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On July 20, 1969, Neil Armstrong became the first man to step onto the surface of the Moon. He was followed by
Edwin Aldrin, both of the Apollo 11 mission. They and other moon walkers experienced the effects of no atmosphere.
Radio communications were used because sound waves can only be heard by travelling through the medium of air. The
lunar sky is always black because diffraction of light requires an atmosphere. The astronauts also experienced
gravitational differences. The moon's gravity is one-sixth that of the Earth's; a man who weighs 180 lbf (pound-force)
on Earth weighs only 30 lbf on the Moon. (The equivalent metric weight (or force) is the Newton, where 4.45 Newtons
equal one pound-force.)
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Four nuclear powered seismic stations were installed during the Apollo project to collect seismic data about the
interior of the Moon. There is only residual tectonic activity due to cooling and tidal forcing, but other moonquakes
have been caused by meteor impacts and artificial means, such as deliberately crashing the Lunar Module into the moon.
The results have shown the Moon to have a crust 60 kilometers (37 miles) thick at the center of the near side. If this
crust is uniform over the Moon, it would constitute about 10% of the Moon's volume as compared to the less than 1% on
Earth. The seismic determinations of a crust and mantle on the Moon indicate a layered planet with differentiation by
igneous processes. There is no evidence for an iron-rich core unless it were a small one. Seismic information has influenced
theories about the formation and evolution of the Moon.
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The Apollo and Luna missions returned 382 kilograms (840 pounds) of rock and soil from which three major surface materials
have been studied: the regolith, the maria, and the terrae. Micrometeorite bombardment has thoroughly pulverized the surface
rocks into a fine-grained debris called the regolith. The regolith, or lunar soil, is unconsolidated mineral grains, rock
fragments, and combinations of these which have been welded by impact-generated glass. It is found over the entire Moon, with
the exception of steep crater and valley walls. It is 2 to 8 meters (7 to 26 feet) thick on the maria and may exceed 15 meters
(49 feet) on the terrae, depending on how long the bedrock underneath it has been exposed to meteoritic bombardment.
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