Artist's portrayal of a massive terrestrial strike,
by artist Don Davis, courtesy of NASA

By Kari Hetcher, Scott Hughes and Martin Horejsi

READING ASSIGNMENT: In your textbook, read Chapter 25, Asteroids, by Clark R. Chapman; and Chapter 26, Meteorites, by Harry Y. McSween, Jr.

Assignment:   Complete task 1 and task 2 and e-mail them to hughscot@isu.edu.

Preview the vocabulary and tasks you will be completing for this module

 

INTRODUCTION

Imagine yourself in the year 1264 AD. You are finishing milking the cow one night in late November when you pause for a moment to check the sky for signs of tomorrow's weather. Suddenly, a bright flash streaks across the sky and leaves in its path an ominous afterglow that hangs in the night for seconds and hangs in your thoughts for days. To you, this was a piece of the sky tumbling towards earth, or perhaps a sign from God, a harbinger of doom. In the early days, you would not have known about meteorites or asteroids, or even the motions of the other planetary bodies sharing our solar system.

In 1492, a large explosion was heard near the village of Ensisheim in France. A 280 pound stoney-meteorite was seen streaking through the sky, and the impact location was soon found. The people of the town thought the stone was supernatural and some believed it was a sign of God's wrath on the French for their wars against the Holy Roman Empire. To this day, the stone is kept on display in Ensisheim.

   

This meteorite is the earliest witnessed fall in the Western Hemisphere, and the myths surrounding it illustrate the fear and confusion once associated with these objects. Up until the 1800's meteorites were thought to have formed within the Earth's atmosphere as part of violent thunderstorms. Some scientists believed they were chunks of rocks blown out of volcanoes. Fortunately, (although some may disagree) we are living in an enlightened age of science and understanding, a time of telescopes and radar, space stations and spacecraft that can travel to the outer reaches of our Solar System.

Meteorites can come from almost anywhere in our solar system. They may be pieces of asteroids that broke off from the asteroid belt when two asteroids collided, leftovers from the passing of a comet. Visit the Astronomy Pictures of the Day for additional views of the cosmos, or even parts of other planets or moons that broke off during asteroid impacts. Another good website for meteorites is the Nine Planets -- Meteorites page provided by the Lunar and Planetary Institute at the University of Arizona.

Meteor Showers

Meteorites fall to the Earth when they get caught in our orbit and are pulled into our atmosphere by our gravitational field. Often, more than 100 meteors per hour can be observed! These so-called meteor showers are named after the constellation in the night sky from which they seem to "originate".

One of the greatest meteor showers ever recorded was on November 17, 1833. The great Leonid shower (from the constellation Leo) produced over 200,000 meteors within a few hours! Astronomers eventually learned that this shower was caused by particles from the tail of comet Temple as it passed near the Earth. Because comets have predictable orbits, many meteor showers are predictable and about a dozen major showers can be seen each year.

The Leonid meteor shower is scheduled to return in on November 13-19, 2003. That is five days earlier than the usual Leonid showers! For more information about this years Leonid vist: Leonid Shower http://leonid.arc.nasa.gov/1998.html. The best time to view meteors in North America is between midnight and dawn. Remember this date and you will see one of the most splendid events the solar system has to offer.

The Perseid meteor shower is also spectacular.

The study of meteorites is one of the most useful tools in increasing our understanding of the solar system. Thousands have fallen to Earth over the years, many not being found until a long time after they had landed. Meteorites are like snapshots of the stages of development of our solar system, and have yielded literally 'tons' of information about the chemical, physical, and developmental characteristics of the planets. We can speculate about the conditions of the solar nebula before and during the accretion of planetary bodies, but meteorites have given scientists the hard mineralogical and chronological data to support our theories.

NOTE: In 1969, a team of Japanese scientists discovered many meteorites frozen in glacier ice in Antarctica. A series of expeditions to the Yamato Mountains and Allan Hills in Antarctica lead to collecting over 4,000 specimens by the mid 1980’s (previously only about 1,000 samples had been collected worldwide). Using helicopters and snowmobiles, scientists literally rode around the white ice and picked up hundreds of dark-colored rocks.


Classification of Meteorites

It is first important to understand a few often confused terms: meteor, meteorite, and meteoroid. When a solid body is still in space and has not yet reached our atmosphere, it is called a meteoroid. When it hits our atmosphere and begins to burn up due to friction, it generates the bright streak in the sky that is known to many as a "shooting star." This is a meteor. A fragment of the meteoroid that survives the atmosphere and lands on Earth as a solid body is called a meteorite.

