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9.6: Since Apollo

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    No humans have visited the Moon from 1972 until now, but some orbital missions have studied the Moon's magnetic field as well as X-ray and gamma-ray emissions, from which some variations of the surface composition could be inferred.

    The Moon was found to have no global magnetic field like the Earth, but its surface was weakly magnetized in some patches. Molten rock can become permanently magnetized if it solidifies in the presence of an external magnetic field, suggesting that in some ancient era the Moon, like Earth now, had a molten metallic core in which electric currents generated a magnetic field. Somewhat similar observations were made near Mars in 1998-2000.

    Some excitement was caused by indications from the Lunar Prospector ( spacecraft, which suggested that ice may exist on the moon, inside a deep crater near the Moon's south pole. A possible explanation was that some time in the past (perhaps long ago) a comet had crashed into the Moon, and comets contain considerable amounts of water ice. The energy of the impact, turned into heat, would of course evaporate the ice. However, some of the water vapor would form a temporary atmosphere around the Moon, and might condense again to ice in very cold locations, like craters near the pole, which are permanently shaded from sunlight.

    At the end of its mission, on July 31, 1999, Lunar Prospector was therefore steered to deliberately crash inside the crater. It was hoped the impact might create (briefly) a cloud of water vapor, which could be observed from Earth, but none was detected (

    There is little doubt that the future will see further lunar exploration, though a “lunar base" is probably far off. Astronomical and other observations can readily be made from Earth orbit, and providing life support on the Moon is not easy. Such a base will probably become attractive only after ways are developed for utilizing local lunar materials for construction and for fuel.

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