Collection Methods of Cosmic Spherules

The Three Main Steps:

  • Jon Larsen collecting micrometeorites from a rooftop

    Collecting - getting the dust

    Although spherules fall all across the Earth’s surface, there are some places to look where it’s much easier to find them. While scientists often organise exhibitions to Antarctica or survey the ocean floors, the most convenient location might just be your roof. So pick up a broom and a magnet and get ready to collect some dust!

  • Spherules on sticky tape

    Picking - using the microscope

    Once you’ve collected enough dust from your roof (or from rocks) you can now begin the hunt for spherules! Look down a reflected light microscope into your searching dish and scan around for something round, shiny, and dark in colour. It may take some time to get use to identifying and picking them, so definitely read some of our tips to make the process go as smoothly as can be!

  • spherule being analysed on a scanning electron microscope

    Analysing - textures and chemistries

    Finally found your spherules? Amazing! Get your camera ready to take some beautiful photos. You can see some really interesting textures under a normal optical light microscope, but you can take it further by analysing your samples using a scanning electron microscope if you have access to one. This is the last step in scientifically confirming you have an extraterrestial cosmic spherule!

dust collected in foil

The amount of dust you collect will vary. This sample was collected from a 400m squared rooftop in Egham, England. Image credit: ICL MM research group.

dustpan and broom on a flat roof with dust

Dust on flat, smooth rooves can be easily collected using a dustpan and broom. Image credit: ICL MM research group.

Urban rooftop spherule hunting

Micrometeorites fall all across the Earth’s surface, from the poles, to the tropics, and even your rooftops. Some places are better to go spherule hunting than others, for example:

  • Deserts contain a lot of sand which dilutes the concentration of cosmic dust and magnetic material.

  • Vegetated lands are hard to search through physically and they often coincide with areas of high rainfall which can chemically weather and alter cosmic spherules.

Rooftops are a great collection site in urban areas because the cosmic dust isn’t often swept away, and their height above ground means they are less likely to contain similar looking dust pollutants such as industrial metal. Here is what to do if you go rooftop hunting:

  1. Look for a large (>150m²), flat, smooth vinyl rooftop which can be accessed safely (make sure you have permissions to do this).

  2. Go to the roof on a dry day where rain hasn’t fallen that week and sweep the dust into a plastic bag using a normal dustpan and broom. Look for areas where dust may accumulate such as crevices near the drainage pipes.

  3. Get a fine mesh sieve with gaps that are between 0.3mm and 2mm in size (a normal tea strainer will do) and start sieving the material. The larger size fraction can be discarded since we are interested in the dust!

Instead of sweeping rooves, you can also place buckets under rain catchers to collect sediment which will drain away during rainy days. Put a strong magnet wrapped in plastic at the bottom of the bucket so that the micrometeorites stay put, and a wire mesh on the top to prevent large debris such as leaves falling through. Come by every few days/weeks to clean the top of the mesh, and after about 6 months you can gather the collected sediment into a plastic bag and start your searching!

Other sources of fallen micrometeorites

Finding micrometeorites isn’t just limited to sweeping up your roof. As cosmic dust falls onto every part of Earth’s surface, micrometeorites can be found in a lot of different places – some of them maybe a little more exciting than your roof:

  • An expedition to Antarctica to hoard dust and dirty ice may not sound like your dream adventure, but scientists have built up a collection of micrometeorites from Antarctic ice. This is thanks to the movement of ice and glaciers, which helps concentrate meteorites and micrometeorites into a small area. Meteorites, and areas rich in micrometeorites, can then be easily identified and collected. Many early studies of micrometeorites are based on samples found in the Antarctic collections.

  • The surface area of Earth is 70% ocean, so a sizeable fraction of micrometeorites falling on Earth end up in the oceans. Micrometeorites, being predominantly made up of metals, are heavy and will sink down the water column to the ocean floor. Therefore, deep sea sediments are rich in micrometeorites.

