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CHAPTER 1
MICROMETEORITES EVERYWHERE
MICROMETEORITES belong to the oldest matter there is. They are mineral remnants from before the planets were formed and may even contain particles older than the sun that have traveled farther than anything else on Earth. We are just beginning to explore these microscopic alien stones, yet they are everywhere.
WHAT IS A SPHERULE?
Most micrometeorites retrieved on Earth are cosmic spherules. A spherule is a rounded object, a solidified melt droplet of stone and/or metal. The sphere is nature's efficient solution for maximum volume and minimum surface. Surface tension while the micrometeorite is still in a liquid state bends the object into a sphere. Raindrops are also formed this way.
In nature, rock can melt and create spherules in three ways: by volcano, lightning, and meteorite. On an uninhabited world, we would only have to distinguish these three types of naturally occurring spherules from one another to identify the extraterrestrial particles. There are, however, traces of human activity everywhere on Earth, in all sediments and layers younger than the Industrial Revolution dating to the 1760s. This is why scientists at the Scott-Amundsen Base at the South Pole drilled down through the ice to the strata from one thousand years ago to melt drinking water. There, however, they found other types of impurities in the ice: micrometeorites.
Power tools and industrial processes produce a huge number of anthropogenic (manmade) spherules, which are everywhere on Earth's surface. The search for stardust is therefore a question of separating the terrestrial (both natural and manmade) from the extraterrestrial. When you know what micrometeorites look like, and what to disregard as anthropogenic spherules, you can find stardust. But where shall we begin the search?
SEARCHING FOR THE RIGHT PLACE
To find micrometeorites, we must understand two concepts. The first is accumulation, the gradual increase in number or amount over time. Despite the total global influx of approximately 100 metric tons of cosmic dust particles per day, the rate on the ground is low, simply because there's so much ground to cover, not to mention that water covers more than 70 percent of Earth's surface. We can expect one extraterrestrial particle with a diameter of 0.1 millimeter per square meter per year.
The longer a skyward surface has accumulated particles from above, the more micrometeorites there are to be found. Contrary to existing literature on the subject, cosmic spherules do not erode "in weeks." In my studies I have found that the older an accumulating area is, the more micrometeorites one is likely to find. The exception is if prevailing winds are especially unfavorable, in which case the yield might be low. Of nearly two thousand cosmic spherules examined, I have seen signs of slight erosion on only a couple of particles. Our hunt for stardust, therefore, begins with a search for a hunting ground with favorable accumulation.
The other concept we must understand is signal-to-noise ratio. In the context of micrometeorites, this means how many micrometeorites (signal) there are compared to terrestrial particles (noise).
For years, micrometeorite hunters have built traps of various types. Few have succeeded because cosmic dust particles are rare. To catch hundreds of cosmic spherules, a trap would have to be the size of a football field and accumulate particles over decades. The challenges connected with such a construction have discouraged more than one good scientist. There are, however, "traps" already in place and ripe for harvesting: roofs.
When I started to hunt for micrometeorites, I began by searching for large, old accumulating areas, such as roads and parking lots. But I did not find anything other than myriad anthropogenic spherules. Not until I moved the search up one floor closer to the sky — to the roof's rain gutter — did I start to find extraterrestrial treasures. The explanation is an improved signal-to-noise ratio.
On the roof there is less human activity, and thus there are fewer anthropogenic spherules. The difference between the signal-to-noise ratio in roof dust and urban road dust is enough for us to find micrometeorites in the former while making the latter very difficult. Consequently, the hunt for urban micrometeorites begins with finding the right roof. Thanks to the stronger signal-to-noise ratio, almost any roof or surface above the turbulent ground will do. A pitched roof with an easily accessible gutter is an excellent place to start. The larger and older a roof is, the better. Particles fall from above, roll down roof tiles or shingles, and accumulate in rain gutters. By placing a bucket under the downspout for a year or two, one may even catch those that the rainwater washes away.
The fewer particles the roof decking contributes to the content of the gutter, the better. Glazed roof tiles, metal plates, stone, wood, and vinyl are favorable roof materials for a successful micrometeorite hunt. With experience you will recognize a promising roof by its accumulating properties and signal-to-noise ratio. Then the result of the hunt accelerates.
Pitched roofs aren't the only place to search. Flat roofs, which usually have safety walls around the edges, serve as micrometeorite traps. On roofs such as this, loose particles accumulate at the lowest points: around the drains, along the walls, and in the corners.
A flat roof is not entirely flat but constructed of slightly pitched sections, each of which leads water into a drain. Drains are usually located in the middle of the roof, making it a bit safer for us to conduct our search away from roof edges. Moreover, the drain itself is often elevated slightly above the lowest point, leaving the small particles to accumulate there for star hunters such as us. Crush or pulverize any lumps and extract the magnetic particles.
