THE MOON: A FOSSIL PLANET
MOONS OF THE SOLAR SYSTEM--WALTZING WITH THE MOON

Unlike the Earth - a dynamic planet where weather has eroded the shifting surface and erased the record of early history - the Moon has hardly changed. Its gravitational force - only a sixth of that of the Earth - is not great enough to grasp or retain an atmosphere. As a result, there is no water, no weather, and no wind.Until recently three competing theories attempted to explain the origin of the Moon, some 4.5 billion years ago. The "fission" theory claimed that the early Earth was rotating so fast that a chunk of it broke away and was flung out into space to form the Moon - leaving, it was even suggested, a scar on the Earth's surface in the form of the Pacific Ocean. The "double planet" theory contended that the twin bodies of Earth and Moon formed independently from the same primordial cloud of dust and gas. And the "capture" theory proposed that the Moon formed elsewhere in the solar system and was pulled into orbit by the Earth's gravity as it flew past the Earth. None of these theories, however, was confirmed by the Apollo missions of the late 1960s and early 1970s. The Apollo findings showed that the chemical compositions of Earth and Moon are very different - that, for example, there is little or no iron, water, or sodium on the Moon. These discoveries raised more questions than they answered.

Over the past few years a fourth theory, which has been tested by computer simulations, has been gaining strong supportfrom scientists. The "giant impact" theory argues that the Moon was created when a planetary body collided with the early Earth. In the crush, hot jets of vapour were blasted into space, where they cooled, coalesced, and eventually condensed into a "protomoon". Iron from the two bodies stayed within the Earth, and volatile materials, such as water and sodium, simply boiled away.

Soil samples collected by the Apollo astronauts record subsequent phases of the Moon's development. The highlands were the first areas to form as the lighter materials in the interior of the molten Moon floated like scum to the surface, where they cooled to form a primitive crust. Giant meteorites bombarded the Moon, scarring the highlands and blasting out great basins in the surface. Although they are waterless, these basins reminded the early astronomers of oceans, and each of them is still known by the Latin word for sea, mare. Between 3.9 and 3.2 billion years ago, lava welled up from the interior of the Moon. These volcanic outpourings flooded the basins and filled them with layers of dark basaltic materials that then solidified into the grey plains of today. Even after the volcanic activityhad died down, meteorites continued to pound and remodel the surface.

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