NOTE: The generally accepted distinction between a meteoroid and an asteroid is size; asteroids are small planetary bodies that orbit the Sun, whereas meteoroids can be much smaller fragments of asteroids, comet tails, dust particles, etc.

We can determine a meteorite’s type and from where it came by examining the kinds of minerals and rocks within it. There are three main types of meteorites, which are described in detail in many textbooks:

stones, irons, and stony irons

Stones are further subdivided into chondrites and achondrites; however, each of the major divisions has several subtypes.

Stones

Resembling Earth rocks, the stones are either primitive, i.e., they are made up of primordial Solar System material, or they are differentiated, i.e., they have igneous textures because they were once molten.

Primitive stones are chondrites, the class that makes up most of the meteorites that fall to Earth (~87% of falls and ~52% of finds). All other meteorite types (achondrites, stony-irons and irons) are differentiated. Chondrites are so-called because of the presence of small round chondrules, which are typically ~ 1 mm diameter and appear to be material condensed from the primordial solar nebula.

NOTE: Chondrites are either carbonaceous or ordinary depending on the amount of carbon, water and other volatiles. Ordinary chondrites are primitive, but have been slightly heated to drive off most of the volatile constituents.

Carbonaceous chondrites: are made of undifferentiated silicates with carbon compounds and water (densities ~2.3 gm/cc). They represent the primordial material of the Solar System from which planets accreted. Besides an aggregate of carbonaceous dust and hydrated silicates, they also contain chondrules, spherical blobs ranging in size from less than 1 mm to nearly 2 cm that have higher temperatures of melting than the surrounding material. A famous specimen is the Allende carbonaceous chondrite that fell as a shower in Chihuahua, Mexico on February 8, 1969.

Allende Meteorite. Note the aggregated mass of particles, several of which can be easily identified as chondrules, that are over 4.6 billion years old, the age of the Solar System. This piece also exhibits a rind of melting caused by atmospheric friction. This represents the most primitive geological material from which planets were formed.  The grid spacing in this and subsequent photographs is one centimeter. Photos are by Scott Hughes, unless other credits are stated.
Allende Meteorite. This is a photomicrograph of a thin-section used for microscopic analysis; field of view is ~4 mm. Dark areas are regions of carbonaceous material that are nearly opaque to transmitted light.

Chemical analyses indicate that, except for volatile elements (H, He, Ne, Ar, N, etc.), the relative chemical proportions of rock-forming elements (e.g. Si, O, Mg, Fe, Ca, Al, etc.) in carbonaceous chondrites closely correspond to the relative proportions in the Sun. Extensive chemical, isotopic, and microscopic analyses of these primitive meteorites have provided geologists with a reasonable assessment of the starting planetary material from which differentiated core-mantle-crust layers are derived.

Ordinary chondrites: are comprised of metamorphosed (heated and recrystallized) silicates and iron left over when water and other volatiles were driven off by early radioactive heating. By far, most of the chondrites are ordinary, comprising about 82 percent of all meteorites observed as falls.

Haxtun Meteorite, Ordinary chondrite, type H/L4. Photomicrograph of a thin-section (compare to the Allende photomicrograph above); field of view is ~4 mm. The Haxtun meteorite was found in August, 1975 in Phillips Co., Colorado, USA. It is a strongly shocked ordinary chondrite as evidenced by flattened chondrules, shock-melt veins and "strain shadows," a feature determined by optical properties that indicates deformation in the atomic crystal lattices of major minerals.

Ordinary chondrites discovered in Idaho:
Wilder Meteorite, Olivine-bronzite chondrite, type H5. found in 1982, Idaho, USA. Total known mass: 27 kg. Note chondrules and melt rind in this 710 g slab.
Achondrites: are differentiated stones (~9% of falls and ~1% of finds), comprised mainly of igneous silicates. These are probably derived from the mantle or crust of a differentiated meteorite parent body. Radiometric dating indicates that most of these igneous meteorites are between 4.5 and 4.6 billion years old, the appropriate time for planetary differentiation. Some are as young as ~1.3 billion years (see SNC chondrites, below).
Cumberland Falls Meteorite, Aubrite. Photomicrograph ~ 4mm across. This meteorite fell on April 9, 1919 in Whitley County, Kentucky, USA. It is a calcium-poor aubrite composed of plagioclase and pyroxene, much like a coarse basalt or gabbro on the Earth. The textural arrangement of these minerals represents igneous processes.

