COSMIC DUST: CHARACTERISTICS, TYPES AND THEORY - ARTE - 2025

The Cosmic dust consists of tiny particles that fill the space between planets and stars, and sometimes accumulates to form clouds and rings. They are particles of matter whose size is less than 100 micrometers, where a micrometer is one millionth of a meter. Larger particles are renamed "meteoroids."

For a long time it was believed that vast interstellar spaces were devoid of matter, but what happens is that not all that exists is condensed in the form of planets or stars.

Figure 1. Interstellar cosmic dust and gas clouds in the Carina Nebula at 7500 light-years in the constellation Carina. Source: NASA via Wikimedia Commons.

There is a large amount of matter of very low density and diverse origins, which with time and the appropriate conditions turns into stars and planets.

But it is not necessary to go that far to find cosmic dust, since the Earth receives about 100 tons of dust and fragments every day that arrive from space at high speed. Most of it goes to the oceans and is distinguished from household dust, from which volcanic eruptions and sandstorms produce in great deserts.

Cosmic dust particles are capable of interacting with radiation from the Sun and also ionizing, that is, capturing or giving up electrons. Its effects on Earth are diverse: from scattering sunlight to modifying temperature, blocking infrared radiation from the Earth itself (heating) or the Sun (cooling).

Types of cosmic dust

Here are the main types of cosmic dust:

Cometary dust

When approaching the Sun and being exposed to its intense radiation, part of the comet disintegrates, the gases are expelled forming the hair and the tails composed of gas and dust. The straight tail seen on the comet is made of gas and the curved tail is made of dust.

Figure 1. The most popular comet of all: Halley. Source: Wikimedia Commons. NASA / W. Liller

Rings

Several planets in our solar system have rings of cosmic dust, originating from collisions between asteroids.

The remnants of collisions travel through the solar system and frequently impact the surface of the moons, breaking up into tiny particles. The surface of our Moon is covered in fine dust from these impacts.

Some of the dust remains around the satellite forming a faint halo, like that of the large Jovian satellites Ganymede and Callisto. And it also spreads along the satellite orbits, forming rings, which is why it is also called circumferential dust.

This is the origin of Jupiter's faint rings, first detected by the Voyager probe. Asteroidal impacts are due on the small Jovian moons Metis, Adrastea, Amalthea and Thebe (figure 3).

Figure 3. Structure of Jupiter's rings. Source: NASA via Wikimedia Commons.

The Jovian system also sends large amounts of dust into space thanks to volcanic eruptions on the moon Io. But the gas giant is not the only one to have cosmic dust rings, as Uranus and Neptune have them too.

As for the famous rings of Saturn, their origin is somewhat different: they are believed to be the remains of an icy moon that collided with the newly formed giant planet.

Interstellar dust

Stars expel large amounts of mass at the end of their lives and then when they explode as supernovae, leaving behind a nebula. A small part of this material condenses into powder.

And although there is barely 1 hydrogen atom for every cubic centimeter of space, the dust is large enough to cause starlight fading and flushing.

Intergalactic dust

The space between galaxies also contains cosmic dust, and as for the galaxies themselves, spirals are richer in cosmic gas and dust than ellipticals. In the former, the dust concentrates rather towards the disk and the spiral arms.

Interplanetary dust

It is found throughout the solar system and comes in part from the original cloud that gave rise to it, in addition to cometary dust and that produced by asteroid collisions and impacts on moons.

Cosmic dust theory

Cosmic dust from the Andromeda galaxy, revealed by infrared light from the Spitzer Space Telescope. Source: NASA / JPL-Caltech / K. Gordon (University of Arizona) Cosmic dust particles are so small that the force of gravity is just one of many interactions they experience.

On particles just a few microns in diameter, the pressure exerted by sunlight is significant, pushing dust out of the solar system. It is responsible for the tails of comets when they get close enough to the Sun.

Cosmic dust particles are also subject to the so-called Poynting-Robertson effect, which counteracts the pressure of solar radiation and causes a slow spiral movement towards the Sun. It is a noticeable effect on very small particles but negligible when the size exceeds meter.

Magnetic fields also affect the movement of cosmic dust particles, deflecting them when ionized, which happens easily, since dust grains are easily electrified by capturing or giving up electrons.

It is not surprising that these forces generate dust streams moving at 70 km per second or more through space.

Composition and relationship with the origin of life

Cosmic dust that comes from stars is rich in graphite and silicon crystallized from high temperatures. On the other hand, that of asteroids is rich in metals such as iron and nickel.

What is surprising is that molecules of biological importance can also settle in grains of cosmic dust. On its surface, hydrogen and oxygen atoms meet to form water, which despite the low temperatures of deep space, can still be mobilized.

Other simple organic compounds are also present, such as methane, ammonia, and carbon monoxide and dioxide. Scientists do not rule out that some living beings such as tardigrades and some plants and bacteria are capable of leaving the planet transporting themselves in the dust. Nor do they rule out the idea that life has come to our planet from some remote place by this same path.

The zodiacal light

Observing the evidence for cosmic dust is simple. There is a band of diffuse light in the shape of a cone or triangle called the zodiacal light, which appears in the sky right where the ecliptic emerges. It is sometimes called the "false dawn" and was studied by Domenico Cassini in the 17th century.

Figure 4. Zodiacal light (right) seen from the Paranal observatory in Chile. Source: Wikimedia Commons. ESO / Y. Beletsky. It is visible mainly at dusk in spring (late January to early April) or dawn in autumn in the northern hemisphere. For their part, observers in the southern hemisphere should look for it at dusk in late summer and early fall or before sunrise in spring.

Finally for those who are in equatorial latitudes, the zodiacal light is visible throughout the year.

The name is due to the fact that the luminosity appears to be over the constellations of the Zodiac and the best time to see it is during clear, moonless nights, away from light pollution, preferably in the two weeks after the full moon.

The zodiacal light is due to the cosmic dust accumulated in the equatorial plane of the Sun scattering the light of the star.

References

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