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Formation of the Moon

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Did Earth Have Two Moons?
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Did Earth Have Two Moons?

Credit: Lunar Reconaissance Orbiter, NASA
Source: http://en.wikipedia.org/wiki/File:Moon_nearside_LRO.jpg
License: CC BY-NC 3.0

Earth’s gravitational pull on the Moon is so strong that the same side of our satellite always faces us. The far side turns out to be shockingly different. How that difference came to be is a mystery scientists have yet to solve.

Why It Matters

Credit: NASA/GSFC/Arizona State University
Source: http://en.wikipedia.org/wiki/File:Moon_Farside_LRO.jpg
License: CC BY-NC 3.0

The far side of the moon is very different from the near side: it is heavily cratered with just a minor maria coating [Figure2]

  • The near side of the Moon, the side we see from Earth, has distinctive flat black maria, lava flows that cooled recently enough that they are not heavily cratered.
  • The far side of the Moon is more heavily cratered and has few maria. There are thick and extensive highlands on the far side.
  • The far side was first photographed in 1959 by a Soviet probe, and seen in 1968 by Apollo 8 astronauts.
  • The NASA Grail spacecraft was launched in September 2011 to study the Moon.

Explore More

With the links below, learn more about the two moon hypothesis. Then answer the following questions.

  1. What is the possible explanation for the differences between the near side and far side of the Moon described in the video?
  2. What is the evidence for this hypothesis?
  3. What evidence does the video suggest that the Grail mission will look for that there was a second moon?
  4. What evidence did Grail find for the existence of the second moon?
  5. What happens to this hypothesis now?

Image Attributions

  1. [1]^ Credit: Lunar Reconaissance Orbiter, NASA; Source: http://en.wikipedia.org/wiki/File:Moon_nearside_LRO.jpg; License: CC BY-NC 3.0
  2. [2]^ Credit: NASA/GSFC/Arizona State University; Source: http://en.wikipedia.org/wiki/File:Moon_Farside_LRO.jpg; License: CC BY-NC 3.0

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