The Moon: Earth's Sneezeguard

Mandi Walls writes "SF Gate is running an article about looking on the moon for pieces of Earth that may have been knocked into space by collisions with asteroids, etc. The article claims the guys responsible for the idea came up with it while stuck in traffic. They were probably digging for change for a toll in the seats."

Comments

Isn't the moon itself a piece of the earth?

[T.Hobbes]

I had heard in SciAm awhile ago that the moon was created when a mars-sized asteroid hit a young (~4bn yrs), molten earth. This sent a v. large mass of rock into orbit which coalesced into what is now the moon.

In fact, after a little searching I found this at NASA:

How did the Moon come to be? The leading theory is that a Mars-sized body once hit Earth and the resulting debris (from both Earth and the impacting body) accumulated to form the Moon. Scientists believe that the Moon was formed approximately 4.5 billion years ago (the age of the oldest collected lunar rocks). When the Moon formed, its outer layers melted under very high temperatures, forming the lunar crust, probably from a global "magma ocean."

A few more links: Perty image and more detailed explanation; a google search on the topic.

Re:Isn't the moon itself a piece of the earth?

[pyr0]

Just to defend radiometric dating somewhat here, I have actually taken a course on the subject so I know a bit about it. Depending on what isotopic system you are working with (U-Th-Pb, Rb-Sr, Sm-Nd, etc.), you have to make several assumptions about the rock you are trying to date. When dating lunar material, many of these assumptions are completely valid, the main one being a closed system. There is no water to leach elements from these rocks, and no atmosphere to speak of on the moon, therefore the system has effectively been closed to the exchange of parent and daughter products since their time of crystallization. Even if there was weathering going on at the Moon's surface, all it takes is very careful sample selection as well as picking the proper isotopic system in order to get a very good date. This is why we can date rocks on the Earth at all.

The other interesting thing is that the same date is found +- a couple millions years or so using different isotopic systems, which is a great check for accuracy.

Archimedes, not Newton

[T.Hobbes]

You're thinking of Archemedies. From school-for-champions.com(link),

Story of Archimedes

The ancient Greek philosopher and scientist, Archimedes was asked by the king to determine if a gold statue he had was 100% gold. Since it was an odd shape, Archimedes could not simply measure the volume to determine the object's density and thus its composition.

Archimedes decided to take a hot bath, to help him think about this problem. When he got in the bath tub, he noticed the water rise. This clue led to the discovery that an object will displace its volume when immersed in a liquid. When Archimedes realized that objects displace their volume in water, he excitedly jumped out of the tub and ran down the streets shouting, "Eureka! Eureka!" which means, "I have found it!" Unfortunately, he didn't notice that he forgot to put his clothes on! When Archimedes put the statue in a container full of water, he measured the volume of the overflow to determine the volume of the statue. Then he measured the weight of the statue and compared its density with the known density of pure gold. He discovered that the statue was not made of pure gold, rather it contained some other metal, like lead.