Dear Editor
Most of your readers know about the fireball that exploded in the sky near Rockhaven on the evening of Feb. 21, and are likely aware numerous people have been searching for meteorites that might have been produced. So far, no meteorite fragment definitely attributed to this event has been found. It seems likely that if a specimen is to be found it will be by a farmer while working his or her land. In fact most of the 62 meteorites known to have fallen in Canada have been found in this way. Most of these include a single specimen only; others are represented by many individual fragments, such as the nearly 2,000 specimens of the Buzzard Coulee meteorite that were found resulting from the fireball over Lloyminster on Nov. 20, 2008. We anticipate that only about 100 meteorite fragments may have been produced by the Feb. 21 fireball.
I represent a team of meteorite scientists who would like to obtain as much information about the Rockhaven event as possible, especially to determine exactly where meteorite fragments fell and what type of meteorite it is. Our best estimate of the fall area is somewhere in the central portion of the "box" outlined by Wilkie, Unity, Cut Knife and the Battlefords. This is based on eye-witness accounts and the video taken by a number of security cameras. More precisely, we anticipate specimens are more likely to be found over an area of perhaps 100 square miles, centered a few miles southwest of Rockhaven. The extinct village of Swarthmore3 is the only named locality likely to be within the fall area (strewn field). This is a projected estimate, and the center point could easily be out by several miles. We ask anyone working their land within the region to be on the look-out for unusual rock specimens that might be meteorite fragments.
Most people have heard fresh fallen meteorites are dark, heavy, and will attract a magnet. As a generality these are true, but a bit vague, so in the interests of accuracy I'll explain a further. Besides, some 'normal' Earth rocks fit this description too.
We recognize three broad categories of meteorites based on the presence of metallic iron: iron meteorites, which contain almost entirely metallic iron and thus attract even weak fridge-magnets; stony meteorites, which can contain up to about 25 per cent grains of metallic iron, and most of which can also attract a small hand magnet (however, about two per cent of all meteorites will not attract a small hand magnet); and what we call stony-iron meteorites, which contain about 50:50 iron and stone.
Regarding their weights, the iron meteorites are about three times as heavy as a normal fieldstone of the same size; the average stony meteorite is about 50 per cent heavier (but these vary); and stony-iron meteorites are more than twice as heavy. Stony meteorites are by far the most common - about 94 per cent of all witnessed falls (eg. Buzzard Coulee) -- so it is most likely that the Feb. 21 fireball produced this type of meteorite. Irons are next and account for less than five per cent of all witnessed falls, but these tend to be the largest meteorites and make up nearly 90 per cent of the meteorite mass we have found. Less than 1.5 per cent of all witnessed falls are of the stony-iron variety.
So, how can you tell iron grains from stone grains? A file will help do the job. Pick a corner of the rock and file through the outer millimetre or so, exposing about 1 square centimetre of the rock's interior. Iron grains will be shiny and will appear much like you expect. In a stony meteorite the iron grains will be a millimetre or so across, but will be larger in the stony-iron type. Stone will normally be pale to medium grey, but the colour can vary from black to dark red, and small irregular white inclusions may be present in some. Roundish particles that vary from white to dark grey are commonly present in stones. In most stony-irons the stone grains will be up to a cm across and may be green or dirty yellow. Gets complex, but most of this is for an expert to figure out. In fact, I recommend you do not mar the surface of a fresh fallen meteorite, leave that for an expert when it comes time for a detailed examination. Some specimens may break upon landing and expose a portion of the interior.
The surface of a fresh-fallen meteorite is often the best first indicator. Meteoroids (small asteroids) enter Earth's atmosphere with cosmic velocities of around 20 km per second, but this slows to landing speeds of a few hundred meters per second. Friction with the atmosphere typically causes an incoming meteoroid to breakup explosively (producing a fireball or a succession of fireballs). The surfaces of the many fragments produced will melt, leaving a trail of smoke-like tiny droplets. When friction has caused a sufficient drop in velocity melting ceases and whatever liquid is left coating the fragments will solidify, producing a thin layer of glass that we call a "fusion crust"; typically less than two millimetres thick. The interiors of the fragments nearly always stay cold because meteoroids enter the atmosphere with temperature colder than - 200 C and do not have time to heat up. Fusion crusts are nearly always black, often with a velvet-like texture, but can be dark brown, and tan ones have been found. With time these crusts will become rusty and, depending on climate, the rock may rust so badly that only an expert can identify it as a meteorite. But fresh falls are often very distinctive and once you've seen one you'll never mistake the look for something else.
Finally, small, typically shallow melt pits may form, called regmaglypts, but more popularly known as "thumb prints" because of their size and appearance.
I hope that you will be able to keep an "eye on the ground" this spring, and that this letter will be of some help. Please email me if you have questions about this or about meteorites in general. I will be glad to identify and authenticate anything you find. Usually I can identify a fresh meteorite from a couple of good photographs, so take pictures and email (or snail mail) them to me. Please determine the location of anything you feel might be a meteorite specimen.
This letter is written as a private individual, not as a legal representative of the University of Saskatchewan.
Mel Stauffer
Saskatoon
Department of Geological Sciences
114 Science Place,
Saskatoon
S7N 5E2