It is important to emphasize HSE (health, safety, and environment). Remember your sunscreen. Also, roofs are the domain of birds, and in some places their droppings may be the main constituents of the roof sample. This can present a biohazard. I always use gloves; a dust mask (N95 type) is also recommended. A disinfecting soak of the sample in rubbing alcohol prior to the rinse will make it even safer to handle.
Micrometeorites are so small that the prevailing winds, not gravity, ultimately determine where they end up on a flat surface. In this respect cosmic dust may be considered a type of aeolian sediment (i.e., carried by the wind). Safety walls along the edges of a flat roof or a new floor rising opposite to where the prevailing winds come from (see photo on page 16) will act as micrometeorite traps, allowing the extraterrestrial particles to accumulate. If, in addition to these factors, the roof is covered with PVC vinyl, the signal-to-noise ratio will be stronger because PVC does not create dust particles that can be confused with micrometeorites, offering an optimal starting point for a successful hunt. Use a dust broom to gather loose debris and extract the magnetic particles. When finished, take a few photos of the roof and the sample in situ.
In a place where you suspect micrometeorites are abundant, it is recommended that you also take a nonmagnetic sample. As mentioned, the micrometeorite collection from the water well at the South Pole base contains around 20 percent nonmagnetic glass spherules — completely melted micrometeorites. To search for these beautiful stones in a promising place where the signal-to-noise ratio is expected to be particularly strong, we can scrape up the remaining particles after having extracted the magnetic ones. Make sure to mark the sample "nonmagnetic."
The rooftop finds in urban micrometeorite collections are found on the roofs of buildings no older than 50 years. Consequently, it can be assumed that these stones have a terrestrial age of 0 to 50 years, which makes them fresh compared to most micrometeorites in other collections. Most of the Antarctic stones, for example, have a terrestrial age of 1,000 to 1,000,000 years and are weathered accordingly. Some are eroded beyond recognition. Therefore, the urban micro meteorites look a bit different — the pristine cosmic spherules are less eroded and have distinct surface textures. This enables us to identify fresh micrometeorites by visual examination.
By monitoring a skyward-facing area at regular intervals, it should be possible to be even more precise in future sampling, perhaps down to the week — or even day — that a micrometeorite fell to Earth. With careful preparation and cleaning of the collection area around annually reoccurring meteor showers, it should be possible to identify material from some of the comets and possibly detect variations in the influx rate over time.
We are still in the beginning stages of understanding micrometeorites. We have managed to break the code and find micrometeorites on roofs, where the signal-to-noise ratio is better than it is on the ground below. But in the future, we may even be able to separate the trillions of micrometeorites from the vast amounts of urban road dust.
Micrometeorites fall like a gentle cosmic rain in equal amounts everywhere on Earth. There is no need for an expensive expedition to find them — the near- est roof will do. To find a lot of micrometeorites, however, it is possible to use Google Earth to search for exceptionally large roofs. Schools, shopping malls, sports arenas, airports, and industrial buildings are superb hunting grounds. The best results are found on large, old, PVC-covered flat roofs with security walls around the edges. In such a place one may find a hundred micrometeorites in a single search.
When you have discovered a promising roof, send an email to the owner, explain the project, and politely ask for permission to take a scientific dust sample from the roof. Some owners will deny access for safety reasons. Others might be interested and welcome you for a brief sampling. Climbing a ladder and working on a high roof is a potential risk, so do not take any chances.
CHAPTER 2
SEVEN STEPS TO HEAVEN
HOW DO WE FIND A MICROMETEORITE hidden among billions of other particles? Micrometeorites offer two clues that will help us in the process.
First, most micrometeorites contain small amounts of iron and nickel. These we can extract from the rest of the particulate we gather with our weapon of choice: a magnet. In the cosmic spherule collection from the South Pole, for example, approximately 80 percent of the micrometeorites are magnetic.
The remaining 20 percent of spherules are nonmagnetic. Here, the metal content has been evaporated completely by the frictional heat created during atmospheric flight. These spherules we have to search for in a different way, which we will get to later.
Meanwhile, the short road to finding 80 percent of micrometeorites is a magnet. Any type will do, but the stronger the better. I use a handheld neodymium magnet that measures 40 millimeters in diameter and has a hook in the center that serves as a grip. The magnet shown on page 20 is the one I have used to find all the urban micrometeorites in my collection.
The other property that will help us separate micrometeorites from terrestrial particles is their size. Most cosmic spherules are between 0.2 and 0.4 millimeter in diameter. Magnetic extraction plus screening for size narrows down the possible candidates in a dust sample sufficiently to start searching with a microscope for the proverbial needle in the haystack. An iron needle can be found relatively easily in a haystack if you have a magnet. But how do we use the magnet out in the field, in the rain gutter or on the roof?
