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Artistry Revealed in the Structure of Iron-Meteorites

We have all looked up into the night sky and wished upon a falling star. The vast majority of these falling stars are pebble-sized objects from space that are heated and vaporized as they pass through the Earth’s atmosphere producing a brief but bright streak in the night sky. Rarely an object that is inches or even yards across will enter the atmosphere. As these larger objects pass through the Earth’s atmosphere the outermost surface is ablated producing a fireball that can be seen even during the day and may be accompanied by loud explosions (sonic booms) as they are slowed down by friction.

Meteoroids, Meteors, Meteorites and Asteroid

So what are falling stars? Meteoroids are pieces of stony or metallic debris which travel in our solar system. These meteoroids are pieces of comets or asteroids, less frequently pieces of Mars or the moon, and very rarely they may represent a piece of interstellar debris.


It is estimated that tens of thousands of tons of natural space debris enters the Earth’s atmosphere each year. Most of this material falls as dust-sized particles (usually less than 0.04 inch in size) that, due to their small size, radiate away the frictional heating produced from the interaction with the atmosphere faster that they melt. This extra-terrestrial dust settles on the earth without notice.


The pebble-sized objects that burn up in the atmosphere are meteors. Sometimes these meteors occur in showers when many may be observed each hour over several days. These meteor showers are commonly associated with the known orbit of comets. 


Still larger objects may fall intact hitting the surface of the Earth. These larger objects are meteorites.


The source of most meteorites is believed to be asteroids. Scientists have based this conclusion on the observed trajectories of meteorites as well as their compositional makeup and a comparison to the compositional makeup to that of asteroids.[1]Asteroids are rocky debris left over from the formation of our solar system which occurred more than 4700 million years ago. Most of these asteroids are located in orbit around the sun between Mars and Jupiter in a region referred to as the asteroid belt. Asteroids are smaller than planets and range in size from less than 30 feet across to more than 300 miles in diameter (Ceres – the largest known asteroid is about 580 miles in diameter)[2].


Unlike the surface of the Earth which undergoes constant change from weathering due to the atmosphere, and subsidence and uplift due to plate tectonics, the structure of the asteroids have remained essentially unchanged since their formation.[3]Meteorites, therefore, represent some of the oldest objects on the surface of the earth and scientists have learned a lot about the early formation of our solar system by studying these meteorites. 


When a meteoroid enters the Earth’s atmosphere, it is traveling between 30,000 to 150,000 miles per hour; however, due to friction with the atmosphere, by the time most meteorites reach a height of 6 to 20 miles, they will have slowed so that the final distance is traversed in free-fall due to the normal effects of gravity (2800 to 5600 mph).[4]During the period of frictional heating, the outer surface of the meteorite is heated and stripped away through ablation producing a trail of incandescent gas and solidified droplets. The ablation process is so efficient that heat is seldom conducted into the interior of the meteorite and when meteorites are found shortly after hitting the earth they are reported to be cool or only slightly warm to the touch. Occasionally, due to their icy cold interiors from being in space, they have been observed to form a white frost (hoar frost).


Meteorites can be classified into three broad categories: Stony meteorites which are composed primarily of silicates but contain some metal, iron meteorites which are principally metal, and stony-irons which contain almost equal amounts of silicates and metals. 


The stony meteorites are further divided into two main categories, the chondrites and the achondrites. Chondrites make up the largest group representing more than 80 percent of the total meteorites found. The chondrites are extremely old and have remained unaltered since their formation nearly 4,600 million years ago. Chondrites formed by the rapid cooling of once molten droplets of rock into aggregates containing densely packed, nearly spherical beads (chondrules) of mainly silicates. These chondrites retain the physical and chemical records of some of the earliest events in our solar system.


After the formation of chondrules in our early solar system, collisions between these chondrules allowed some of them to accrete into up to kilometer-sized chondrites. Once these planetesimals reached this size, gravitational effects became significant and these planetesimals became attracted to each other making collisions more frequent and their growth much easier. The larger bodies grew hot enough to melt (possibly due to radioactive heating or from impact energy) and, under the effects of gravity, the dense metallic elements sank to form a core and the more buoyant rocky silicates rose and solidified to form the mantel. Unlike the Earth which is still cooling down, these smaller planet-like bodies cooled much faster and have a solid core. Some minor modifications to their structure may have resulted from impacts with other asteroids but generally the structures have not changed since their formation. Pieces resulting from impacts between these planetesimal bodies are the source of the iron and stony-iron meteorites.


Analyses based upon the radioactive half-lives of certain elements in the iron meteorites suggest the separation of the metal, and the formation of the solid metallic cores by cooling, took place within a few million years of their initial melting. Dating of certain groups of the iron meteorites indicates they solidified on the order of 4,500 million years ago.


