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Re: A Look At The Age of The Earth.

	In 1956 a bold step was taken in the realm of dating the Earth.
Geoscientists had long been working on a way to date the age of the Earth,
but they could not get to the absolute age. Due to continent formation,
and plate tectonics, most of the early history of the Earth had long since
been erased.
	Meteorites seemed to be the alternative to looking for the answer
in the Earths crust. Meteorites are from the Solar System, that was well
established at this point, but were they from the very beginning was the
question? That was answered by looking at Chondrites.
	Chondrites are undifferentiated meteorites, which means they have
undergone none to minor heating since their formation. On the other hand
the achondrites had all experienced planetary differentiation. This means
they had been melted, and recrystalized. This heating and recrystalizing
tends to restart the atomic clocks of elements like Lead, argon, and
potassium. The ages of achondrites would give a time only when the
asteroid that spawned them finally cooled. Of course that was too late in
the history of the planet Earth.
	Claire Patterson, from the California Institute of Technology went
ahead in 1956 to make a absolute statement about the age of the Earth. Dr.
Patterson wrote: " It seems we now should admit that the age of the Earth
is known as accurately and with about as much confidence as the
concentrations of aluminium is known in the Westerly Rhode Island
granite." 
	What Dr. Patterson was saying is basicly it's time to stop beating
around the bush, and just say the age of the Earth is 4.55 billion years
old. He points out that Uranium isotopes have been used to date chondrites
for sometime, and it seems that all tests came to the same answer. Also he
points out other isotopes such as Argon, and Strontium were being used to
determine ages as well. Granted isotopic age dating was not as precise as
it would become in later decades, but what was known was known well,
and within small experimental error. 
	Dr. Patterson admits that it is difficult to defend the
calculations used, but logical reasoning seemed to be the support. The
support comes from these assumptions made about meteorites:

1. Meteorites formed at the same time, and in closed systems.
2. They originally contained lead of the same isotopic composition.
3. They contain Uranium that has the same isotopic composition as that
found in the Earth.
	These assumption bolster the idea that the Earth and meteorites
were made in similar ways, and similar materials. At the time however,
this was difficult to "prove". 
	Dr. Patterson was close to the currently accepted age of the Earth
which is 4.6 billion years old. Not bad for some one working with minimal
equipment. This is the first occurrence of an age being firmly argued for
in the literature known to this author, and is quite a monumental
statement.
	It seemed as though modern science had solved a long standing
question that is fundamental to human existence. Now the Earth had a
beginning nestled in deep time. Since the advent of new analytical
methods, the age stayed relatively the same. This date puts a time
constraint on the origin of the Solar System, origin of life, and  gave
a frame of reference on how the Earth evolved up till the present.
	Meteorites tell us how our planet formed, and processes that
controlled the formation. They help us to see beyond the everyday, and
into a time far removed from the senses. They are truly a gift from the
infinite reaches of time.


Reference: Patterson, Claire, Age of meteorites and earth
              Geochimica Et Cosmochimica, 1956, Vol 10 pp.230-237  


Frank Stroik
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