^aT»»»\' ' • ' ^.•.< 



ability, objects of our own solar system and do not come 

 from beyond that. Now, since we know by radioactive dat- 

 ing methods, that meteorites were formed generally around 

 A]/2 billion years ago, we may also tentatively deduce that 

 the composition of the Sun itself had remained fairly con- 

 stant for that period of time otherwise the Sun's composition 

 today would be difTerent from meteorites. This can only 

 be a tentative deduction at this point because the argument 

 is, as you will notice, quite circular. 



Finally, since the earth's crust is so difTerent from meteo- 

 rites and the Sun's atmosphere we conckide that some proc- 

 ess must have taken place on the earth that did not take 

 place in meteorites. From a large amount of geophysical 



back to a little over 3 billion years are known. Thus, we 

 conclude that it took about 1 — billion years for the crust to 

 form. From the differences in chemical composition of the 

 crust relative to meteorites we can tell what chemical ele- 

 ments had to have been separated out of the original matter 

 to form the crust, and also make some deductions about the 

 elements that separated out in the downward direction to 

 form the core of the earth. What arises from all this is the 

 view of the earth as a layered planet with an iron-nickel 

 core, surrounded by a thick mantle with the composition of 

 a rock called peridotite (consisting of olivine and pyroxene 

 with some feldspar, and very much like stone meteorites in 

 composition except for the elements extracted to make the 



The Navajo iron meteorite (Hall 35) is about three feet long and contains a pre-ierrestrial crack. 

 Gouge marks were made by Indians attempting to break off pieces for use in tool-making. 



Widmanstalten pattern is illustrated 



in this iron meteorite from LaPorte, 



Indiana. 



evidence we know that the earth consists of various layers 

 going downward : the crust, the mantle, and the core, and 

 that these layers represent difTerent rock types with difTer- 

 ent chemical compositions and minerals. We also know that 

 the mantle makes up about 88% of the volume of the earth, 

 with the core making up over 10% and the crust, on which we 

 live, making up less than 2%. We may assume that the 

 earth was once a homogeneous object with no original layers 

 and with the same composition as meteorites and the solar 

 atmosphere, that is, formed from elements boiled off the 

 Sun also. Since astronomers believe that all the objects in 

 the solar system, planets and meteorites, were formed at 

 approximately the same time, then the earth must also be 

 4}/^ billion years old. This is, in fact, the basic method for 

 determining the age of the earth — from meteorites. Thus, 

 we may conclude that the layers formed during this 41/^-bil- 

 lion-year period. Now, by measuring the age of the oldest 

 crustal rock which can be found we can obtain a measure of 

 how long it took for the crust to form. Crustal rocks dating 



core and crust). Over this is a thin crust consisting of a 

 basaltic base (basalt consists of mostly feldspar and pyrox- 

 ene) with a granitic outer portion (feldspar and quartz). 

 During this large scale chemical reconstitution of the earth 

 obviously any original structures from the earth's early days, 

 namely, chond rules, would be destroyed. 



Thus, a study of the minerals and chemical compositions 

 of meteorites has permitted us to make some deductions re- 

 garding their origin, the origin of the earth, the chemical 

 history of the Sun, and a view of the interior of the earth. 

 Each of these considerations is, however, constantly being 

 reviewed by research workers in meteoritics as new data 

 come to light. At the present time some alternative views 

 are showing promise and may alter these deductions over 

 the coming years; however, the basic arguments will not 

 change. Even after the moon and other planets are visited 

 and sampled by men the key to the origin and early develop- 

 ment of the solar system will come from the study of me- 

 teorites. 



NOVEMBER Page 7 