[1]The composition of many asteroids have been determined by spectroscopic analyses.

[2]The Earth has a diameter of more than 7900 miles and the moon a diameter of 2160 miles.

[3]The exception being alterations which occur as a result of the pressure of impact with each other.

[4]Large meteoroids, such as the one responsible for the extinction of the dinosaurs, are not appreciably slowed by interaction with the Earth’s atmosphere.

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Iron Meteorites

  

  

As a metallurgical engineer, I am fascinated by the structure of iron meteorites. Iron meteorites come from the cores of planet-like bodies in the asteroid belt and represent less than 5 percent of the total meteorites found.


The artistry presented in my images is a creation of the universe and the solar system. My role is to polish, etch and photograph the complex and intricate structures revealed within the meteorites I have studied. While I do not collect the meteorites, I purchase them from reputable collectors who are also members of the Meteoritical Society, and who certify the origin of the meteorite.


In contrast to their general appearance, all metals, including iron in meteorites, are crystalline solids. The iron atoms within the crystals are neatly arranged in an orderly fashion. However, both terrestrial and meteoric metals are seldom a single crystal but rather composed of a myriad of tiny crystals which must be imaged using a microscope. Each of these crystals (which metallurgists call grains) will have their own orientation, and where two crystals join, there is an area of mismatch which is called a grain boundary. 


When viewed through a microscope, after polishing and etching the meteorite to enhance the grain boundaries, I can image the array of microstructures which formed nearly 4.5 billion years ago in the cores of asteroids. I am not the first person to do this. In fact, the microstructures of iron meteorites have been studied for more than a century, and a comprehensive study of iron meteorites can be found in the three volume Handbook of Iron Meteorites, Their History, Distribution, Composition and Structure, by Vagn F. Buchwald.[1]Other excellent references on meteorites in general include:


  • Meteorites, A Journey through Space and Time, by Alex Bevan and John De Laeter, 2002.[2]
  • Meteorites, by Caroline Smith, Sara Russell and Gretchen Bendix, 2011.[3]
  • The Metallography of Meteoric Iron, Stuart H. Perry, 1944[4]


Not all crystals within the meteorite have the same crystal structure or composition. A phase is defined as a chemically and physically uniform quantity of matter. Iron in meteorites can be present as one of two phases, taenite and kamacite. When the iron core of the asteroid originally solidified, it formed an iron-nickel alloy called taenite; however, as the iron core continued to cool to below 1400°F, some of the metal transformed to a second phase known as kamacite. The maximum nickel content of kamacite is less than approximately 6% so iron meteorites containing greater than 6% nickel will contain both taenite and kamacite. The microstructures of these meteorites will consist of large regions of kamacite with islands of a two-phase mixture called plessite. These islands frequently form a small number of common arrangements that have been given unique names (see the photomicrographs in the side ribbon). Which of these arrangements forms is highly dependent upon the local nickel content as well as the cooling rate.


Iron meteorites also frequently contain minor amounts of other phases such as Schreibersite (Fe,Ni)3P, Cohenite (Fe,Ni,Co)3C and Trolite (FeS). 


Each of the microstructures I capture in the Art of the Gods images is unique, just like the structure of a snowflake. The composition of the iron core within each asteroid will vary somewhat as will the cooling rate. Even within a given asteroid, there will be minor variations in both chemical composition and cooling rate. As a result, while the structure of meteorites may have similar overall appearances, the detailed arrangement of the grains will be unique to that meteorite and even to the sectioning plane prepared for examination.


The majority of the images I capture are created through the use of tint etchants, although a few are obtained using specialized lighting techniques (such as differential interference contrast or DIC). The tint etchants produce sulfide films on the meteoric grains, producing various shades of brown and blue, while leaving other areas unaffected and thus remaining white.


The images often take an hour or more to create as they are commonly captured at a magnification of from 200x to 1000x. To create an image suitable for producing a 20 inch x 40 inch print requires 50 or more fields-of-view to be stitched together into a composite image. At 1000x, a 20 inch x 40 inch print represents a 0.01 inch x 0.02 inch area on the sample surface.


The completed images are then printed as a metal print on aluminum through a process known as dye sublimation. Dye sublimation uses heat and pressure to infuse the image directly into the hard coating on the metal surface producing a durable, high definition print with vibrant colors.


[1]Published for the Center for Meteorite Studies, Arizona State University, by the University of California Press, ISBN 0-520-02934-8, 1975.

[2]Published by the University of New South Wales Press, ISBN 1-58834-021-X, 2002.

[3]Firefly Books, ISBN-13: 978-1554078332, 2011.

[4]Smithsonian Institution, United States National Museum, Bulletin 184. 

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