MINERAL SCIENCES EXHIBITS: The E. Hadley Stuart, Jr. Hall of Gems and Minerals

METEORITES

This exhibition  provides an introduction to the fascinating story of meteorites. Fine specimens of all of the major types of meteorites are exhibited. Featured in the exhibition are the three meteorites found in Los Angeles County including the Los Angeles Meteorite, the first Martian Meteorite known to have hit in the U.S.

Main topics covered:

• METEORITES: What they are and where they came from
• METEORITES FROM LOS ANGELES COUNTY: Descriptions of the first three
• METEORITES FROM MARS: The new Los Angeles Meteorite
• METEOR-WRONGS: How to tell if you've got a meteorite

METEORITES: What they are and where they came from

Great Balls of Fire
Space isn't really so empty. Debris from colliding asteroids, dust trailing from comets
and material ejected from impact craters on Mars and the Moon regularly bombards
our planet. Depending on where an object is in its journey determines what we call it.

• Meteoroids are small rocky objects traveling through interplanetary space.

• Meteors are bright streaks of light seen briefly in the sky.

• Fireballs are especially bright meteors.

• Bolides are fireballs that explode at the end of their paths.

• Meteorites are meteoroids that have landed on the Earth.

Fast and Hot
A meteoroid enters the Earth's atmosphere at tremendous speed. Air friction raises its surface temperature to as much as 3000°F causing the surface to begin to melt and vaporize. Light is produced by the glow of the vaporizing meteoroid and the glow of the heated atmosphere immediately surrounding it. The vaporization process usually lasts only a few seconds and only takes place on the surface of the meteoroid. Any portion of the meteoroid that survives to strike the earth as a meteorite will seldom feel even warm to the touch.

Is Smaller Better?
Size counts. The smaller the meteoroid the more effective the atmosphere is at slowing its speed. Large meteoroids will retain much of their speed, but resistance with the atmosphere will often cause them to break up into smaller pieces. Meteoroids over 10 tons that strike the Earth will be destroyed by the force of their impacts. Meteoroids larger than pea-size may survive their trip through the atmosphere to land on the Earth as meteorites. Slightly smaller bodies usually vaporize completely. Dust-size particles survive the trip because they are so light that they slow down very quickly. Microscopic dust survives, but may take years to reach the ground.

Who's Who and What's What?
There are three basic types of meteorites: iron meteorites, stony-iron meteorites and stony meteorites. The following examples of these are displayed in the exhibition:

Laguna Manantiales Meteorite (Iron, Octahedrite)
Found in 1945 at Laguna Manantiales, Santa Cruz, Argentina
On loan from UCLA Institute of Geophysics & Planetary Physics
Iron meteorites are composed mostly of two nickel-iron alloys, kamacite (high iron) and taenite (high nickel), which may be intergrown in a crisscross (Widmanstätten) pattern. They may be analogous to the core of our own planet.

Imilac Meteorite (Stony-Iron, Pallasite)
Found in 1822 at Imilac, Atacama Desert, Chile
On loan from UCLA Institute of Geophysics & Planetary Physics
Stony-iron meteorites contain both silicate and nickel-iron. Pallasites contain approximately equal proportions of silicate and nickel-iron. They are considered to be analogous to the Earth's core-mantle boundary.

Gao Meteorite (Stony, Ordinary Chondrite)
Fell on March 5, 1960 at Gao, Sissili, Burkina
Ordinary chondrites are the most common stony meteorites. Named for the silicate spheres (chondrules) found inside them, they formed about 4.6 billion years ago from the original dust and gas of our solar system.

Millbillillie Meteorite (Stony, Achondrite)
Fell in 1960 at Millbillillie, Western Australia
On loan from Charles and Cecille Schoettlin
Achondrites are stony meteorites without chondrules. These are volcanic rocks composed of silicate minerals such as plagioclase, olivine and pyroxene and some are considered to be the analogous to the Earth's crust.

Allende Meteorite (Stony, Carbonaceous Chondrite)
Fell on February 8, 1969 at Allende, Chihuahua, Mexico
Carbonaceous chondrites are stony meteorites that consist mostly of silicate minerals. They contain particles that were among the first solids to form in the early solar system.

Los Angeles Meteorite (Stony, SNC)
Found on October 30, 1999 at Sunland, Los Angeles Co., California
On loan from Robert S. Verish
SNC Meteorites are the rarest stony meteorites. Only 30 have been found on Earth and this is the one known to have hit in the United States. (The Lafayette Meteorite was found in the collection at Purdue University, but it is not known where it fell.) Composed of the minerals plagioclase, olivine and/or pyroxene, evidence indicates that these meteorites originated on Mars and were formed by volcanic processes. SNC stands for the names of the locations where the first three Martian meteorites were discovered: Shergotty (India), Nakhla (Egypt) and Chassigny (France).

