What is Dust? Between the Dust on Our Shelves and the Dust that Builds Planets

Cosmic and terrestrial dust — *10-micron dust particle captured in the stratosphere (12–50 km altitude) by flat collectors on a U2 plane (2001). Image source: NASA (new window)*

Terrestrial Dust: A Particle of Daily Life

Atmospheric dust consists of microscopic grains, typically ranging from \(\,0.1\,\mu m\) to \(\,10\,\mu m\). These grains include mineral fragments, pollen, textile fibers, and even nanoparticles from human activities. In physics, a dust particle is defined as an object whose surface-to-volume ratio is so high that surface forces dominate over gravity. Thus, dust with a diameter <10 µm can remain suspended in the air for several days.

Different types of terrestrial dust
Dust type	Origin	Average size	Main composition	Context	Density (g/cm³)
Atmospheric dust (general)	Mix of natural and anthropogenic sources	0.01 to 100 µm	Visibility, radiative balance, condensation nuclei	Forms the urban/regional aerosol background: diurnal and seasonal variability, regulatory monitoring (PM10/PM2.5), mix of ultrafine and coarse modes.	0.001 to 10^-4 g/m³ (suspended), solid 1 to 2.7
Mineral desert dust	Wind erosion (e.g., Sahara)	0.5 to 50 µm	Quartz, iron oxides, clays	Transported by wind over thousands of km	2.5 to 2.7
Volcanic ash	Explosive eruptions	0.1 to 200 µm	Silicates, volcanic glass	Global climatic impact after major eruptions	2.3 to 2.8
Marine aerosols	Bursting of oceanic bubbles (salty water droplets)	0.05 to 10 µm	Sea salts NaCl, sulfates, water	Influence cloud formation and Earth's radiative balance	2.1 to 2.2
Biological dust	Pollen, spores, cellular fragments	1 to 100 µm	Organic compounds, cellulose, proteins	Responsible for seasonal allergies	1.0 to 1.4
Urban and industrial dust	Combustion, traffic, industrial processes	0.01 to 10 µm	Carbon, heavy metals, hydrocarbons	Air pollutants, health risks	1.8 to 7.0
Resuspended dust	Soil tillage, agricultural traffic, harvests, human activities	1 to 100 µm	Massive input of coarse particles, local dust	Peaks related to activities (plowing, harvesting), significant contribution to local coarse dust loads and surface deposits.	1.0 to 2.7

Sources: Seinfeld & Pandis, Atmospheric Chemistry and Physics (3rd ed.), IPCC reports, Draine and specialized aerosol reviews.

N.B.:
Resuspended dust refers to the re-suspension of particles deposited on the ground due to mechanical action of wind, road or agricultural traffic, or human activities. This mechanism strongly contributes to local concentrations of coarse PM (particulate Matter) (diameter >2.5 µm), with short but intense peaks. The dynamics depend on particle size, soil moisture, and surface roughness, and can lead to transport over several kilometers under dry and windy conditions.

Cosmic Dust: Between Science and Mystery

On an interstellar scale, dust consists of silicate grains, ices, and amorphous carbon. These particles play a crucial role in star formation: they absorb ultraviolet radiation, cool the gas, and allow the aggregation of complex molecules. The typical density of cosmic grains is on the order of \(10^{-26}\,\mathrm{g.cm^{-3}}\). In dense regions, called "molecular cores," these dust particles trigger gravitational contraction that leads to the birth of stars.

Different types of cosmic and stratospheric dust
Dust type	Origin	Typical size	Main composition	Context	Density (g/cm³)
Diffuse interstellar dust	Residual matter from the interstellar medium (molecular clouds)	0.005 to 0.25 µm	Amorphous silicates, carbon, ice (H₂O, CO, NH₃)	Present in interstellar clouds and the Milky Way, absorbs and scatters starlight	2.0 to 3.0
Circumstellar dust	Ejection from giant stars (AGB, supernovae)	0.01 to 1 µm	Silicates, graphitic carbons, metal oxides	Formation of protoplanetary disks and stellar envelopes	2.5 to 3.5
Cometary dust	Evaporation and sublimation of comets	0.1 to 100 µm	Silicates, ice, organic compounds, carbon	Forms cometary tails and contributes to meteor showers	1.0 to 2.5
Interplanetary/zodiacal dust	Mix of cometary and asteroidal debris	1 to 100 µm	Silicates, carbons, metals	Forms the zodiacal cloud visible in the inner solar system, contributes to micrometeorites	2.0 to 3.0
Micrometeorites	Fragments of asteroids or comets entering Earth's atmosphere	1 to 500 µm	Silicates, iron-nickel, sulfides	Captured in the atmosphere or on the ground; study of the origin of the solar system	3.0 to 3.8
Stratospheric dust	Volcanic or transported from the Earth's surface to high altitude	0.1 to 20 µm	Volcanic ash, sulfates, minerals	Suspended in the stratosphere, contributes to global radiative forcing and solar radiation scattering	2.3 to 2.8

Sources: Tielens (2013), The Physics and Chemistry of the Interstellar Medium, NASA Cosmic Dust Catalog, Draine (2003) Reviews of Modern Physics, volcanological reports.

Comparison between the dust that accumulates on our shelves and the dust that builds planets

Dust is everywhere, on the Earth's surface and in space. Terrestrial dust comes from erosion, volcanic activity, traffic, agriculture, and resuspension. It typically ranges in size from 0.01 to 100 µm and has solid densities of 1 to 2.7 g/cm³, up to 7 g/cm³ for some urban metal particles. The average density of a terrestrial dust cloud suspended in the atmosphere is between 10^-9 and 10^-6 g/cm³, depending on location and source intensity. This dust influences cloud formation, radiative balance, and air quality, and can be transported into the stratosphere after major volcanic eruptions.

Cosmic dust circulates in the interstellar medium, circumstellar disks, comets, and the solar system. Its size ranges from 0.005 to 500 µm, and the density of solid grains is between 1 and 3.8 g/cm³, depending on composition (silicates, carbons, ice, iron-nickel). However, the average density of a cosmic dust cloud is extremely low, typically from 10^-26 to 10^-22 g/cm³ for the interstellar medium and up to 10^-12 to 10^-9 g/cm³ in the zodiacal cloud near Earth.

Some cosmic dust enters the Earth's atmosphere, joining the continuum of stratospheric dust and contributing to a mix of particles between the Earth's surface and space.

Summary table of dust

Terrestrial and cosmic dust: summary
Dust type	Origin/source	Grain size	Grain density (g/cm³)	Average cloud density (g/cm³)	Role/Context
Terrestrial dust	Erosion, volcanoes, traffic, agriculture, resuspension	0.01 to 100 µm	1 to 2.7 (up to 7 for metal particles)	10^-9 to 10^-6	Influences cloud formation, radiative balance, air quality, stratospheric transport
Cosmic dust	Interstellar medium, circumstellar disks, comets, solar system	0.005 to 500 µm	1 to 3.8 (silicates, carbons, ice, iron-nickel)	10^-26 to 10^-22 (interstellar); 10^-12 to 10^-9 (zodiacal)	Formation of zodiacal clouds, cometary tails, micrometeorites, mixing with stratospheric dust

Sources: Tielens (2013), The Physics and Chemistry of the Interstellar Medium, NASA Cosmic Dust Catalog, Draine (2003) Reviews of Modern Physics.

What is Dust? Between the Dust on Our Shelves and the Dust that Builds Planets

Terrestrial Dust: A Particle of Daily Life

Cosmic Dust: Between Science and Mystery

Comparison between the dust that accumulates on our shelves and the dust that builds planets

Summary table of dust

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