A Rock from the Earth from the Moon
“And God made two great lights; the greater light to rule the day, and the lesser light to rule the night: he made the stars also.”
As a child, I was so enthralled by the Apollo moon landings. For many people, interest waned after Apollo 11 reached the Moon in 1969; not mine! I followed the adventures of every one. So it is of great interest to me that scientific news is still being made by researchers looking at rocks from the Moon gathered by the Apollo astronauts. Recent news has concerned a rock sampled by the astronauts of Apollo 14, who traveled to the Moon in 1971.
Much of the interest, however, has concerned the supposed age of the rocks. One rock – sample 14321 – has been dated by the rubidium-strontium method to 4.1 billion years old. This method – similar to the uranium-lead method – relies on measuring the relative amounts of radioactive rubidium-87 and stable strontium-87.
Rock 14321 has a piece in it that contains minerals that lunar geologists think do not normally occur on the Moon but do occur on the Earth. Therefore, they suppose that an object striking the Earth 4.1 billion years ago chipped a little piece of Earth away, which floated through space until it landed on the Moon. They further suppose that this little piece was re-melted by being struck by a meteorite.
The whole cosmic game of pool described sounds far fetched but is the sort of story that can be woven in order to explain difficult observations. For our part, we accept that the Earth is, in fact, older than the Moon – by three days!
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