WHAT ARE METEORITES?

من طرف Admin* الأربعاء سبتمبر 28, 2011 10:25 pm

WHAT ARE METEORITES?

The first in a series of articles by Geoffrey Notkin, [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط]

Welcome!

Welcome to the first edition of Meteorwritings.

I am a science writer, photographer, and professional meteorite hunter.
I travel the world looking for actual rocks from outer space, and I write
about my adventures for both scientific and popular magazines. I have
also made documentaries about meteorite hunting for PBS, National Geographic,
The History Channel, and the Travel Channel. I was thrilled by the invitation
to contribute to Dr. King's Geology.com and in the months ahead I will
discuss the origins of meteorites, what they are made of, why they are
valuable to science and collectors, and even how to find and identify
them. I hope to share my enthusiasm for these amazing visitors from deep
space.

What are Meteors?

Every year hundreds of hopeful people contact me because they believe
that an unusual or out-of-place rock they have found is a meteorite. I
frequently receive emails which contain an amusing but impossible statement
along the lines of: "I think I've found a meteor."

In order to appreciate the humor inherent in this sentence we must first

understand the difference between meteors and meteorites. Meteor is the
scientific name for a shooting star: the light emitted as
fragments—usually rather small—of cosmic material which we sometimes
see at night, burning high up in the earth's atmosphere. The bright,
and typically very short-lived flame, is caused by atmospheric pressure
and friction as pieces of extraterrestrial material become so hot they
literally incandesce, as does the air around them. Manned spacecraft
such as NASA's space shuttle and the Mercury,
Gemini, and Apollo capsules experienced similar heating during re-entry
into our atmosphere, which is why they employ heat shields to protect
the astronauts and cargoes inside.

Meteor Showers

There are a number of periodic meteor showers visible each year in the
night sky: the Perseids in August, and the Leonids in November usually
being the most interesting to observe. The annual meteor showers are the
result of our planet passing through debris trails left by comets. The
meteors we see during those annual displays are typically small pieces
of ice which rapidly burn up in the atmosphere and never make it to the
surface of our planet.

Sporadic Meteors

An sporadic is a meteor which is not associated with one of the periodic
showers and the majority of those meteors also burn up entirely in the
atmosphere which acts as a shield, protecting us earthbound humans from
falling space debris. Any portion of a meteor which does survive its fiery
flight and falls to the surface of the earth is called a meteorite. So,
meteorite scientists and hunters understandably chuckle to themselves
when a hopeful person claims to have discovered a meteor. The excited
people who ask me to help them identify a strange rock should actually
be saying: "I think I've found a meteorite."

A polite and charming lady once telephoned the Aerolite Meteorites office
and asked if we had, for sale, any meteorites from the constellation of
Castor and Pollux. I explained to her that most—or possibly all—meteorites
found on earth originate from within the Asteroid Belt between Mars and
Jupiter, but there is a chance that some meteorites come to us from farther
afield. It has been theorized that rare carbon-bearing meteorites known
as a carbonaceous chondrites—such as Murchison which fell in Victoria,
Australia in 1969—may be the remnants of a comet nucleus, but that
remains conjecture. The stone meteorite Zag, which was seen to fall in
the Western Sahara in 1998 and later recovered by nomads, contains water
and so a slightly more fanciful but intriguing theory developed which
suggests that large meteorites may have carried both water and amino acids
(the so-called "building blocks of life") to our planet in the
distant past.

What are Meteorites?

Meteorites are rocks, usually containing a great deal of extraterrestrial
iron, which were once part of planets or large asteroids. These celestial
bodies broke up, or perhaps never fully formed, millions or even billions
of years ago. Fragments from these long-dead alien worlds wandered in
the coldness of space for great periods of time before crossing paths
with our own planet. Their tremendous terminal velocity, which can result
in an encounter with our atmosphere at a staggering 17,000 miles per hour,
produces a short fiery life as a meteor. Most meteors burn for only a
few seconds, and that brief period of heat is part of what makes meteorites
so very unique and fascinating. Fierce temperatures cause surfaces to
literally melt and flow, creating remarkable features which are entirely
unique to meteorites, such as regmaglypts ("thumbprints"), fusion
crust, orientation, contraction cracks, and rollover lips. These colorful
terms will be discussed and examined in future editions of Meteorwritings.

Meteorites: Very Rare and Very Old

Meteorites are among the rarest materials found on earth and are also
the oldest things any human has ever touched. Chondrules—small,
colorful, grain-like spheres about the size of a pin head—are found
in the most common type of stone meteorite, and give that class its name:
the chondrites. Chondrules are believed to have formed in the solar nebula
disk, even before the planets which now inhabit our solar system. Our
own planet was probably once made up of chondritic material, but geologic
processes have obliterated all traces of the ancient chondrules. The only
way we can study these 4.6 billion year old mementoes from the early days
of the Solar System is by looking at meteorites. And so meteorites become
valuable to scientists as they are nothing less than history, chemistry,
and geology lessons from space.

Gemstones from Space

Some meteorites even contain gemstones. The beautiful Brenham pallasite,
found in Kiowa County, Kansas is packed with sea-green olivine crystals,
which is also known as the semi-precious gemstone peridot. Both the Allende
meteorite which fell in Chihuahua, Mexico, and the Canyon Diablo iron
which formed Arizona's immense and erroneously named Meteor Crater (craters
are formed by large meteorites, not meteors) contain micro diamonds.

