instance, a higher \(\text{CO}\) content suggests a comet formed in a colder region of the protoplanetary disk compared to one dominated by water ice.
The Coma and Tails: Gaseous Transformation
As a comet moves closer to the Sun, solar heat causes the ices in the nucleus to turn directly into gas, a process known as sublimation. This escaping gas drags dust particles outward, creating the expansive, fuzzy atmosphere called the coma. The coma’s chemical composition initially reflects the parent molecules sublimating from the nucleus, such as \(\text{H}_2\text{O}\), \(\text{CO}\), and \(\text{CO}_2\).
Once in the coma, these parent molecules are exposed to solar ultraviolet radiation, which breaks them down into smaller fragments called daughter molecules. Water, for example, rapidly photodissociates into products like hydroxyl (\(\text{OH}\)), hydrogen (\(\text{H}\)), and oxygen (\(\text{O}\)). Other parent molecules yield radicals like cyanogen (\(\text{CN}\)) and diatomic carbon (\(\text{C}_2\)), which are easily observed using spectroscopy.
The two distinct tails that define a visible comet are chemically separate phenomena. The dust tail is formed by larger, solid particles released from the nucleus, which are pushed away from the Sun by solar radiation pressure. This tail often appears yellowish or white because the dust particles scatter sunlight.
In contrast, the ion or plasma tail is composed of gases that have become ionized by the Sun’s ultraviolet light. These charged particles, which include water ions (\(\text{H}_2\text{O}^+\)) and carbon monoxide ions (\(\text{CO}^+\)), are then strongly swept away by the solar wind. The ion tail always points directly away from the Sun and typically glows a bluish color due to the fluorescence of these ionized molecules.
Refractory Materials and Organic Molecules
The non-ice components of a comet are refractory materials because they can withstand high temperatures without vaporizing. These materials are intimately mixed with the ices and constitute the “dirty” fraction of the nucleus. The inorganic refractory fraction includes tiny dust grains made of silicate minerals, such as olivine and pyroxene, which are common rocky materials.
These rocky grains also contain metal sulfides and other components thought to be remnants of the interstellar dust cloud that preceded the Solar System. The nucleus structure is a porous matrix where these silicate and metallic dust grains are locked together by the surrounding ices. As the ices sublimate, this non-volatile material is released into the coma and tails.
A significant component of the refractory material is complex organic matter, which holds carbon, hydrogen, oxygen, and nitrogen. This organic material exists in two forms: insoluble organic matter (IOM), a macromolecular polymer, and more labile, soluble organic molecules. IOM is thought to be the most abundant carbon component in comets, often forming robust structures within the dust grains.
The presence of specific complex organic molecules is of great interest, as it suggests comets may have delivered the chemical building blocks of life to early Earth. Missions have detected the simplest amino acid, glycine, along with methylamine and ethylamine, directly in cometary matter. Analysis of the refractory organic material has shown that it is relatively rich in hydrogen compared to carbon, indicating a less chemically saturated composition than similar material found in primitive meteorites.