The Terrestrial Planets

A new world in our hands.
Apollo 17 astronaut Harrison Schmitt uses a special rake to collect rock and soil samples from the Moon Littrow Valley. Pieces of the Moon, brought back to Earth, yield a detailed picture of the nature and history ofour nearest neighbor world. Samples from the lunar surface also bear clues to the history of the Sun: trapped gas atoms that have been sprayed out from the Sun over billions of years.

The Moon

Is the Moon a planet? Technically no, for it revolves around the Earth rather than directly around the Sun. But the Moon is a large world, comparable in size to the planet Mercury. In composition, origin, and history, the Moon is like a terrestrial planet, resembling the Earth in many ways, different in others. These similarities and differences make the Moon an important starting point for the study of other worlds.

Because it is close to the Earth, the Moon is our most intensely studied "new" world. It has been reached by many spacecraft from both the United States and the USSR. On each of six missions, two American astronauts landed and studied the lunar surface. Manned and unmanned satellites placed in orbit around the Moon have made detailed photographs and mapped the gravity, magnetism, and chemical composition over much of its surface. Instruments placed by our astronauts on the surface have investigated the once wholly inaccessible interior of the Moon. Especially important has been the return of actual lunar samples from nine sites (six American, three Russian) on the Moon. By analyzing these samples in laboratories on Earth, we have measured the Moon's precise composition, deduced its history, and inferred the nature and variations of the space environment around the Moon over long intervals of ancient time.

The composition of the Moon is much like that of the Earth. Its rocks are similar to Earth rocks and contain many but not all of the same minerals. However, the Moon has no atmosphere, it shows no trace of past or present life, and its rocks contain no water.

The Moon rocks show that the lunar surface is very ancient. Its rocks are older than most of the rocks now found on Earth, and the Moon has preserved a record of the earliest history of the formation and development of the planets. In its oldest rocks, we detect indications of an intense primordial melting that left the new born Moon covered with a seething ocean of molten lava hundreds of kilometers deep. Gradually, the lava ocean cooled and solidified, forming the light-colored crust we call the lunar highlands, which still covers 80 percent of the Moon.

Lava from the lunar "seas".
This chunk of bubble-rich lava, collected by the Apollo 15 astronauts, typifies the surface rock of the dark maria (or "seas') on the Moon. Tremendous volcanic eruptions about 3.5 billion years ago flooded much of the Moon with molten lava resembling the volcanic rocks found in Hawaii and Iceland. Unlike Earth rocks, the lunar specimens contain no water; the nature of the gas that made the bubbles remains a mystery.

The newly formed lunar crust was subjected to long-term catastrophic bombardment by meteoroids and asteroids, some as large as Rhode Island or Delaware. These violent impacts formed overlapping craters and huge circular basins that can still be seen on the Moon. Then, when the bombardment subsided, about 4 billion years ago, radioactivity began to heat the inside of the Moon. The lunar rock melted at depths of hundreds of kilometers, and fresh lava rose to the surface, spreading to form the great, dark regions, or maria, of the Moon.

The lavas from these eruptions resemble lavas found on Earth in places like Hawaii and Iceland. But, unlike terrestrial lavas, they contain no water, and the crystals in them are as clear and gemlike as if they had been erupted yesterday rather than 3.5 billion years ago.

The rubble of ages.
Collected by Apollo 16 astronauts on the lunar highlands, this light-colored rock (or breccia) was formed from pieces of many different rocks, shattered, melted, and mixed together by the great meteorite impacts that rocked the Moon during its early years. Me complex breccias are the key to understanding how the Moon and other planets developed. Some fragments in this specimen may be pieces of the original lunar crust that formed 4.5 billion years ago.

rough shapped light colored lunar rock with dark patches

For the last 3 billion years, the Moon has been a very silent world. A steady, gentle bombardment by tiny meteorites has gradually chipped away at the lunar bedrock, creating a slowly stirred surface layer of powdery rubble, the lunar "soil." During these billions of years, the soil particles have been recording the space environment, retaining traces of the steady bombardment of the Moon by cosmic dust, by charged atomic particles from the Sun, and by cosmic rays from the stars. Unlike the geologically active, continually changing Earth, the Moon has been a virtual "museum world", a key to understanding the history of the Sun and the planets.