Part 10

Because the particles that make a meteor shower are moving through space along parallel paths as they come into the earth’s atmosphere, the meteors all seem to shoot out from a single small area in the sky. You may have seen something like this in the case of the sunrise or sunset effect known as “the sun drawing water.” In this more familiar phenomenon, the sun’s disk is the area from which shafts of sunlight radiate out in a beautiful, if somewhat irregular, fan-like pattern. The area from which the meteors of a given shower seem to come is the radiant of that shower.

Meteor showers are named for the constellation in which their radiant lies. The suffix “-id” (Greek for “daughters of”), or some modification of this suffix, is added to the name of the constellation from which the meteors seem to radiate. The Orionid radiant, for example, is in Orion, the Hunter; the Leonid radiant is in Leo, the Lion; and the Lyrid radiant is in Lyra, the Harp. Exceptions to this rule do occur, however. Astronomers may refer to a shower sometimes appearing on the night of October 9 as the “Giacobinid” shower in honor of the comet Giacobini-Zinner, which is associated with this particle-swarm.

In the course of each year, the earth passes through a number of particle-swarms of varying densities. Some of the resulting meteor showers, like the Leonids and Giacobinids, are very feeble in most years, but sometimes produce spectacular displays.

The more important recognized meteor showers are:

NAME OF SHOWER DATE OF MAXIMUM Quadrantids January 1-3 Lyrids April 21 Eta-Aquarids May 4-6 Perseids August 10-14 Giacobinids (Nu-Draconids) October 9 Orionids October 20-23 Leonids November 16-17 Geminids December 12-13

Certain daytime streams are also known to be active during June and July. These daytime showers are, of course, invisible in the glare of sunlight, but they can be picked up by radar devices like those used in World War II to spot enemy airplanes.

Some meteor showers have been splendid enough to make a place for themselves in the historical record. Examples are the Leonid returns of 1833 and 1866, and the Giacobinid showers of 1933 and 1946. During these displays, meteors fell in a veritable fiery snowstorm, several hundred meteors sometimes appearing within a minute.

Not every annual return of a meteor shower is spectacular, however, since conditions may not be favorable each year for a brilliant display. After all, both parties to a traffic collision at an intersection must try to pass through the intersection at the same time. Our earth, like a well-managed train, always goes through the intersection on schedule, but the particles responsible for meteor showers are much more erratic. They may be early or late—or they may not show up at all. Of the meteor showers seen annually, the Perseids are the most dependable. The Leonids put on their best shows at intervals of 33 years (1799-1800, 1832-33, 1866, etc.). The Giacobinids at intervals of 6½ years (1933, strong; 1939-40, poor; 1946, magnificent).

If you plan to observe a meteor shower, here are some suggestions. You will need:

Acquaintance with the stars, both faint and bright, in the region containing the radiant of the shower.

Comfortable reclining lawn-chair.

Warm clothing (including blankets) for winter showers or summer ones at high elevations.

A patient family that will not only approve of your observing but will help you get up to watch after midnight, when most showers are at their best.

A corner of your back yard (or sun roof) where you can shade your eyes from street lights and other illumination.

Timepiece, preferably with radiant dial.

Sit back and watch Nature put on her show. Any records you make may have some scientific value even if you note only these two things: Hourly number of meteors seen. Condition of the sky (clear, hazy, cloudy, etc.) during each hour of your watch. At present, we know of only one instance in which it seems probable that a meteorite came to earth during a meteor shower. The Mazapil, Mexico, iron meteorite fell at 9:00 p.m. on November 27, 1885, during a return of the now very weak Bielid meteor shower. Scientists still cannot decide whether or not a mere coincidence was involved in this case.

As we have already mentioned, most of the cosmic particles rushing into our atmosphere evaporate and do not reach the earth at all except as the tiny congealed droplets and spherules of their own melt. Some cosmic particles, the micro-meteorites, are so tiny that they “stall” rather than fall down. These minute objects do not melt or disintegrate and so preserve their original cosmic form unchanged. Scientists have developed various methods for the collection of both of these types of material in order that at least rough estimates of their rate of accumulation on the earth can be made.

One of the simplest methods of collecting this so-called “meteoritic dust” is to expose a sticky glycerine-coated glass microscope slide for at least a 24-hour period in a protected spot well away from locations where any industrial contamination is in the air. At the end of the period of exposure, the “catch” on the slide is examined microscopically, and the individual trapped particles are counted and classified. Meteoritic dust is also carried down to the ground by rain, snow, and hail and can therefore be obtained by filtering rainwater or melted glacier-ice, snow, and hail.

Such collection efforts have been plagued by the difficulty of identifying the particles. How can a collector be sure that the dust he has trapped, even though magnetic and possibly even in part metallic, does not come from some smelter or other industrial plant? Because of such uncertainties, the current estimates of the annual deposit of meteoritic dust for the world range from approximately 20 tons to several million tons. We need improved collection and identification techniques if we are to obtain trustworthy figures.

Recent analyses of rainfall records indicate that the infall of meteoritic dust produces at least one interesting weather-effect. These analyses show that rainfall peaks often occur some 30 days after the appearance of important meteor showers. Apparently, as meteoritic dust particles from the meteor showers filter down through the cloud systems in the lower layers of the atmosphere, the individual particles serve as centers about which atmospheric moisture condenses to form raindrops. The time lag of approximately a month is considered to be due to the very slow rate of fall of such tiny particles. It looks very much as if Mother Nature had beaten man to the idea of “seeding” the clouds to produce rainfall!

9. THE NATURE OF METEORITES

So far in this book we have dealt with meteorites indirectly, chiefly in connection with their fall, distribution, and recovery. In this chapter, however, we are shifting our attention to the meteorites themselves, and will tell what the main types of meteorites are, what meteorites are made of, what they look like, and how to tell them from ordinary rocks.

First of all, meteorites neither all look alike nor have the same composition. The general term “meteorite” applies to any mass that reaches the earth from space. Such masses are made up of metals and minerals in varying proportions. The term “meteorite” is nearly as general in meaning as the word “rock,” which geologists apply to bodies, large and small, that are formed by earth processes and are composed of various kinds of minerals. Actually, there are almost as many different kinds of meteorites as there are kinds of rocks; so you can see that in meteorites a wide range of composition and appearance is possible.

All recognized meteorites belong to one of three main divisions, irons, stones, and stony-irons.

The irons are composed of an alloy of iron and nickel which may contain small inclusions of nonmetallic minerals.

After a cut section of an iron meteorite has been polished, the flat surface, except for possible inclusions, is mirror-like and resembles stainless steel. It appears to be remarkably uniform and uninteresting, but this appearance is misleading. A characteristic and beautiful structural pattern develops when such a polished nickel-iron surface is treated with, for example, a special mixture of nitric acid, alcohol, and Arabol glue.

This process of treatment is known as “etching.” The different structural patterns brought out by such etching give us the basis for classifying the iron meteorites.

If the etching process reveals certain features from which we can infer a cubic, or 6-faced, crystalline structure, we classify the iron meteorite as a hexahedrite.