SNC Meteorites: are rare chondrites about 1.3 billion years old believed to be pieces of Mars. They are classified according to the original group of specimens used to chemically and lithologically suggest a Martian origin: Shergotty, Nakhla, and Chassigny. Now there are several examples of each type classified as shergottites, nakhlites and chassignites. Sample ALH 84001 (found in the Allan Hills region of Antarctica in 1984) is the meteorite that is currently being investigated for signs of life on Mars. Learn more at: http://cass.jsc.nasa.gov/lpi/meteorites/mars_meteorite.html

Lunar Meteorites: are pieces of the lunar surface that were ejected during impact and subsequently fell to the Earth. These were first discovered in 1982 when a regolith breccia sample (see Fig. 18, Chapter 19 in the text) was evaluated chemically and petrographically (textures and minerals) and determined to have a lunar origin. We know from the Apollo and Luna missions to the Moon what compositions and rock types to expect.

Irons

Metallic nickel-iron masses, the irons are derived from the differentiated core of a planetary body, and their densities are quite high (>7 gm/cc). These meteorites were one of the original sources of iron during the transition from the bronze age to the iron age. Most metals are in an oxidized state on the surface of the Earth, so the metallic characteristic is good evidence for an extraterrestrial origin. Less than half (~42%) of all meteorite finds are irons, but only about 3 percent of all observed falls are irons.

Iron Meteorite, Unknown. The 945g chunk of iron shown in this picture is an iron meteorite approximately 13 cm long of unknown origin. Perhaps a local collector threw it into a box with other rocks, or a geology teacher purchased it at a rock shop. Many such meteorites exist in personal collections. Some folks believe that meteorites have holes in them, similar to vesicles in chilled lava or a sponge. Actually the "holes" are ablation pits ("thumb prints") formed by removal of the melted skin as the rock fell through the Earth’s atmosphere. The gridlines have a 1 cm spacing for scale.
Meteor-Wrong, Phosphate Slag. This is a piece of slag formed when a molten mass of metal was poured into a bowl-shaped crucible and allowed to solidify. Note the elongate bubbles caused by gases that exsolved during cooling. Pieces of slag from an old foundry or chunks of iron ore (magnetite) are often believed to be meteorites. We call such falsely identified specimens "meteor wrongs." Nearly every professional geologist has caused minor disappointment for someone who thought they had found a meteorite! Specimen provided by FMC Corp., Pocatello, Idaho, USA.
Canyon Diablo Meteorite, Octahedrite, type IA. The famous Meteor Crater in Arizona was created by the impact of the Canyon Diablo iron meteorite from which this 738 g specimen was obtained. The impact occurred ~25-50 thousand years ago into the Coconino Sandstone of northern Arizona, USA although the finding of the meteorite is reported to have been in 1891. The total known mass is about 30,000 kg.
Gibeon Meteorite, Octahedrite, type IVA. Found in 1837 in Great Nama Land, Namibia, Africa, this iron meteorite has a total known mass of 15,000 kg. This 192 g slice has been etched with dilute nitric acid and polished to display the widmanstätten texture caused by intersecting layers of various iron alloys.

Stony-Irons

As their name implies, the stony-irons are comprised of differentiated igneous silicates plus iron and nickel metal. They are fairly rare (~1% of falls and ~5% of finds) and thus are valuable to scientists as well as collectors. The stony-irons provide a glimpse of the region within a planetary body near the core-mantle boundary where metallic core components have not entirely separated from the silicate mantle.

Brenham Meteorite, Pallasite. Found in 1882 in Kiowa County, Kansas, USA. Each piece, 48.0 g and 30.1 g, is part of a large polished slice of the original that weighed more than 900 kg.
Huckitta Meteorite, Pallasite. Photomicrograph of a thin section (field of view ~4 mm). Dark regions are opaque Fe-Ni metal veinlets and stringers similar to that found in iron meteorites. Lighter regions are olivine, a dominant mantle mineral formed during planetary differentiation.

The Meteoritical Society. An international, non-profit organization devoted to the study of extraterrestrial materials and history, including asteroids, impact craters, interplanetary dust, lunar samples, meteors, meteorites, tektites, and the origin of the solar system. Our 925 members from 32 countries include scientists specializing in aspects of geology, chemistry, physics, and astronomy, as well as amateurs interested in meteorites and impact craters.

