The Story of Dust, Through Space and Time


What is dust?

It feels ridiculous just asking that question sitting in India. Dust is everywhere. On the roads, in your nose, in your lungs. You lock up your house, go on a month-long holiday and come back, and there’s a fine patina on the table. It’s inside your laptop, driving the cooling fan nuts.

It is also in the atmosphere, in orbit around Earth, in outer space even. It makes up nightmarish storms on Mars. Philip Pullman and Steven Erikson have written books fantasising about it. Dust is omnipresent. (The only dustless places I’ve seen are in stock photos strewn across the internet.)

But what exactly is it, and where did it all come from?

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Earth

The Saharan dust plume in June 2018. Credit: NASA/Earth Observatory

The Saharan dust plume in June 2018. Credit: NASA/Earth Observatory

Dust is fine particulate matter. It originates from a tremendous variety of sources. The atmospheric – or aeolian – dust we are so familiar with is composed of small particles sheared off of solid objects. For example, fast-blowing winds carry particles away from loose, dry soil into the air, giving rise to what is called fugitive dust. Another source is the smoke from exhaust pipes.

Yet another is mites of the family Pyroglyphidae. They eat flakes of skin, including those shed by humans, and digest them with enzymes that stay on in their poop. In your house, exposure to their poop (considered a form of dust) can trigger asthma attacks.

Winds lift particulate matter off Earth’s surface and transport them into the troposphere. Once dust gets up there, it acts like an aerosol, trapping heat below it and causing Earth’s surface to warm. Once it collects in sufficient quantities, it begins to affect the weather of regions below it, including rainfall patterns.

Dust particles smaller than 10 microns get into your lungs and affect your respiratory health. They conspire with other pollutants and, taking advantage of slow-moving winds, stagnate over India’s National Capital Region during winter. Particles smaller than 2.5 microns “increase age-specific mortality risk” (source) and send hospital admissions soaring.

There is also dust that travels thousands of kilometres to affect far-flung parts of the world. The “Sahara is the world’s largest source of desert dust”, according to one study. In June this year, the Atlantic Ocean’s tropical area experienced its dustiest period in 15 years when a huge billow blew over from northeast Chad towards the mid-Americas. According to NASA’s Earth Observatory, Saharan dust “helps build beaches in the Caribbean and fertilises soils in the Amazon.”

But speaking of dust that migrates large distances, the transatlantic plume seems much less of a journey than the dust brought to Earth by meteorites that have travelled hundreds of thousands of kilometres through space. As these rocks streak towards the ground, the atmosphere burns off dust-like matter from their surfaces, leaving them hanging in the upper atmosphere.

Atoms released by these particles into the mesosphere drift into the planet’s circulation system, moving from pole to pole over many months. They interact with other particles to leave behind a trail of charged particles. Scientists then use radar to track these particles to learn more about the circulation itself. Some dust particles of extraterrestrial origin also reach Earth’s surface in time. They could carry imprints of physical and chemical reactions they might have experienced in outer space, even from billions of years ago.

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Orbit

Dust in the Andromeda Galaxy, as seen by the Spitzer Space Telescope. Credit: NASA/JPL-Caltech/K. Gordon (University of Arizona)

Dust in the Andromeda Galaxy, as seen by the Spitzer Space Telescope. Credit: NASA/JPL-Caltech/K. Gordon (University of Arizona)

In the mid-20th century, researchers used optical data and mathematical arguments to figure that about four million tonnes of meteoric dust slammed into our planet’s atmosphere every year. This was cause for alarm: the figure suggested that the number of meteorites in space was much higher than thought. In turn, the threat to our satellites could have been underestimated. More careful assessments later brought the figure down. A 2013 review states that 10-40 tonnes of meteoric dust slams into Earth’s atmosphere every day.

Still, this figure isn’t low – and its effects are exacerbated by the debris humans themselves are putting in orbit around Earth. The Wikipedia article on ‘space debris’ carefully notes, “As of … July 2016, the United States Strategic Command tracked a total of 17,852 artificial objects in orbit above the Earth, including 1,419 operational satellites.” But only one line later, the number of objects smaller than 1 cm explodes to 170 million.

