METEORITES AND TEKTITES
If you've ever wondered just what "shooting stars" looked like, or what they are, the following information and photos will satisfy your curiosity.
The "shooting stars," or "falling stars" that you've seen streaking across the night sky are called meteors. Most of them are composed of particles outgassed from comets. Every time a comet enters the solar system, it encounters the solar wind, and as it nears the sun, gaseous material is vented. The vented material is pushed outward by the solar wind, creating the famous comet tails. Bits of rocky material and metals are broken off during these "outgassings," and many bits are left behind in the wake of the comet's orbital plane. As the Earth makes its way around the sun, it occasionally encounters the wake of previously passing comets. The result of our atmosphere plowing through these particulates is a meteor shower, as these particulates are, for the most part, burned up. For the most part, meteor showers are predictable, and occur at the same time each year. The Perseid Meteor Shower in mid-August is from the passage of Comet Swift-Tuttle. The Orionids, in late October, are from Halley's Comet, as are the Eta-Aquarids in mid-May. The Leonids, in mid-November, are from Comet Tempel-Tuttle. This comet has just started to break up nicely, and its debris is especially rich every thirty-three years, just after each cometary pass through our solar system.
Ancient people also noted these showers, and meteoric iron was often used to make ceremonial and serviceable weapons and tools. The dagger that rested on King Tut's chest is made from meteoric iron.
Occasionally, larger bits broken off from planetary or satellite (various moons) impacts are hurled through space and are caught by Earth's gravity. If the piece is large enough to survive its passage through our atmosphere, it will fall to the earth as a meteorite. Meteors are atmospheric. Meteorites actually land upon the Earth. Meteoroids are simply bits of debris in space, too small to be called an asteroid. Asteroids are bodies large enough to be detectable by telescope.
Meteorites generally arrive with a crusty outer shell (the fusion
crust) due to the fact that their exterior has been melted during their entry into Earth's
atmosphere, and the molten material is cooled after impact. Meteors can be rocky,
metallic, or a combination of both. The composition of meteor samples taken as a
whole suggests the possibility that a small planetary body or bodies may have been broken
up by an enormous impact with another body or each other. Much of the material found
in meteorite samples suggests that the material came from a differentiated body--a small planet or
moon that had a core (molten metal that crystallizes), a mantle, and a crust (crustal
material includes carbonaceous chondrites--carbon being the important element).
Since the material seems to be differentiated in some samples, and may be less
terrestrial in others, some meteors may be more icy in composition. This
raises the question of where these particles came from. An obvious answer
is the asteroid belt. That, however, brings up the larger
question..."Where did the asteroids come from?" It would appear
that one or more small planetary bodies or moons have collided and have
scattered debris throughout the solar system. Much of that debris has
found a relatively stable orbit between Mars and
Jupiter. Naturally, if such an impact occurred, particles and chunks
of both worlds would have bombarded the entire solar system, and some may have
even escaped the gravity of our system to travel through space until caught by
the gravity of a distant star, planet, or moon.
Most meteors are tiny...no larger than grains of rice or pencil erasers. They burn up completely when they enter Earth's atmosphere, but as they are incinerated, we enjoy the spectacular "trail" that streaks across the night sky. Most begin glowing at about 100 miles up in the Ionosphere, but are totally vaporized as they reach the Stratosphere about 40 miles above Earth's surface. A few larger pieces may actually make it through the atmosphere, and fall to Earth as meteorites.
Bolides are larger pieces that explode, and leave behind a magnificent glowing trail. Some of their larger fragments may reach the ground as meteorites. Some break up spectacularly, with chunks falling away from the main body. They may resemble an aircraft on fire, and many have been seen in broad daylight.. Those which are seen to explode and the pieces are immediately recovered are called "falls." These are scientifically the most valuable, because they are fresh and uncontaminated by soil erosion. Scientists study the differences between meteorites and Earth-based rocks and minerals to try to understand the composition of our planet, as well as other celestial bodies.
There is some discussion in the astronomical community of the true
nature of tektites. These pieces resemble meteorites, but are generally composed of
obsidian or other glassy materials, and appear to be formed as a result of violent
impacts. Material at "Ground Zero" is melted and blown above the
atmosphere, and is re-melted upon re-entry into Earth's atmosphere. Many of the
pieces show nodule formations on the surface from the melting and cooling of the
material. The bright lunar rays of the newer Craters like Copernicus and Tycho are
formed of this same type of material--impact glass. Much of this material forms
small beads, but occasionally larger pieces are found. These may be brown or
orange-brown, black, or green. Black and green forms are illustrated below.
When fractured, tektites display a glass fracture pattern much like obsidian (a volcanic
glass), rather than simple cracking or crumbling.
Antarctica is an ideal collection ground, for tektites and meteorites are found sitting on the surface--the only color in an immense, white landscape. A meteorite that was subsequently discovered to have come from the surface of Mars was found on the ice fields of Antarctica. Some great planetary impact threw fragments of Mars out into space where Earth's gravity captured some of them. A few moon rocks have also been found in this manner.