  • Micrometeorites have been falling on Earth ever since the Earth’s formation – so where have they gone?

    Unfortunately, most of them would have weathered away after being exposed to elements for millions of years. However, a few would have been preserved within sediments – think insect enclosed in amber. They can now be found as “fossil micrometeorites” within sedimentary rocks, which is an exciting field with a lot of active research at the moment.

    These fossil MMs have the potential to tell us a lot about Earth and the environment back then – for example, an MM’s degree of oxidation can tell us about how much oxygen was in the Earth’s atmosphere when it fell! However, not all of them will look like they fell yesterday and it’s more challenging to analyse heavily weathered ones.

A chalk quarry in Stevns Klint, Denmark. The ICL MM research group visited this location to obtain fossil micrometeorites from the white Cretaceous chalk. Image credit: ICL MM research group.

Sampling techniques

There are a number of sampling techniques used. Most research groups choose a method based on the type of host sediment, as well as the expected size and fragility of the micrometeorites (if known).

For urban rooftop samples, the most common practice is magnetic separation:

  1. Prepare a searching dish - this should be a ~10cm diameter clear glass petri dish with a labelled grid stuck to the bottom and a removable lid. Set it on top of an A4 sheet of paper.

  2. Use a sieve to dust a thin (~1mm) layer of your acquired sediment on a flat, smooth surface.

  3. Place a neodymium magnet inside a small plastic bag with no holes.

  4. Hover the covered magnet just above the layer of dust.

  5. When you’ve successfully gone over the entire layer, move your covered magnet over your searching dish and carefully remove it from the bag. The magnetic particles should fall onto the dish. The paper beneath will catch any stray particles which can also be put into the dish.

  6. Clean up the remaining sediment and repeat the process as necessary until all your collected dust has been gone through.

Other than simple magnetic separation, check out some of the other sampling techniques that have been used below!

  • Fossil micrometeorites are often iron-rich and unaffected by weak acids such as diluted HCl. Acid dissolution is useful for processing acid-soluble rock samples (e.g. chalk, limestones), but not so for clay-rich rocks as it will form an insoluble, sticky residue which will be difficult to search through. Strong acids can alter susceptible S-types, causing a bias in the collection process. This method is often only used when processing many kilograms of host chalk rock as it can be a relatively low effort way in removing the majority of the host sediment, but the resultant clay-slurry is extremely difficult to process.

  • Micrometeorites mainly consist of silicates and/or metals, which can have a density significantly higher than that of its surrounding sediments. Density separation makes use of this property: within a mixture of samples, components with different densities will separate – the lighter portion, such as chalk, will float whereas the heavier portion, which contains the MMs, will sink to the bottom. This method usually involves the use of “heavy liquids” e.g. methylene iodide (diiodomethane), sodium polytungstate and zinc chloride, and/or a centrifuge.

    WARNING: most heavy liquids are hazardous. Any procedures involving heavy liquids must be properly risk-assessed in advance, and carried out in an appropriate environment.

  • This is a composite method which involves the use of alcohol and magnets. The sample is first immersed in alcohol (>90% ethanol would do) in a container. A magnet, sealed in plastic, is slowly immersed into the mixture, just above the bottom where the sediment will settle. Magnetic particles will become attached to the magnet, and the sample is put aside to rest until the alcohol has evaporated to expose the magnet to air. The magnet is then removed from the container and the plastic carefully unwrapped to collect the adhering particles.

  • A water bath, or a sonic bath, is very useful in cleaning samples by removing debris on the surface of particles. This is particularly the case for urban rooftop sediments/samples. However, this does not significantly affect the concentration of the magnetic particles, therefore, further processing by sieving and magnetic separation is advised.

A simplified methodology for cosmic spherule collection.

A simplified methodology for cosmic spherule collection. Image credit: ICL MM research group.

Neodymium magnets in a plastic bag, hovered over a thin layer of micrometeorite-bearing dust.

