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The Moon: Not Green Cheese After All

In 2011, the twin GRAIL space probes were launched to map what the moon is made of. Upon reaching the moon exactly one year ago, they began to carefully map the density of the entire crust of our largest satellite.

The mission was a complete success; the map was finished late this year. At that point, NASA put the probes into what's called a decaying orbit. The craft spiraled down towards the surface, falling lower each revolution until they crashed into the surface earlier this month to end the mission. The map they left behind (shown in the NASA image below) tells some interesting stories about the moon.

NASA Moon Map Sm.png

The Power of Asteroid Impacts

When you look at the map, craters pop out dramatically. The rings of color are the patterns left in the moon's crust by space rock impacts. Blue areas are less dense and red areas are more so. Since the moon doesn't have erosion (no wind, rain, rivers, living organisms to shape its surface), craters stay forever. An amazing 12% of the moon's surface is actually empty space, smashed in by impacts. This level of destruction is even greater than expected by astrophysicists.

The early solar system was a place where massive space rock impacts were frequent. The moon's surface is an indelible map showing the ultra-violent ferocity of this time.

The Moon Grows and Shrinks

Some features on the density map resemble giant slashes or lines. These are called dikes, and they indicate volcanic activity related to the growing and shrinking of the moon's crust. During its first billion years, the moon was very hot, and grew in size from volcanic activity. Eventually it cooled down, and now it is actually shrinking. These maps are the only known way to find and study this phenomenon.

The Moon is Made of Earth

No one knows for sure where the moon came from and how it ended up orbiting the earth. The most likely theory however, is that early in the planet's history, an enormous object (roughly the size of the moon) smashed into it. The resulting explosion was so violent that a moon's-worth of earth's rock was blasted into space. All that rock slowly coalesced into the moon.

The GRAIL data supports this theory, showing that both the earth and the moon contain about the same amount of aluminum. This would probably not be the case if the moon formed in some far away place and was "captured" by the earth's gravity.
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Tom Hartsfield
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