Visit their website at: http://www.meteoriticalsociety.org/

New England Meteoritical Services. This is a fun site to visit for information on the meteorite market as well as information on other planetary features: http://www.meteorlab.com/homepage.htm

Task 1: Meteorite Study Questions and Activities

1.  Why would more meteorites be available on the glacier ice of Antarctica than other places in the world?

2.  Inspect these images of the lunar and Martian surfaces. Many of the small craters were caused by meteorite impacts (some may be volcanic craters). Why are there so many more craters on the Moon and Mars compared to the Earth? List at least 2 reasons.

Surface of the Moon

Surface of Mars

3.  Define the three types of meteorites and their major sub-types.

4.  Give three scientific reasons that will calm people's fear of a large asteroid impact with Earth.

5.  Visit the Frequently Asked Questions About Meteorites page at the University of Arkansas: http://www.uark.edu/depts/cosmo/metfaqs.htm and Exploring Meteorite Mysteries - a NASA site, and maybe http://www.meteorite.com .   Write a paragraph describing how one distinguishes between meteorites and "meteor-wrongs."  Include descriptions of objects that are often mistaken for meteorites.

6.  If possible, go outside after dark and watch the sky for a specified length of time, say 30 minutes or one hour.  Count the number of "falling stars."   Give the number as a ratio, x/minute, or x/hour, the location you made your observation, the date and time.


Asteroids

As mentioned previously, many of the meteorites that fall to the Earth are
broken off pieces of asteroids from the asteroid belt between Mars and Jupiter

The group of main belt asteroids lies between the orbits of Mars and Jupiter and contains nearly 5000 bodies that are numbered and have well-documented orbits. A smaller group of Apollo asteroids, including about 200 with diameters of 1 km or more, travel in orbits that cross the Earth's orbit. These are of obvious interest to scientists. Perhaps you have seen any of the TV-movies or have read fictional stories about an asteroid collision with and devastation of the Earth. It is possible that there used to be many more Apollo asteroids, but they have collided with the inner planets long ago. With this knowledge, why shouldn't we be afraid of an asteroid impact on Earth now?

NASA Photo (Galileo, P-40450c)

Galileo image of asteroid 951 Gaspra.

Links to check out:

Gaspra, Ida, Phobos, and Deimos

These four large asteroids discussed in your text and in many other literature resources. There are several internet sites for learning more about each of them:

 

 

Gaspra and Ida

Asteroid 951 Gaspra. Shown in false color imagery provided by NASA from the Galileo Mission. This irregular body is approximately 19 km x 12 km x 11 km.

Asteroid 243 Ida, with its tiny moon, Dactyl. False color image provided by NASA from the Galileo Mission. The long dimension of Ida is about 58 km.

Phobos and Deimos

Phobos and Stickney crater. Viking 1 Orbiter view of the Martian satellite Phobos with the 10 km diameter Stickney crater at the left. Radiating grooves and crater chains can be seen around Stickney. Phobos is roughly 20 km across in this view. Image by NASA.

Phobos from 480 km, anti-Mars side.   Viking 1 Orbiter image of the "back-side" of the Martian satellite Phobos. The south pole is just off the lower center of the frame. In this NASA image Phobos is roughly 18 km from top to bottom.
Deimos from 500 km. Viking 2 Orbiter image of Mars satellite. The surface appears smooth, but many craters are partly hidden by regolith: soil and broken up material. Closer images reveal a heavily cratered surface. Deimos is about 10 km across and 16 km in its longest dimension.
Task 2: Asteroid Study Questions

1.  Compare and Contrast Phobos and Deimos according to the following parameters:

  • date of discovery
  • radius
  • composition

2.  How do Phobos and Deimos differ from Ida and Gaspra?

3.  Plot the locations of each planet according to its distance from the Sun. Now predict the approximate location of a would-be planet between Mars and Jupiter. Is this where the asteroid belt is really located? Why do scientists believe the asteroid belt is not a broken up missing planet?

4.  Why are asteroids such irregular shapes?

Other Site of Interest:

NOTE: You should have other favorite web sites that you have collected from previous modules that provide additional information on meteorites.

Additional Reading:

After finishing this module: Go to Module 6-the Moon.

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