The Moon has been critical to Earth and the life that inhabits it. If Earth did not have its Moon, things would be very different!

• Earth would spin three times as fast, making a day last only 8 hours instead of 24.
• The rapid spinning would create gale-force winds and a magnetic field nearly 3 times as strong as we currently have.
• Tides would be much lower with only the Sun pulling on our oceans.
• No moonlight. Night would always be as dark as it currently is during the new moon.
• All of these differences would have large impacts on the development of life, which would have to adapt to short, alternating periods of light and utter darkness; constant ferocious winds; and much reduced tides. Chances are that humans as we have come to know them would not exist.
• No solar eclipses. If intelligent life did develop, how much longer would it have taken to discover the Sun's corona? (By the way, Earth's total solar eclipse is one of the most startling coincidences in nature.)

Importance to understanding of terrestrial planets

The Moon has the lumpiest known gravitational field in our solar system.

When you're trying to learn the nature of an object, it is instructive to examine more than one example.

We know the Earth well because we live here, but we need to look at the other planetary bodies in the inner solar system if we really want to understand how they all developed.

The Moon is especially useful because, unlike Earth, Mars, and Venus, it's thought to have changed very little after it first formed. So it gives us a unique opportunity to look into the distant past of planetary evolution.

Just as the discovery of other solar systems has caused us to rethink our ideas about how they form -- previously based only on the one example of the solar system in which we live -- investigating the structure, composition, and processes involved in the development of neighboring worlds such as the Moon will help us to understand how our own planet came to be.

Unresolved lunar mysteries

• What is the full story of the Moon's origin?
• The Moon's shape appears to have frozen into place when it was much closer to Earth, but that doesn't fit other observations of the Moon's development. What really happened?
• Why is the side that faces Earth so different from the side that faces away? On the near side, the crust is thinner and there is evidence of much more volcanic activity (which created the vast dark "seas" responsible for the "man in the moon").
• Why does the Moon have magnetized rocks? Did it ever have a dynamo, like Earth, generating a magnetic field?
• What are the "mascons" that give empty craters as much gravity as mountains?
• What created the South Pole-Aitken basin, the largest and deepest known impact crater in the Solar System? And what can it tell us about the Moon's structure and history?
• What is the story behind KREEP -- rocks rich in potassium, rare Earth elements and phosphorous which have no counterpart on Earth?