Neodymium magnets in a plastic bag, hovered over a thin layer of micrometeorite-bearing dust. Image credit: ICL MM research group.

Micro-manipulation (finding the hidden spherules)

Recognising a tiny cosmic spherule surrounded by sediment is not an easy task for the untrained eye. There are many tiny particles, some natural and some man-made, that can easily be mistaken as micrometeorites. The main criteria for identifying micrometeorites using a light microscope are:

  1. Morphology – a spherical shape. Much rarer are sub-spherical, teardrop and dumbbell shapes, and multiple spherules fused together.

  2. Colour – black or extremely dark in colour, or white and translucent if glassy

  3. Lustre – a shiny, metallic lustre; or a glassy, vitreous lustre

Unfortunately, there are plenty of industrial spherules that can be mistaken as cosmic spherules. Typically, these are black, spherical and have similar sizes, but subtle differences. Check out Micrometeor-Wrongs for more information!

Once you have found a contender, you will need to successfully pick up the spherule…

  1. Prepare a container to hold your samples - this can be a small plastic box with a lid and double sided sticky tape stuck inside at the base, or an SEM stub.

  2. Use a picking tool such as a micromanipulator needle or a simple artist’s paint brush with most hairs cut off.

  3. Slightly wet the brush and touch the spherule under the microscope with the bristles to so it can latch on.

  4. Now place your container under the microscope and focus the light onto the tape.

  5. Carefully transfer the spherule over to the container and stick the spherule down onto the tape.

Congratulations! You just picked a spherule! Check out some hints and tips below to make this process as smooth as possible.

If you are confident of your micrometeorite identification skills, start building up a micrometeorite collection of your own! Once you have plenty of samples in your collection, you might want to reach out to researchers to verify your collection – we are all very friendly and don’t bite, so don’t hesitate to get in contact!

A photo taken down a light microscope of a collection of Antarctic cosmic spherules placed on double sided sticky tape. It is normal to accidentally stick other grains on the tape which may have been attached to the picking brush. To avoid this, try cleaning your brush before you pick up a suspected cosmic spherule. You do not have to place your spherules onto a grid like this, but it can help you keep track of where they are (since they are so small)! Image credit: ICL MM research group.

Hints and Tips

  • It is very common to lose spherules, even for professionals! You can practice picking and transferring by using any small particle so you can be more confident with the real ones.

    Cosmic spherules also take a lot of practice to recognise so train your eyes by looking at some photos in our collections!

  • Sometimes spherules can hide under other dust grains. To disperse particles and reveal a stronger shiny lustre, wet your searching brush and gentle tap the clump or surrounding area with the bristles. Water is fine for rooftop or Antarctic samples, but ethanol is more useful when searching through clay-rich sediment to prevent further clumping.

  • If your brush is too wet, you can create a pool of water in your dish which can cause your spherule to wonder off, but if your brush is too dry you can accidentally fling the spherule away. Make sure your brush is only slightly damp by wetting it and drying the residual water with a tissue.

  • Having a labelled grid underneath your searching dish can save you a lot of time by helping you systematically search through the magnetic separates. It can also help you remember your last visited location if you take a break.

  • Take eye breaks every 30 minutes or less to avoid straining your eyes, and keep the brightness on the microscope to a level you are comfortable with.

  • A reflected light microscope is necessary to search for micrometeorites. The best for lab uses are from Zeiss or Leica, but most others can work too.

    Any artists paint brush with thin bristles that are cut down to reduce their density will be fine for picking.

    Petri dishes and small single jewelry plastic boxes from Amazon can be very useful for searching and holding picked samples.

Magnetic particles clumped together in a searching dish

Magnetic particles tend to clump together after magnetic seperation. There can be hidden cosmic spherules within these areas! Use water or ethanol to disperse the grains and find those sneaky micrometeorites! Image credit: ICL MM research group.