Comet dust refers to cosmic dust that originates from a comet. Comet dust can provide clues to comets' origin. When the Earth passes through a comet dust trail, it can produce a meteor shower. Physical characteristics Size The majority of dust from comet activity is sub-micrometer to roughly micrometer in size. However, this fraction is short-lived, as radiation pressure causes them to blow out of the Solar System or spiral inwards. The next size class is large, "fluffy" or "cluster-type" aggregates of the above grains. These are typically 20-100 micrometers, a size not arbitrary but observed as the porous aggregates tend to fracture or compact. Larger particles are micrometeoroids, not dust. In the absence of a definition from the IAU, groups devised their own definitions of dust: smaller than 100 micrometers, 50, 40, 30, and 20 microns, and <10 μm. Some of these dust/micrometeorite definitions are approximate or ambiguous, some overlapping or self-conflicting. The IAU released a formal statement in 2017. Meteoroids are 30 micrometers to 1 meter, dust is smaller, and the term "micrometeoroid" is discouraged (though not micrometeorite). The IMO noted the new definition, but still displays a prior definition on their site. The Meteoritical Society site retains its prior definition, 0.001 cm. The AMS has posted no rigorous definition. Composition Dust is generally chondritic in composition. Its monomers contain mafic silicates, such as olivine and pyroxene. Silicates are rich in high-condensation temperature forsterite and enstatite. As these condense quickly, they tend to form very small particles, not merging droplets. As with chondritic meteoroids, particles contain Fe(Ni) sulfide and GEMS (glass with embedded metal and sulfides) Various amounts of organics (CHON) are present. Though organics are cosmically abundant, and were widely predicted to exist in comets, they are spectrally indistinct in most telescopes. Organics were only confirmed via mass spectrometry during the Halley flybys. Some organics are in the form of PAHs (Polycyclic Aromatic Hydrocarbons). Very small inclusions of presolar grains (PSGs) may be found. Dust and comet origin The models for the origin of comets are: the interstellar model, the Solar System model, primordial rubble piles, aggregation of planetesimals in the dust disk around the Uranus–Neptune region, cold shells of material swept out by the protostellar wind. Bulk properties of the comet dust such as density as well as the chemical composition can distinguish between the models. For example, the isotopic ratios of comet and of interstellar