The rarity of meteorites, along with the fact that they are the only
way in which most of us will ever have the chance to touch a piece of
an alien world, make them of great interest to an ever-expanding network
of private meteorite collectors. Meteorite collecting is an exciting and
growing hobby and there are perhaps a thousand active enthusiasts in the
world today. The international space rock market is something else we
will explore in the months ahead.

About the Author

Photograph by
Leigh Anne DelRay

Geoffrey Notkin is a meteorite hunter, science
writer, photographer, and musician. He was born in New York City,
raised in London, England, and now makes his home in the Sonoran Desert
in Arizona. A frequent contributor to science and art magazines, his
work has appeared in Reader's Digest, The Village Voice, Wired, Meteorite, Seed, Sky & Telescope, Rock & Gem, Lapidary Journal, Geotimes, New York Press,
and numerous other national and international publications. He works
regularly in television and has made documentaries for the BBC, PBS,
History Channel, National Geographic, A&E, and the Travel Channel.
He is currently at work on a book about his adventures as a meteorite
hunter, which is expected to be published by Stanegate Press in 2009.

[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط]
WE DIGSPACE ROCKS™

[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط]

A fine example of a Sikhote-Alin iron meteorite,
weighing 409.9 grams, which was seen to fall in eastern Russia in
February of 1947. This meteorite is described as an individual, as it is
a complete mass (rather than an exploded shrapnel fragment) which fell
to earth on its own. Note the abundant overlapping regmaglypts
(thumbprints) caused by surface melting as the meteorite flew through
our atmosphere. Regmaglypts are one of the key surface features used to
identify meteorites. Sikhote-Alin is a coarsest octahedrite iron in the
group IIB, and is comprised of approximately 93% iron, 6% nickel and 1%
trace elements. Photo © Aerolite Meteorites / Geoffrey Notkin. Click
image to [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط].

Meteorwritings Menu

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WHAT ARE METEORITES? Clear

[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط]

A 1,179-gram specimen of the Canyon Diablo iron
meteorite, found at the famous impact site, Meteor Crater, near Winslow,
Arizona. The crater is erroneously named - craters are formed by
meteorites, not meteors - but it is spectacular nonetheless: the best
preserved impact crater in the world and something of a mecca for
meteorite enthusiasts. Unfortunately meteorite hunting is not allowed at
the privately owned site, but specimens of this historic meteorite are
sometimes available from old collectors and prospectors who hunted at
the site decades ago. This specimen is a twisted fragment - its
interesting shape a result of the tremendous impact and explosion which
made the crater. Such pieces are described as "sculptural" because of
their attractive, aesthetic shapes, and are highly sought after by
meteorite collectors. The impact is believed to have occurred
approximately 25,000 years ago, and this specimen is in as-found
condition and displays an orange/ochre patina, caused by long term
weathering and oxidation in a desert environment. Photo © Aerolite
Meteorites / Geoffrey Notkin. Click image to [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط].

[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط]

The Seymchan meteorite was originally discovered
near Magadan in Russia in 1967 and was classified as a IIE iron.
Recently, colleagues of mine mounted a new expedition and returned to
the original find site, where they were happily surprised to discover
pallasites (attractive stony-iron meteorites containing olivine
crystals) in the same zone, making Seymchan similar to the Glorieta
Mountain (New Mexico) and Brenham (Kansas) meteorite, both of which have
produced examples of pallasites and siderites (irons) from the same
fall. Although the pallasite and siderite specimens are somewhat
different in composition, they did originate within the same mass, which
likely exploded high up in the atmosphere. This complete slice is an
extremely rare and interesting transitional specimen which shows
characteristics of both pallasites (olivine crystals) and irons within
the same specimen. This slice has been polished and then etched with a
mild solution of nitric acid to reveal its beautiful interior
crystalline structure known as a Widmanstatten Pattern. Picture ©
Aerolite Meteorites / Geoffrey Notkin. Click image to [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط].

[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط]

A handful of small, freshly fallen stone meteorites
which landed in the Republic of Mali in Northwest Africa during the
fall of 2007, making them one of the newest extraterrestrial arrivals on
earth. Meteorites are typically named after the nearest town or
geological feature to the point of impact, and although this meteorite
was originally and unofficially known as Mali, it will likely be renamed
Erg Chech, once official classification has been approved by academia.
These stones were picked up immediately after their fall and display a
rich, black fusion crust. Picture © Aerolite Meteorites / Geoffrey
Notkin. Click image to [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط].

[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط]

An artist's impression of meteoroids (potential
meteorites) about to enter the earth's atmosphere. Most meteorites which
land on our planet are believed to have originated within the Asteroid
Belt. Click image to [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط]. © iStockphoto / duuuna

[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط]

A complete slice of a rare carbonaceous chondrite
found in the Sahara Desert. This stone meteorite has been cut and
polished to reveal an abundance of brightly colored and densely packed
chondrules. These grain-sized inclusions are believed to have formed in
the solar nebular more than 4.6 billion years ago, and therefore
pre-date the formation of our own planet. While chondrites are the most
abundant type of meteorite, it is very unusual to find specimens which
contain such a wealth of distinct chondrules. Photograph © Aerolite
Meteorites / Geoffrey Notkin. Click image to [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط].

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من طرف **geo_Alger** الخميس أكتوبر 06, 2011 11:10 pm

WHAT ARE METEORITES?

WHAT ARE METEORITES?

رد: WHAT ARE METEORITES?