If a mote of dust weighing 0.00001 kg carried by a 1.4 m/s breeze strikes your face, you are not going to feel anything. This is because its momentum – the product of its mass and velocity – is very low. But when a particle weighing one-hundredth of a gram strikes a satellite at a relative velocity of 1.5 km/s, its momentum jumps a thousandfold. Suddenly, it is able to damage critical components and sensitively engineered surfaces, ending million-dollar, multi-year missions in seconds. One study suggests such particles, if travelling fast enough, can also generate tiny shockwaves.

Before our next stop on the Dust Voyage, let’s take a small break in sci-fi. The mid-century overestimation of meteoric dust flux may have prompted Arthur C. Clarke to write his 1961 novel, A Fall of Moondust. In the story, a cruise-liner called the Selene takes tourists over a basin of superfine dust apparently of meteoric origin. But one day, a natural disaster causes the Selene to sink into the dust, trapping its passengers in life-threatening conditions. After much despair, a rescue mission is mounted when an astronomer spots a heat-trail pointing to the Selene’s location from space, from onboard a spacecraft called Lagrange II.

This name is a reference to the famous Lagrange points. As Earth orbits the Sun, and the Moon orbits Earth, their combined gravitational fields give rise to five points in space where the force acting on an object is just right for it to maintain its position relative to Earth and the Sun. These are called L1, L2, L3, L4 and L5.

A contour plot of the effective potential of the Earth-Sun system, showing the five Lagrange points. Credit: NASA and Xander89, CC BY 3.0

A contour plot of the effective potential of the Earth-Sun system, showing the five Lagrange points. Credit: NASA and Xander89, CC BY 3.0

The Indian Space Research Organisation (ISRO) plans to launch its Aditya satellite, to study the Sun, to L1. This is useful because at L1, Aditya’s view of the Sun won’t be blocked by Earth. However, objects at L1, L2 and L3 have an unstable equilibrium. Without some station-keeping measures now and then, they tend to fall out of their positions.

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But this isn’t so with L4 and L5, objects at which remain in a more stable equilibrium. And like anything that’s been lying around for a while, they collect dust.

In the 1950s, the Polish astronomer Kazimierz Kordylewski claimed to have spotted two clouds of dust at L4 and L5. These nebulous collections of particulate matter have since been called Kordylewski clouds. Other astronomers have contested their existence, however. For example, the Hiten satellite could not find any notable dust concentrations in the L4 and L5 regions in 2009. Some argued that Hiten could have missed them because the dust clouds are too spread out.

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Space

An artist's impression of dust formation during a supernova explosion. Caption and credit: ESO/M. Kornmesser, CC BY 4.0

An artist’s impression of dust formation during a supernova explosion. Caption and credit: ESO/M. Kornmesser, CC BY 4.0

Only two weeks ago, Hungarian astronomers claimed to have confirmed the presence of dust clouds in these regions (their papers here and here). Because the L4 and L5 regions are of interest for future space missions, astronomers will now have to validate this finding and – if they do – assess the density of dust and the attendant probabilities of threat.

Unlike Kordylewski, who took photographs from a mountaintop, the Hungarian group banked on dust’s ability to polarise light. Light is electromagnetic radiation. Each wave of light consists of an electric and a magnetic field oscillating perpendicular to each other. Imagine various waves of light approaching dust, their electric fields pointed in arbitrary directions. After they strike the dust, however, the particles polarise the waves, causing all of the electric fields to line up with one particular orientation.

When astronomers detect such light, they know that it has encountered dust in its path. Using different instruments and analytical techniques, they can then map the distribution of dust in space through which the light has passed.

This is how, for example, the European Space Agency’s Planck telescope was able to draw up a view of dust around the Milky Way.

A map of dust in and around the Milky Way galaxy, as observed by the ESA Planck telescope. Credit: NASA

A map of dust in and around the Milky Way galaxy, as observed by the ESA Planck telescope. Credit: NASA

That’s billions upon billions of tonnes. Don’t your complaints about dust around the house pale in comparison?

And even at this scale, it has been a nuisance. We don’t know if the galaxy is complaining but Brian Keating certainly did.

In March 2014, Keating and his team at Harvard University’s Centre for Astronomy announced that they had found signs that the universe’s volume had increased by a factor of 1080 in just 10-33 seconds a moment after its birth in the Big Bang. About 380,000 years later, radiation leftover from the Big Bang – called the cosmic microwave background (CMB) – came into being. Keating and co. were using the BICEP2 detector at the South Pole to find imprints of cosmic inflation on the CMB. The smoking gun: light of a certain wavelength polarised by gravitational waves from the early universe.

