Comet dust is a type of cosmic dust that comes from a comet. This dust can help scientists learn where comets came from. When Earth moves through the dust trail left by a comet, it can create a meteor shower.
Physical characteristics
Most dust from comet activity is smaller than a micrometer or about the size of a micrometer. However, these tiny particles do not stay in the Solar System for long. Radiation pressure pushes them out of the Solar System or makes them spiral inward.
The next size group includes larger, "fluffy" or "cluster-type" groups of the smaller grains. These are usually 20 to 100 micrometers in size. This size range is not random; it is observed because porous aggregates often break apart or become more compact.
Larger particles are called micrometeoroids, not dust. Without a clear definition from the International Astronomical Union (IAU), different groups created their own definitions for dust. Some definitions describe dust as smaller than 100 micrometers, 50 micrometers, 40 micrometers, 30 micrometers, 20 micrometers, or less than 10 micrometers. Some of these definitions are unclear, overlapping, or conflicting.
In 2017, the IAU provided an official statement. It defined meteoroids as particles between 30 micrometers and 1 meter in size, with dust being smaller. The term "micrometeoroid" is no longer encouraged, though "micrometeorite" is still used. The International Meteor Organization (IMO) noted the new definition but still shows an older one on its website. The Meteoritical Society also keeps its previous definition, which states dust is smaller than 0.001 centimeters. The American Meteor Society (AMS) has not provided a clear definition.
Dust is generally similar in composition to chondritic meteorites. Its individual particles contain mafic silicates, such as olivine and pyroxene. Silicates are rich in high-condensation temperature materials like forsterite and enstatite. These materials form quickly, creating small particles rather than merging droplets.
Like chondritic meteoroids, dust particles contain Fe(Ni) sulfide and GEMS (glass with embedded metal and sulfides). Various amounts of organic materials (CHON) are present. Although organic materials are common in space and were predicted to exist in comets, they are hard to detect using most telescopes. Organic materials were confirmed through mass spectrometry during the Halley comet flybys. Some organic materials exist as PAHs (Polycyclic Aromatic Hydrocarbons).
Very small amounts of presolar grains (PSGs) may be found in dust.
Dust and comet origin
The models that explain how comets formed are based on the physical and chemical features of comet dust, such as density and composition. For example, the chemical patterns found in comet dust and in dust from space are very similar, suggesting they may have a shared origin.
The first model, called the interstellar model, suggests that ice formed on dust particles in a dense cloud of gas and dust that existed before the Sun was born. This ice and dust then combined to form comets without major chemical changes. This idea was first proposed by J. Mayo Greenberg in the 1970s.
The second model, the Solar System model, explains that ice from the interstellar cloud turned into gas as part of the disk of gas and dust that formed around the early Sun. Later, this gas reformed into ice and combined to create comets. In this model, comets would have a different chemical makeup compared to comets that formed directly from interstellar ice.
The third model, the primordial rubble pile model, suggests that comets formed by clumping together in the area where Jupiter was forming.
The Stardust mission discovered tiny crystals made of silicates in the dust of comet Wild 2. These crystals formed at very high temperatures (over 1,000 K) near a young, hot star. The dust from comet Wild 2 has a chemical makeup similar to dust found in the outer parts of disks around newly forming stars.
Comets and their dust help scientists study the Solar System beyond the main planets. Comets have different types of orbits. Long-period comets have very long, oval-shaped orbits that are tilted at random angles compared to the plane of the Solar System and take more than 200 years to complete one orbit. Short-period comets have shorter, more circular orbits that are tilted less than 30 degrees from the plane of the Solar System, move in the same direction as the planets, and take less than 200 years to orbit the Sun.
As comets travel through space, they experience many different conditions. Long-period comets spend most of their time far from the Sun, where it is too cold for ice to evaporate. When they pass near the inner planets, ice evaporates quickly, causing small dust particles to escape, but larger particles may remain on the comet’s surface, forming a thin layer of dust. When comets get very close to the Sun, the intense heat causes all dust to escape, leaving no dust layer behind. The thickness of dust layers on a comet’s nucleus can show how often and how close the comet has come to the Sun. A thick dust layer may indicate that the comet has passed near the Sun frequently without getting too close.
Short-period comets are often covered in thick dust layers, which can be several meters deep. Over time, these layers change the physical properties of the comet. The dust layer blocks sunlight from reaching the ice below, reducing the amount of heat that can warm the ice. This also slows the release of gas from the comet’s nucleus. A comet with a thick dust layer might appear to astronomers as a dark, rocky object similar to an asteroid, even though it is a comet.
Further assemblages and bodies
Dust particles, with help from ices and organic materials, form "aggregates" (sometimes called "agglomerates") that range in size from 30 micrometers to hundreds of micrometers. These structures are fluffy because large dust particles do not fit together perfectly, and smaller groups of them also do not pack tightly when forming aggregates.
The next size group is "pebbles," which range from millimeters to centimeters in size. Pebbles have been observed in comet 103P/Hartley 2 and directly imaged in comet 67P/Churyumov-Gerasimenko. In space science, the word "pebble" has a different meaning than in geology. The geological term "cobble," which refers to larger rocks, is not used by scientists from the Rosetta mission.
Larger objects are called "boulders" (at least decimeters in size) or "chunks." These are rarely seen in the coma of a comet because gas pressure is often too weak to lift them to high altitudes or escape the comet's gravity.
The basic building blocks of comets are called "cometesimals," which are similar to "planetesimals," the building blocks of planets. Scientists have debated whether these objects were the size of pebbles, boulders, or something else. This topic is important in research about the Solar System and exoplanets.
The word "dust" is used in astronomy to describe the non-gas part of a comet's coma and tail. However, the term can be confusing because it is a general term used by astronomers but may not be clearly understood by others, such as teachers or scientists in other fields. Larger solid objects are better described as "debris" or, more generally, as "particles" or "grains."
Comet Encke is officially classified as a comet with little dust and more gas. In reality, Encke releases most of its solid material as meteoroids or "rocks," not dust. Observations from the ISO spacecraft found no infrared evidence of a typical dust tail made of small particles.