METEORITES FROM LOS ANGELES COUNTY
As of December 1999, 22,507 meteorites were known, but only three had been found in Los Angeles County. These three L.A. County meteorites are on display in this exhibit. Recent efforts by local meteorite collectors have turned up many more L.A. County meteorites, mostly from the Mojave Desert region.

Neenach Meteorite (Stony, Ordinary Chondrite)
In April of 1948, this meteorite weighing 30 pounds was found on a ranch near Neenach in the western Antelope Valley. Mr. Elden Snyder unearthed it with his plow, in the process breaking it into four pieces. The rock was unusual enough that Mr. Snyder saved it and placed it on the porch of his ranch house. In the fall of 1952, Mr. Charles Johnson of Lancaster, California, brought it to Dr. Robert Webb, at the University of California, Santa Barbara. Later, it was added to the collection at UCLA's Institute of Geophysics & Planetary Physics.

Littlerock Meteorite (Stony, Ordinary Chondrite)
In April of 1979, this meteorite weighing 42 pounds was found on farmland at Littlerock, a small town east of Palmdale in the Antelope Valley. The farm owner, Mr. Donald Reed, broke a piece off the top and sent it to the Smithsonian for identification. The freshness of the fusion crust suggests that the meteorite landed only about 100 years ago. Mr. Reed sold the meteorite to the Natural History Museum of Los Angeles County in 1979.

Los Angeles Meteorite (Stony, SNC, Shergottite)
About twenty years ago, Mr. Robert Verish picked up two "peculiar volcanic rocks" somewhere in the Mojave Desert. He then threw them into his box of rocks "to be identified later". On October 30, 1999, while cleaning out his collection in his backyard in Sunland, Mr. Verish took another look at the two rocks and suspected they were meteorites. Scientists at UCLA's Institute of Geophysics & Planetary Physics confirmed the rocks to be meteorites and very rare ones at that. They are classified as a shergottite, a meteorite type identified as being of Martian origin. The smaller of the two pieces, weighing 234 grams, is displayed courtesy of Mr. Verish. The other piece weighs about 453 grams.

LINK: Los Angeles Meteorite

METEORITES FROM MARS

How Do We Know They Are From Mars?
Martian meteorites are relatively young. They are volcanic rocks that range in age from a few hundred million years to about 1.3 billion years old in contrast to the 4.5 billion years for most other meteorites. Confirmation of their Martian origin came when the gases trapped in glassy nodules on these meteorites were found to be identical to the Martian atmosphere as measured by the Viking Lander.

How Did They Get Here?
Millions of years ago, large meteorites impacted the surface of Mars blasting chunks of the Martian crust into space. To escape the gravity of Mars, these rocks had to reach speeds of 11,000 miles per hour, more than five times the muzzle velocity of a hunting rifle. The only known natural process that could do this is a large meteorite impact. We can tell from cosmic ray damage that Martian meteorites spent millions of years in space before landing on Earth.

LINK: Mars Meteorite Home Page (JPL)

Is There Life On Mars?
In 1996 a Martian meteorite found at Allan Hills in Antarctica in 1984 (ALH84001) was reported to have evidence of possible life on Mars dating to about 3.6 billion years ago. NASA scientists reported microscopic structures that appear to be fossils, in close association with an apparently unique pattern of organic molecules that are the basis of life and several unusual mineral phases that are known products of primitive microscopic organisms on Earth. Further investigations have cast serious doubts on the plausibility of this interpretation.

LINKS: Life on Mars? (NASA); On the Question of the Mars Meteorite (LPI)

METEOR-WRONGS: Is It, Or Isn't It?
Some ways to tell if you've found a meteorite.

Is It Magnetic?
Except for those from Mars and the Moon, almost all meteorites contain some iron-nickel alloy. Iron meteorites will be very strongly attracted to a magnet, while stony meteorites will be only weakly attracted. Some common terrestrial rocks, notably those containing magnetite, and some industrial slags will also be attracted to a magnet.

Is There A Fusion Crust?
The 15 seconds of heat generated during its trip through the atmosphere flash-fries the outside of a meteorite, resulting in a thin, often glassy, ash-black fusion crust. Over time this crust may weather to a rusty brown and eventually disappear.

What's Its Shape?
A meteorite may develop the shape of a rounded cone as parts of its surface are removed by melting – a process called ablation – as it moves through the Earth's atmosphere. The surface of a meteorite is usually smooth and featureless, but due to ablation it often has shallow depressions and cavities resembling thumbprints in wet clay.

Does It Have Chondrules?
Chondrules, microscopic to marble sized silicate spheres found in most stony meteorites, are not found in Earth rocks. Recognizing chondrules takes practice, as there are various types of spherical structures that do occur in Earth rocks.

Does It Show A Widmanstätten Pattern?
When cut, polished and brushed with a dilute solution of acid, most iron meteorites show a crisscross structure known as a Widmanstätten pattern. The crisscross structure represents the intergrowth of two nickel-iron alloys. The Widmanstätten pattern is not found in manmade iron objects.

LINK: Meteorite magazine