While the announcement was made with great fanfare – as the “discovery of the decade” and whatnot – their claim quickly became suspect. Data from the Planck telescope and other observatories soon showed that what Keating’s team had found was in fact light polarised by galactic dust. Just like that, their ambition of winning a Nobel Prize came crashing down. Ash to ash, dust to dust.

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You probably ask, “Hasn’t it done enough? Can we stop now?” No. We must persevere, for dust has done even more, and we have come so close. For example, look at the Milky Way dust-map. Where could all that dust have come from?

This is where the story of dust takes a more favourable turn. We have all heard it said that we are made of stardust. While it would be futile to try and track where the dust of ourselves came from, but understanding dust itself requires us to look to the stars.

The storms on Earth or Mars that stir dust up into the air are feeble breaths against the colossal turbulence of stellar ruination. Stars can die in one of many ways depending on their size. The supernovae are the most spectacular. In a standard Type 1a supernova, an entire white dwarf star undergoes nuclear fusion, completely disintegrating and throwing matter out at over 5,000 km/s. More massive stars undergo core collapse, expelling their outermost layers into space in a death-sneeze before what is left implodes into a neutron star or a black hole.

Any which way, the material released into space forms giant clouds that disperse slowly over millions of years. If they are in the presence of a black hole, then they are trapped in an accretion disk around it, accelerated, heated and energised by radiation and magnetic fields. The luckier motes may float away to encounter other stars, planets or other objects, or even collide with other dust and gas clouds. Such interactions are very difficult to model – but there is no doubt that these interactions are all essentially driven by the four fundamental forces of nature.

One of them is the force of gravity. When a gas/dust cloud becomes so large that its collective gravitational pull keeps it from dispersing, it could collapse to form another star, and live to see another epoch.

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Together

The Cat's Paw Nebula, imaged here by NASA's Spitzer Space Telescope, lies inside the Milky Way Galaxy and is located in the constellation Scorpius. Its distance from Earth is estimated to be between 4,200 light years and 5,500 light years. The green areas show regions where radiation from hot stars collided with large molecules and small dust grains called polycyclic aromatic hydrocarbons, causing them to fluoresce. Caption and credit: NASA/JPL-Caltech, Wikimedia Commons

The Cat’s Paw Nebula, imaged here by NASA’s Spitzer Space Telescope, lies between 4,200 lightyears and 5,500 lightyears from Earth. The green areas show regions where radiation from hot stars collided with large molecules and small dust grains called polycyclic aromatic hydrocarbons, causing them to fluoresce. Caption and credit: NASA/JPL-Caltech, Wikimedia Commons

This way, stars are cosmic engines. They keep matter – including dust – in motion. They may not be the only ones to do so but given the presence of stars throughout the (observable) universe, they certainly play a major part. When they are not coming to life or going out of it, their gravitational pull influences the trajectories of other, smaller bodies around them, including comets, asteroids and other spacefaring rocks.

The Solar System itself is considered to have been condensed out of a large disk of dirt and dust made of various elements surrounding a young Sun – a disk of leftovers from the star’s birth. Different planets formed based on the availability of different volumes of different materials at different times. Jupiter is believed to have come first, and the inner planets, including Earth, to have come last.

But no matter; life here had whatever it needed to take root. Scientists are still figuring what those ingredients could have been and their provenance. One theory is that they included compounds of carbon and hydrogen called polycyclic aromatic hydrocarbons, and that they first formed – you guessed it – among the dust meandering through space.

They could then have been ferried to Earth by meteors and comets, perhaps swung towards Earth’s orbit by the Sun’s gravity. When a comet gets closer to a star, for instance, material on its surface begins to evaporate, forming a streaky tail of gas and dust. When Earth passes through a region where the tail’s remnants and other small, rocky debris have lingered, they enter the atmosphere as a meteor shower.

Dust really is everywhere, and it seldom gets the credit it is due. It has been and continues to be a pesky part of daily life. However, unlike our search thus far for extraterrestrial companionship, we are not alone in feeling beset by dust.





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