ohOn the roof of Canterbury Cathedral, two planetary scientists search for cosmic dust. While the red brick parapet hides the streets, buildings and trees far below, only puffy clouds block the deep blue sky that stretches into outer space.
The roar of a vacuum cleaner breaks the silence and researcher Dr. Penny Wozniakiewicz, dressed in a hazmat suit with a bulky vacuum cleaner backpack, carefully traces a chute with the tube of the vacuum machine.
“We’re looking for small microscopic balls,” explains her colleague, Dr Matthias van Ginneken from the University of Kent, also dressed in protective gear. “Right now we’re collecting thousands and thousands of dust particles, and we hope there will be a minuscule number that came from space.”
Most of the extraterrestrial dust that bombards Earth each year evaporates in the atmosphere – some models suggest that 15,000 tons reach the Earth’s atmosphere (the equivalent of about 75 blue whales). But about 5,200 tons of micrometeorites falling to Earth, based on an estimate from Antarctica. These particles, which most likely come from comets and asteroids, are small, between 50 microns and two millimeters in diameter.
“You have to be a bit of a detective,” says Van Ginneken. The extreme heating on atmospheric entry changes many of the minerals and “you have to figure out the nature of the original particle based on the limited information you have”.
Researchers turn to micrometeorites for clues about the chemistry of asteroids and meteorites. By looking at chemical variants known as isotopes, scientists can understand more about the parent body from which the cosmic dust came – and what happened to it when it entered Earth’s atmosphere.
Also in the past, cosmic dust was more abundant, as there were many more collisions between objects in the solar system when the Earth was young. That dust is trapped in rocks, and it can point to what has happened in our planetary environment throughout Earth’s history, and how it has changed.
Van Ginneken and Wozniakiewicz are trying to understand how the flux of micrometeorites changes, among other science questions.
“If you can get an understanding of how many dust particles are arriving across the surface, you can make some estimates about how much material is arriving at Earth over time, and therefore, potentially, what contribution space dust is making to Earth chemistry. ,” says Wozniakiewicz.
“And it is in two ways – some of [the cosmic materials] survive on the surface, and they can participate in surface chemistry. Some of them burn in the atmosphere, and they can participate in atmospheric chemistry.”
Micrometeorites can seed the land and seas with elements that are not commonly found on Earth’s surface, as well as in the atmosphere – which can affect how these systems behave.
Wozniakiewicz and Van Ginneken are looking for a specific type of extraterrestrial substance: cosmic spherules. These small spheres are relatively easy to identify compared to other matter, due to their characteristic shape, but it takes a microscope to make sure that a cosmic spherol does not come from Earth. This makes them useful for estimating how much cosmic dust has fallen at a particular location over a given period of time.
Cosmic dust was thought to be impossible to collect in the urban environment – it was limited to pristine places, such as the Antarctic, or in fossilized sediment. But in 2009, Norwegian jazz musician, cosmic dust hunter, Jon Larsen began combing through hundreds of kilograms of urban dust particles, looking for cosmic dust. In 2017, Larsen and colleagues, including Van Ginneken, published a seminal paper in the journal Geologyshowing that anyone with a microscope and patience can discover cosmic spheres.
But it is difficult to collect micrometeorites for scientific study, even though they are constantly falling to the Earth’s surface. The particles are easily contaminated, which can compromise their use in research. (But that’s not why Van Ginneken and Wozniakiewicz look like white-clad aliens – they’re protecting themselves from bird flu, possibly contained in the fog and bird bones we see on the roof.)
Larsen “started the whole era of urban micrometeorites”, says Van Ginneken. “Since then, more and more people are doing it as a hobby. Part of what Penny and I want to do is bring the science into it.”
Cathedral roofs, like Canterbury’s, are ideal for cosmic dust hunting as they are vast, inaccessible and largely untouched. We go through a wooden door, usually barred, at the back of the cathedral’s main Trinity Chapel, up hundreds of tightly winding steps, and then through another specially unlocked door to reach one of the cathedral’s roofs. It was Van Ginneken and Wozniakiewicz’s last roof of the day – they moved up to several other roofs on the cathedral. They have also collected dust from Rochester Cathedral, and hope to add Salisbury and Winchester to their list.
Van Ginneken is keen to sample many rooftops, to understand the biases that creep into urban micrometeorite collections, such as the effect of rainwater. The advantage of roofs is that they are easily accessible, he says. Going to Antarctica, where a lot of micrometeorite research has been undertaken, “is very expensive, it requires a lot of preparation, and there is a limit to the amount of samples you can bring back”. The research is also limited to a specific climate and latitude. Roofs broaden the opportunities to investigate how these tiny dust particles behave in different environments.
The abundance of urban micrometeorites also opens up planetary science to those who don’t necessarily have access to the transportation of larger space missions. And there is increasing interest in the bounties of outer space. Nasa’s OSIRIS-REx missionfor example, last year brought back to earth material from the asteroid Bennuwhich is more than 4.5 billion years old.
“Those missions are great,” Wozniakiewicz says. “They go to a single object, and they tell you a lot about that one object. Micrometeorites tell you about thousands, millions of objects… They tell you more about the population of asteroids as a whole, a snapshot of all the different processes, all the different bodies that are out there. And then you can compare those samples, along with meteorites, to the samples that are brought back from these missions.”
Cosmic dust may also hold clues about our own planet in the distant past, says Dr Martin Suttle, a lecturer in planetary science at the Open University. It could also have created an inhospitable environment on the early Earth that sparks life spontaneously a new paper published by Suttle and colleagues in Nature Astronomy.
“There was more dust coming to Earth, maybe 1,000 times more dust, than today,” he says. “That dust carries a lot of stuff that’s attractive as a raw material for early prebiotic chemistry, things like iron metal, which is otherwise not found on Earth’s surface.”
But collecting the cosmic dust is just the beginning of the research process, and probably the easier part – despite all the cathedral steps. The bags of dust will now be sterilized so that they are safe to work with, and then the scientists will examine each particle under a sterile microscope.
“We’ll spend hours and hours and hours and hours just extracting spheres and hoping that one is a cosmic spherol,” says Van Ginneken.
