Why haven't you gotten "wet"
from the meteorite rain?

Alan Hildebrand

Once in a while you read that a meteorite has hit a building or car. Or landed on the ground next to someone. Why has it never happened to you? The Canadian Meteorite Observation and Recovery Project (MORP) that operated on the prairies from 1974 to 1985 recorded meteorite falls and Halliday et al. (1989) established from the MORP data that ~7,000 meteorites weighing more than 1 kg fall on the Earth each year (note that these are masses on the ground, not pre-atmospheric masses). However, only about five meteorites are recovered as fresh falls each year. Why only five?

A 1-kg meteorite is about the size of a large man's fist. Most recoveries occur because someone sees a meteorite strike the ground near them; less frequently a meteorite is recovered because it fell in the "space" that they use - their house, yard, or driveway - and is known to be out of place. Assume that the Earth's human population (potential meteorite fall witnesses) is five billion, but that only one billion are really effective, because of age, sedentary occupation/habits, or high population densities (many people living in same building in a city); further assume that only one third are "awake" at any given time (otherwise are asleep, eating, reading or visiting relatives in the city). For the remaining 300 million potential witnesses let us assume that an area with a radius of 20 m is the effective cross-sectional awareness of the average person. (Most meteorites noticed to fall are closer to the witness than this. In the extraordinary case of the fall of the large Baszkowka L5 chondrite in Poland, August 24, 1994, Halina Grodzki saw the soil disturbance of the meteorite striking the ground ~200 m away.) This yields an effective collection area of ~400,000 km2 or 0.00075 of the Earth's surface. So the fraction of 7,000 meteorite falls that should be seen to hit the ground is 5.3 per year. It is that simple.

Assuming that you are one of the one billion effective observers, your chance of seeing a meteorite land near you during your 70-year lifetime is about 1 in 2.5 million. Most Ottawa RASC members are somewhat redundant city dwellers, though, so that our chances are reduced. In Canada, our 6 million effective observers represent 0.006 of the world's billion witnesses or 0.03 meteorite falls per year (or a predicted ~30 years between witnessed falls). The most recent fall (St-Robert) was seen by Stephane and Mme Forcier on June 14, 1994 (whose awareness cross-section was extended by a herd of dairy cattle). We have had 12 recovered falls during our nation's history vs. an expected three, so our rural nature and comparatively high scientific awareness has given us a few extra falls, but targeted meteorite research efforts are responsible for the recovery of several: Innisfree (1976), Vilna (1967) and Revelstoke (1965).

So, if all these meteorites are falling and nobody is seeing them, then how many are out there rusting away in the dirt at Rolf and Linda Meier's place?

Let us consider the Canadian prairies where most of Canada's meteorite finds occur (most recent recovery: Hodgeville, Sask., 1996). An area of ~700,000 km2 is bounded by 96°-114° W longitude and 49°-54° N latitude and encompasses the Canadian prairies. The glaciers that scrubbed the recording surface back to zero withdrew sequentially across this area, and periglacial lakes kept sedimentation rates high in some areas for a while, but 10,000 years of postglacial exposure is a reasonable average for the terrain. Approximately 10 falls/year occur in this area for meteorites >1 kg in mass. One hundred thousand have fallen since the ice left. Extending the mass range to >100 g, increases the total by a factor of ~4. One >1 kg meteorite occurs in every 7 km2 on average. This does not include the effects of fragmentation which happens in almost every sizable fall; an effect that would multiply these totals by 10 to 100 if my intuition is correct (although many of the fragments will be <1 kg). So, simplistically, a meteorite occurs in every ~0.7 km2 using the conservative fragmentation factor, although the strewnfield-limited dispersion pattern does not spread these fragments randomly. So while Rolf and Linda may not have one >1 kg on their ~0.4 km2 property, they could well have one >100 g.

It might also be of interest that the largest single fall on the prairies since the glaciers withdrew would have been of ~1,000 tonnes mass - an 8-m diameter object on the ground if it had stayed in one piece.

The Ottawa Centre will be looking for a new meteorite coordinator in the fall. Has anyone got a geological background or an interest in meteorites? The pay is good and the fringe benefits are out of this world. Call Alan Hildebrand at 563-1092.


Reference:
Halliday, I., Blackwell, A.T., and Griffin, A.A., 1989, The flux of meteorites on the Earth's surface: Meteoritics, v. 24, p. 173-178.
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