Note: This program first aired on November 23, 2013.
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One of the questions that is inevitable when we contemplate the moon is how did the moon get there? Was it always there? And what does “always” mean anyway? The moon’s origins date back to almost the origin of the Earth 4.6 billion years ago. The Earth formed from material that slowly coalesced, gas and dust that were part of a nebula that collapsed and became our sun. All the planets were formed from the dregs of the remaining left over material. Early Earth would have been a difficult place to visit, as it was covered by a magma ocean. The whole planet was liquid rock, heated by the radio active decay of elements in the core, and heat transferred to the surface by near constant asteroid impacts. The early days of the solar system were chaotic, and asteroid strikes were very common.
Because the Earth was at that time, entirely molten, fluids of different densities were able to sort themselves out, following the laws of physics. Denser materials were able to sink towards the center of the Earth, less dense materials floated outwards in response. By the time of the moon’s creation, 100 million years or so after the creation of the Earth, the Earth was already stratified, the layers we all learned in grade school, the core, the mantle and the crust were differentiated by their differing densities. This fact is im portant because it relates to one of the key clues we have about the origins of the moon.
The current most widely accepted theory of the origin of the moon is the impact theory, which states that the moon formed when a very, very large asteroid, or small planet collided with the Earth, and the force of that impact caused a huge mass of material to be ejected from the opposite side of Earth. That ejected material coalesced and became the moon. So the first point to remember is that the moon is made of Earth material. At the time of this collision, the earth was still covered by the magma ocean, meaning, it was still so hot at the surface that the rocks were still molten. The material that would make up the crust, when it cooled down enough to form, was what was at the surface, but in fluid form. When the asteroid hit, it was this material, and some of the underlying liquid mantle rocks, that were thrown up into space to become the moon.
We know this because the moon has differentiated layers, a crust, a mantle and a core, just like the Earth. This tells us that also just like the Earth, the moon was at one time, entirely molten. The only way the materials of a planet can differentiate by physical properties is if those materials are fluid enough to move. The moon is a great deal smaller than the Earth though, so those layers have cooled much faster than Earth’s layers. The moons layers also differ from Earth’s in their composition. The Earth’s core is made of metal, iron and nickel primarily, the heaviest of commonly occurring elements present when Earth formed. The moon’s core lacks large quantities those heavy metal elements which tells us that though the moon came from Earth material, most of that Earth material came from what would end up as Earth crust and upper mantle. The heavy material on Earth had already sunk down to the center of the Earth, forming the core, and was thus not available to be ejected from Earth when the asteroid hit. On a side note, the lack of iron in the moon’s core means that the moon as an incredibly weak magnetic field, as it is the spinning of the iron in a planet’s core that generates that field.
So when I said last week that the moon is made of us, I meant it. The stuff of our bodies, the elements we take in from our food, we breath in with the air came from no where else but the surface of the Earth itself. We are literally made of Earth, and Earth is made of star dust and elements formed inside long dead stars. And the moon is also made of Earth, so when you look up on these winter nights, see her for what she really, on some level, is: your sister.
References:
Age of the solar system http://www.universetoday.com/15575/how-old-is-the-solar-system/ They also have many other terrific short articles on the age of the moon, and the Earth, and all kinds of other things astronomical.
From the Planetary Science Research group, info about lunar geology: http://www.psrd.hawaii.edu/April04/lunarAnorthosites.html
Many scientists are very devoted to the moon, and fund their own institutes pursuing lunar science: http://www.psi.edu/epo/moon/moon.html
Here’s an entire e-course on Astronomy from the University of Northern Iowa: http://www.uni.edu/morgans/astro/index.html
They like to chat it up at NASA, here’s the transcript of a NASA chat about the moon: http://www.nasa.gov/connect/chat/moon_core_chat.html#.UoGWIyR4OHk
