The Moon Does Not Yield her Secrets Easily

Recent lunar missions have shown that there is still much to learn about Earth's closest neighbor.

Although Earth's satellite was the first planetary object to be explored by spacecraft and the only body ever visited by astronauts, scientists have many unanswered questions about its history, composition and internal structure. In recent years, researches have called for renewed exploration of the moon; the European Space Agency and Japan are planning to send probes into lunar orbit, and NASA is considering landing an unmanned spacecraft on the moon*s far side. By studying the moon, these missions may also illuminate the history of all the rocky planets in the inner solar system: Mercury, Venus. Mars and especially Earth. Because the moon's surface has remained relatively unchanged for the past three billion years, it may hold the key to understanding how the inner planets formed and evolved.

When astronomers first gazed at the moon through telescopes 400 years ago, they found that its surface consists of two principal types of terrain (местность): bright, rugged, (неровный) heavily (массивно) created highlands and dark, more sparsely (разбросанный, редкий) cratered lowlands. Galileo Galilei, the 17 - century astronomer, called the lowlands maria - Latin for "seas" - because of their smooth, dark appearance. One of the biggest surprises of the space age came in 1959. when the Soviet spacecraft Luna 3 photographed the moon's far side, which had never been seen before because it is always turned away from Earth. The photographs showed that it almost completely lacks the dark maria that are so dominant on the near side. Although scientists now have some theories that could explain this dichotomy (фаза луны) terrain, it remains an unsolved puzzle.

Analisys of the lunar rocks and soil brought back to Earth by the Apollo astronauts and by unmanned Luna landers allowed researches to get a glimpse of the moon's evolution. The evidence (факты) suggests (наводить на мысль) that the moon was created about 4.5 billion years ago when a Mars-size body hit the early Earth. This collision sent a spray (пыль) of rocks into orbit around Earth, and these small bodies rapidly coalesced (срастались, объединялись) into the moon. They accumulated so quickly that the heat generated by the process melted the outer portion of the nascent (образующийся, возникающий) moon and formed a global ocean of liquid rock, or magma. The lunar crust then formed low-density (разреженный) minerals that floated to the surface of this magma ocean.

This early phase was followed by a violent pelting (удар. проливной) of the moon's surface by comets, asteroids and meteoroids. Some of the larger objects blasted out (взорвать, образовать) enormous basins more than 2.000 kilometers in diameter. Most craters and basins, at least on the near side, were filled with iron-rich basaltic lava over the next 300 million to 400 million years, forming the dark maria seen today. As time went on, the bombardment eased, with impacts becoming less frequent and less powerful. This fact explains why the maria. which are younger than the highlands, have fewer and smaller craters. Little has occurred on the moon since about three billion years ago; after the volcanic fires died, the only activity has been the occasional formation of an impact crater, the constant rain of micrometeorits and the six blink-of-an-eye visits by a dozen astronauts more than 30 years ago.

Because the moon has experienced impact, volcanism and tectonic activity, it can serve as a touchstone (критерий) for understanding those processes. In particular, the moon's companionship (соседство) to Earth makes it an ideal place for studying the extraplanetary events that occurred in this part of the solar system during its early history. Nearly all traces of the asteroids and comets that struck Earth billions of years ago have been erased from our planet's geologically active surface. Yet this record is preserved on the moon, where it can be recovered and read.

Scientists learned much from the Apollo explorations, but many mysteries remained after that program ended. Researches realized that they needed to map the moon globally with a variety of remote-sensing instruments (дистанционный измерительный прибор, средство дистанционного зондирования). A hint of the fascinating discoveries awaiting global reconnaissance (исследование) came from two flybys of the Earth-moon system in the early 1990s by the Jupiter-bound (направляющийся на) Galileo spacecraft. In the southern hemisphere of the moon's far side, mission scientists (ученые, выполняющие задание) saw an unusual signature of high-iron rocks in the floor of the South Pole-Aitken (SPA) basin, the largest basin on the moon. Galileo also mapped some of the maria using spectral filters that provided information on surface composition: the results suggested that researchers could use remote spacecraft data to delineate (начертить) the sequence (следование) of lava flows in the maria.

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