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The hunt for cosmic dust
Every day the Earth is showered with extraterrestrial material. Luckily for us, most of it arrives as harmless dust. Despite their small size, these particles contain valuable information about our solar system.
Planetary scientist Dr Penelope Wozniakiewicz tells us why she’s hunting for this cosmic dust on a small island in the mid-Pacific.
Most of you will probably have heard of meteorites before, but you may not be aware of micrometeorites - extraterrestrial dust particles that make it through our atmosphere and land on Earth’s surface. These particles are less than 2mm in size, a lot smaller than their larger relatives, making them notoriously difficult to collect.
In 2011, I began a project with colleagues at Lawrence Livermore National Laboratory, NASA Johnson Space Centre and the University of Hawaii to develop a novel way of collecting micrometeorites. This project took us all the way to a remote island in the middle of the Pacific Ocean.
It’s raining space particles
But why are we collecting cosmic dust in the first place?
Most of the meteorites and micrometeorites that reach Earth each year are thought to come from asteroids or comets. They are extremely important to planetary scientists like me because the materials that they are composed of preserve details of the processes taking place on these solar system bodies.
We think that most meteorites and micrometeorites come from asteroids, like 4Vesta (left), having been ejected during impacts. © NASA/JPL-Caltech. Others may be shed from comets like 1P/Halley (right), after the loss of binding icy materials when they travel through the inner parts of our solar system. © ESA/MPAe Lindau
For example, certain minerals require liquid water to form, so if you find those minerals in a meteorite or micrometeorite it means that liquid water must have been present at some point on the comet or asteroid that they originated from.
One of the reasons we’re interested in studying micrometeorites is that they vastly outnumber meteorites. It’s been estimated that 20,000-30,000 tonnes of extraterrestrial dust reach Earth each year compared to 50 tonnes of larger rocks.
Needle in a haystack
The problem with collecting micrometeorites on Earth is that they are difficult to find among the massive amounts of dust produced by the natural world. Think of the amount of dust created when a volcano erupts or a wildfire tears across the land. Human activities like mining, burning coal and even shedding skin are also sources of terrestrial dust.
Although the majority of micrometeorites have chemistries, textures and shapes that distinguish them from this terrestrial dust, searching for them is like looking for a needle in a giant haystack.
So the best chance we have of collecting micrometeorites is to go to locations where terrestrial dust concentrations are really low.
Huge amounts of dust are released by volcanic eruptions (left, photo by Austin Post, courtesy of the US Geological Survey) and wildfires (right, courtesy of Yosemite National Park), as well as human activity such as open-pit mining.
One place micrometeorites have been successfully collected is in the Antarctic, where they are found encased in ice or snow. However these particles often exhibit signs of alteration due to the prolonged exposure to Earth’s environment. We also can’t pin down the exact time when they arrived on Earth, which means we can’t associate them with any particular comet or asteroid.
So, micrometeorite scientists are trying to come up with new and inventive ways of collecting cosmic dust.
Gathering dust
Kwajalein Atoll in the Republic of the Marshall Islands is a remote island in the mid-Pacific, more than 1,000 miles away from the nearest continent and associated dust sources. So we thought it would be a good place to try collecting micrometeorites.
On the island we used a dust sampler that pulls air through an extremely fine filter. Collecting samples directly from the atmosphere like this significantly reduces local terrestrial dust contamination.
We changed the filters in the dust sampler once a week and this means we can work out roughly when the collected micrometeorites arrived on Earth. We may even be able to match the specimens with celestial events such as meteor showers.
Me with the dust sampler on Kwajalein atoll, 1,000 miles away from the nearest continent.
Good things come in small packages
I am now working at the Museum as a Marie Curie Fellow, examining the samples we collected from the Kwajalein Atoll.
To study these dust particles we have to look at them under powerful microscopes. It takes a very steady hand to prepare them for this as it involves using a single hair on a brush to pick up a dust particle with static forces. Some of us find it best to take sable paint brushes and cut off the hair leaving just a single strand, while others mount an eyelash (clean of course) on a brush handle. As we are working with static forces, we just have to hope that the particle doesn’t fall off before we can examine it under the microscope.
At the moment we are identifying lots of cosmic spherules, which are micrometeorites that have been melted as they enter the Earth’s atmosphere. These dust particles have a characteristic spherical shape, which makes them easy to spot among the more angular pieces of coral and sand.
I am always amazed by the intricate surface textures I can see on such small particles. These textures are the result of fast heating and cooling in the atmosphere.
My cosmic spherule candidates, imaged by a scanning electron microscope. All of them show amazing surface textures but each is less than 10 micrometres in size.
My next plan is to study the insides of these samples. This will involve embedding them in resin blocks and then carefully polishing away material until the particles are exposed at the surface. This may sound like a simple process but many of the particles are so small (around 10 micrometres - similar in size to a single human blood cell) that they can be polished away completely in a couple of swipes.
There is some complicated work ahead of us but we’re already seeing positive results, so I’m looking forward to sharing the results with everyone.
