![]() Large bodies slamming into the planet produced immense heat in its interior, melting the cosmic dust found there. The process of accretion had significant thermal consequences for Earth, consequences that forcefully directed its evolution. Wetherill of the Carnegie Institution of Washington suggests that about 100 million years could pass between the formation of an object measuring 10 kilometers in diameter and an object the size of Earth. Fewer items available for accretion meant that it took a long time to build up a large planet. ![]() Consequently, the number of collisions between planetesimals, or meteorites, decreased. As the planetesimals became larger, their numbers decreased. The Russian geophysicist had suggested in 1944 that planets grew in size gradually, step by step.Īccording to Schmidt, cosmic dust lumped together to form particulates, particulates became gravel, gravel became small balls, then big balls, then tiny planets, or planetesimals, and, nally, dust became the size of the moon. ![]() This observation rejuvenated the theory of accretion postulated by Otto Schmidt. Thereafter, the number of impacts appeared to have quickly decreased. Studies of moon craters revealed that these gouges were caused by the impact of objects that were in great abundance about 4.5 billion years ago. In the 1960s the Apollo space program changed this view. Scientists used to believe the rocky planets, including Earth, Mercury, Venus and Mars, were created by the rapid gravitational collapse of a dust cloud, a deation giving rise to a dense orb. Exploring this past offers us the only possibility of understanding the origin of life and, perhaps, its future. These forces were gradually joined by the global effects of the emergence of life. From the outset, heat and gravity shaped the evolution of the planet. Such constant change has characterized Earth since its beginning some 4.5 billion years ago. Plate tectonics shift the continents, raise mountains and move the ocean floor while processes not fully understood alter the climate. Earth and its atmosphere are continuously altered. Yet this constancy is an illusion produced by the human experience of time. Continents and oceans, encircled by an oxygen-rich atmosphere, support familiar life-forms. Like the lapis lazuli gem it resembles, the blue, cloud-enveloped planet the we recognize immediately from satellite pictures seems remarkably stable.
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