Click on the images to link to larger pictures of some of Draco's Meteorite / Tektite sample display. Use your "Back" button to return to this page. The entire collection contains small pieces of impact debris thrown off of the Moon and Mars. These specimens are small enough that they are displayed under a magnifier. Some of the larger pieces are shown here. The majority of the collection, however, ranges from sample material coming from the central core of a differentiated body all the way to the crustal material.
1) The larger Barringer Crater sample weighs in at about 7.5 pounds. This is part of the material that formed the great impact crater in Arizona near Flagstaff. Its composition is predominantly nickel and iron. It is about three times more dense than typical Earth crustal rocks. After picking up a piece of rock of similar size, you immediately notice the great increase in weight. The surface is rusted because it has been eroded by Earth's atmospheric forces.
2) This sample is also from Barringer Crater. Erosion can often bring out the metallic crystalline structure found in iron meteorites. These large crystals, called Widmanstatten Figures, are typical of very slow cooling processes deep in the core of a planetary body. When you hold an iron meteorite, you are holding the heart of a shattered planet in your hand.
3) In 1947, there was a bolide brighter than the sun that produced a fall of several tons of meteoric material in Siberia. These pieces have not had time to rust, and are quite shiny. Atmospheric entry forces have produced beautiful fluted convolutions and flow patterns on the surface of these iron meteorites.
4) The Gibeon Meteorite comes from perhaps the largest meteor field on earth which was created in 1836 in Namibia. Gibeon meteorites are iron and nickel with a resulting unique crystallization structure due to cooling in the zero gravity and very low temperatures of space. This is revealed after fashioning the piece and etching in nitric acid. The resulting "Widmanstatten lines" prove that it is a meteorite. Mineral inclusions are sometimes found including graphite, enstatite, and tridymite. Iron/Nickel meteorites will rust. Cut samples are coated with a clear, spray on finish to provide some protection.
5) This Nantan meteorite, an iron/nickel meteorite, was named after the place where it was found in southern China. The Nantan meteorite was discovered by the local residents of Nandan, China. They tried to melt them, assuming by the weight that they were iron ore. They brought them for processing and to their surprise they would not melt. They were studied by the government, and it was decided that these VERY heavy rocks were actually iron meteorites with a composition of approximately 85% iron and 6-7% nickel.
6) This polished Imilac Pallasite meteorite is composed of equal parts olivine and Iron/Nickel. The yellow/orange colored olivine crystals have an average size of about 10 mm. Of 2,600 known meteorites, only 73 are stony-irons, and fewer yet are Pallasites. Pallasites are some of the most sought after meteorites because of their striking beauty. Pallasites are mixtures of nickel-iron and silicate minerals, primarily Olivine. Pallasites are thought to have formed in the mantle of a differentiated body.
7) This olivine crystal fell out of its iron/nickel matrix due to weathering. Many of the Imilac Pallasites have been found strewn across a barren desert in Chile. Many have been highly eroded...enough so that some pieces no longer contain the olivine crystals, and resemble a strange Swiss-cheese construction. This olivine crystal has an orange coloration...one of the most common.
8) The term "astrobleme" derives from the Greek, and may be translated, "star wound," with inferences that it is the type of wound consistent with impact hits from javelins or stones. The material found in the Sudbury basin of Ontario is thought to be the result of a massive asteroid or comet impact. The area has been mined for over 100 years, mainly for copper and, and later for nickel. Other minerals and impact polymorphs have also been found, and it is one of the richest sites for natural fullerines (a rare form of elemental carbon) found on Earth. The smaller stone appears to be a slab of pyrite (fool's gold), and is just one of the deposits found at the impact site.
1) The Moldovite Tektites are formed of a transparent green impact glass. The samples shown were found in Czechoslovakia, where they were deposited after Central Europe suffered a major impact. The center of the impact lies near Prague, and the ring of mountains that mark the crater walls extend about 300 miles in an oval crater which is now distorted by the upward thrust of the Alps.
2) Moldavites may be used as jewels. Traditionally, Moldavites were used in ceremonial pieces in Egypt and other large trade centers of the world. The above image is of a faceted Moldavite that is of a very high quality. There are fewer inclusions than in most commercial emeralds.
3) There is much debate over the origin of the Libyan "Desert Glass." It ranges from lemon yellow, as pictured, to orange and brown tones. This tektite shows a lot of bubble streak inclusions, but also contains clear sections that are translucent. The origin of the material that formed this beautiful tektite are in dispute, as well as the probable forces that produced the bubble streams in its interior. The outside of most Libyan tektites are weather-sculpted by the sands of the Sahara.
4) The Indochinite Tektite is a glob of molten, black, glassy material that was blown up by a mysterious impact somewhere near Australia. The impact site is unknown, and the age of the sample is uncertain, but the material is found all around Southeast Asia and Polynesia.
5) A close view of the large fracture shows a "glass fracture" pattern...circular in nature. The stone does not merely crumble or break along a fault line.
6) Indochinites are rarely faceted. Just like obsidian, impact glass is more fragile than many other stones used for making jewelry. This piece, however, was faceted on one side, and formed into a rough cabochon on the other.
|Weather permitting, the final Sky Interpretation session each year hosted by the Escambia Amateur Astronomers' Association for Ft. Pickens (Gulf Islands National Seashore) will include a Perseid Meteor Watch. Draco Productions often loans some of the samples shown above for display for this and other special events.|
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