5. Lunar Secrets

No place beyond Earth is more well studied than the Moon. We went there, stomped around. Sifted the soil. Brought some rocks back. But the Moon still holds many secrets. The most profound might be rocks launched from Earth billions of years ago by asteroid impacts.

These storehouses of terrestrial information have been presumed for years to exist on the Moon; this July an attempt was made to quantify them. The estimate: 11,000 pounds of Earth stuff sits within a few inches of the surface for every square mile on the Moon.

The rocks should hold information about the composition of the young Earth and its atmosphere, and possibly even the origin of life. This information is not available anywhere else because, unlike the Moon, Earth continually recycles its surface material, folding it inward and melting it beyond recognition.

Nobody can say for certain this stuff is there, or whether it can be retrieved, but researchers are optimistic.

"This [new study] gives us a compelling reason to go back -- to look at the Moon as a window to early Earth," said study leader John Armstrong of the University of Washington. He added that it would be the fastest and cheapest way to learn about planet's early years and the formation of the whole solar system.

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