Not only do we want to catch micrometeorites with a magnet, we want to release them again in a controlled way at the right moment, in the right place. And we want to avoid contamination of particles from one field search to the next. There are seven steps in the process of separating the micrometeorites from the terrestrial dust.
STEP 1. Put the magnet inside a small plastic ziplock bag; this is bag number 1 (see photo on page 21). Hold the magnet by the hook from the outside. The plastic bag keeps the magnet clean and avoids transmitting particles from one place to another. Change it after each field search, or more often. These small plastic bags are sold in packages of a hundred for a few dollars.
STEP 2. Put your entire hand with the magnet inside a larger plastic bag. The outside of bag number 2 will be the magnet's contact with the ground. Rough particles in the dust will act like sandpaper and make small holes in the plastic. If complete separation of particles from one place to another is required, bag num- ber 2 should also be changed often. Bag number 2 must be held tightly around the magnet, so it gets as close contact with the dust on the ground as possible.
STEP 3. Hold a small, empty ziplock bag in the other hand, ready to collect the dust sample. This is bag number 3. Put the magnet with the dust particles ("magnetic catch") from the ground at the opening of bag number 3. Hold back bag number 2 with one hand while pulling the hand with the magnet out of bag number 2. When the magnet is removed, any magnetic particles will be released and fall into bag 3. Repeat the sequence until the required sample is obtained.
Use the magnet to take dust samples in places where small particles have accumulated over time. Wind and rain sort particles by size and weight (density). If the particles are moist, the magnet will not work because the adhesion of water is stronger than the magnet. In these cases, use a spoon to obtain a sample and put it in a plastic bag marked with the place and date. Later on, dry and rinse the sample (see step 5); then you may use the magnet.
Sampling dust on a flat roof is more efficient when you use a dust broom to gather loose particles before using the magnet. Hard lumps of dry dust should be broken apart. Around a drain or in a pitched roof's gutter, a stiff dishwasher brush can be useful. Alternatively, collect the entire contents in a large plastic sack and later rinse the sample by flotation (see step 5, alternative method).
STEP 4. Mark the field sample in plastic bag number 3 with a permanent marker. Write the finding place, date, and any additional notes. This may seem insignificant, but sometimes there is not time for an immediate examination of the field sample. Then it is important to have marked it properly for later.
It's advisable to follow the weather forecast and try to choose a dry day for your field search. This will save extra work. If the sample is moist, the magnet will not extract the magnetic particles. If you must collect a moist sample, an alternative solution is to gather it in a plastic sack and clean it later, either by flotation or magnetic extraction followed by rinsing (see two methods described in step 5).
STEP 5. This step is all about cleaning the sample with water. On a dry, sunny day it is easy to extract magnetic particles in situ, whether the field search is conducted in a rain gutter or on a flat roof.
For the step 5 rinsing process, use a bowl, hot water, some dish soap, and a plastic spoon. The micrometeorites are mainly black, so whenever possible I use white porcelain or plastic buckets, bowls, plates, and plastic spoons for the entire process.
Fill the bowl halfway with the hot water and some dish soap. Pour in the sample and stir; as the water turns black, stir more. Any organic matter will float, while the mineral particles will sink. Stir again. After a while, let the mineral particles sink and slowly pour off the froth and dirty water. Make sure to keep the heavier particles at the bottom. Repeat this process until the organic particles are gone and the water remains clear.
STEP 5 (FLOTATION ALTERNATIVE). Sampling for micrometeorites on a dry day is efficient. After a while with a magnet on a roof following steps 1 through 4, you might extract a sample of magnetic particles from the place where they accumulated. When the time comes to rinse the sample (see step 5 above), a couple of grams will take only a few minutes. However, if conditions on the roof are moist and the sample is a plastic bag full of mud, we must clean it in a different way: by flotation.
An old roof may have accumulated so much debris that the lumpy sediments in the corners or around the drain never dry out. Or you may only get permission to take a sample from a roof on a rainy day. Or maybe you live in an area with a lot of rain. And on a pitched roof the rain gutter may be so full of windblown soil, moss, leaves, pinecones, or sand that it is always wet. In all four cases it is still possible to separate and collect micrometeorites.
In the case of the pitched roof, we must empty the entire contents of the gutter. This should be done once a year anyway, and many homeowners will even thank you for doing it for them. Follow ladder safety procedures and climb up to the gutter, then hang a plastic bag on the ladder. Wearing rubber gloves, use one hand to get out most of the content from the gutter while gripping the ladder with the other. Put the lumpy gutter sample into the bag and use a stiff brush to gather the rest of the mud from the bottom of the gutter. This may be where the heavier micrometeorites are hiding. It is a dirty job, but someone has to do it, and the reward is celestial.
(Continues…)
Excerpted from "On the Trail of Stardust"
by .
Copyright © 2019 Jon Larsen.
Excerpted by permission of The Quarto Group.
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