Urban micrometeorites – a myth?

Is it possible to find micrometeorites in populated areas? The question has been raised for nearly a century and, despite numerous attempts to find them, the answer up to this day has been a very short “no”. Meanwhile, our knowledge about these amazing stones has gradually increased. There is a continuous evolutionary line in the research on micrometeorites, from the early pioneers, John Murray and Adolf Erik Nordenskiöld in the nineteenth century, to Lucien Rudaux and Harvey H Nininger. With Donald E Brownlee and Michel Maurette in the 1960s, micrometeoritics became real science.

During the past two decades, this research has accelerated thanks to, among others, Susan Taylor, who extracted micrometeorites from the water well at the South Pole, Matthew Genge, who figured out the classification, and other splendid researchers, in addition to the space probes that have returned to Earth with dust samples from comets and asteroids. Today, there is a growing literature about micrometeorites, but still the answer to the initial question is “no” and urban micrometeorites have been considered an urban myth.

Micrometeorites have been found in the Antarctic, but also, to some extent, in prehistoric sediments, remote deserts and in glaciers – places that are clear of the confusing anthropogenic influence. The wall of contamination has been considered insurmountable. It is therefore with pride and joy that I can report here about a project involving the systematic examination of all sorts of anthropogenic and naturally occurring spherules in an empirical search for micrometeorites in populated areas. This research has resulted in a new urban collection of pristine cosmic spherules. The findings have been analysed at several different institutions and, in January 2017, a randomly selected subset of 47 objects from the new collection was prepared for wide beam electron microprobe analysis at the Natural History Museum, London by Dr Matthew Genge (of Imperial College). Nine porphyritic olivine, 23 barred olivine and 15 cryptocrystalline spherules were identified and have textures and mineral compositions identical to Antarctic cosmic spherules. A scientific paper about these new discoveries (Genge et al.) is pending publication, but meanwhile, I can present the results here in Deposits.

The project was initiated in 2009 with a minimum of equipment: a magnet, plastic bags, a sieve and a microscope. To begin with, I sampled accumulated mineral particles from skywards facing hard surfaces like roads, roofs and parking lots in Oslo, and then graduated to look in industrial areas, other cities, countries, mountains, soil, glaciers, beach sand, volcanoes and deserts – that is, everywhere. Now, seven years later, I can look back on nearly with one thousand field searches of about 50 to 5,000µm size particles from nearly 50 countries, all continents represented. The samples were examined in a Zeiss binocular microscope, and interesting particles picked out and photographed with a USB microscope with higher magnification. Promising candidates were analysed using SEM/EDS. I established a photo database (now containing photos of more than 40,000 individual objects) and kept an illustrated journal while I tried to look for patterns. Due to consistently contradictory data in the literature, I put my complete trust in pure empiricism.

Nine new micrometeorites
Fig. 1. A range of various micrometeorite. (Illustration by Jan Braly Kihle/Jon Larsen.)

To begin with, the various types of anthropogenic and naturally occurring terrestrial spherules seemed infinite and chaotic, but with time, I started to recognise the most common ones. Gradually, I could start the process of systematisation. There are surprisingly small variations in the types of spherules found in comparable environments around the globe. My recently published book, In Search of Stardust, is an atlas of the various types of spherules, and the approximately 30 most common types that represent most of all spherules found anywhere on Earth. In our search for micrometeorites, the knowledge of these contaminants makes it possible for the first time to separate the extra-terrestrial particles from the terrestrial ones. The most recent field searches with improved methodology for processing the samples before the microscopy have given up to one micrometeorite per gram, which is a near match of the Antarctic results.

Cosmic dust belongs to the oldest matter there is: mineral remnants from before the planets were formed. They may even contain real stardust – interstellar particles older than the Sun, that is, particles which have travelled further than anything else on Earth. There is a widespread misconception that micrometeorites are fragments of ordinary meteorites, ablated during their atmospheric flight, but these ablation spherules are not real micrometeorites in the scientific sense. Furthermore, there is a common misunderstanding that micrometeorites are “metal spheres”, but that is only about 2% of them. Most of the cosmic spherules are stony, mainly olivine/orthopyroxene in glass with interstitial magnetite. Their next of kin are the primitive C-chondrites and their origin may lie beyond Pluto. We are just beginning to explore these alien stones, yet they are everywhere around us.

The breakthrough in the search for micrometeorites in populated areas came at last in February 2015, with the discovery of an approximately 0.3mm barred olivine beauty with dendritic magnetite crystals sprinkled over the surface. I started immediately to search for similar stones and found them. At the end of the first season, I had a collection of more than 500 pristine micrometeorites in the size range of between 150 to 600µm, with all the most common types from the classification represented.

For many years, meteorite hunters have built micrometeorite traps of various types. Some have succeeded, like water pools to catch interplanetary dust particles (IDPs), but, given the low influx rate, a really efficient trap would have to be much larger. To catch thousands of cosmic spherules, the trap would have to be the size of a football field (or larger) and accumulate particles over decades. The challenges connected with the construction of something like that have discouraged more than one good scientist. However, there are such areas already in place, possibly in your neighbourhood, and ripe for harvesting: roofs.

The micrometeorites in the new collection were mainly found on the roofs of buildings with a maximum of 50 years of age, so it can be assumed that the stones have a terrestrial age of 0 to 50 years, which make them fresh compared with most of the micrometeorites in the other collections. As a result, some of the surface structures of the micrometeorites in the new collection are different from previous observations, with the glass still intact. With the exception of the Concordia collection from melted snow, most of the Antarctic micrometeorites have a terrestrial age of one thousand to one million years and are weathered accordingly.

By monitoring a skyward facing area like a roof at regular time intervals, it should be possible to be even more precise in future sampling, perhaps down to the week (or even day) that the micrometeorite fell to Earth. With careful preparation (that is, cleaning the collecting area) around the annually reoccurring meteor showers, it should be possible to identify material from some of the comets and possibly also to detect variations in the influx rate over time.

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