CHAPTERS EARTH's ORIGINS he origin of Earth has been a subject of great interest among scientists for a long time By the late 1800s geolo gists and biologists had collected evidence suggesting that Earth is at least hundreds of millions of years old much older than they had thought Geologists reasoned that vast stretches of time were needed for the slow work of erosion and sedimentation to have had the effects they apparently had Biologists felt that a long time was also needed for evo lution to have resulted in the great diversity of life seen today There were dissenting opinions though Physicists such as Lord Kelvin calculated that Earth apparently still molten inside should have cooled to a solid throughout if it were more than about 40 million years old Similarly astronomers figured that the Sun could not be more than a few tens of mil lions of years old They believed that the Sun shone because of energy from gravitational contraction so it could not have sustained its energy output for so long without a drastic reduction in size 10
EARTH's ORIGINS The discovery of radioactivity by Henri Becquerel in 1896 led to a solution to both problems Earth could be heated internally by radioactive decay of elements such as ura nium deep inside and this could last billions of years The Sun it turned out shines by nuclear fusion giving it a potential lifetime of at least 10 billion years Radioactive dating of rocks played a cru cial role in calculating Earth's age Some isotopes forms of an element with different atomic masses of some elements such as uranium decay by emitting radiation in the form of smaller particles such as electrons and alpha particles This leaves behind a suc cession of daughter products such as lead The rates of decay have been measured with great precision in laboratories By measur ing the ratios of the daughter products to the original radioactive elements and assum ing that the materials were trapped in a rock from the time it solidified the age of the rock can be calculated Rocks from the seafloor are rarely found to be older than 200 million years but those from the continents are occasionally found to date back more than 3 billion years In 2001 Australian researchers announced the dis covery of a small grain of zircon a mineral II
INVESTIGATING THE HISTORY OF EARTH containing zirconium that they dated to 4 4 billion years ago Moreover meteorites which likely formed at about the same time as Earth have been dated to about 4 5 billion years ago These findings help pin down a likely age of about 4 55 billion years for Earth itself THE FoRMATION OF EARTH It is believed that Earth formed along with the rest of the solar system from a cloud of gas and dust called a nebula Nebulae are seen today in other parts of the Milky Way and also in other galaxies Astronomers have found evidence of stars and planetary sys tems forming in many of these nebulae as the mutual gravitational attraction of the nebula's particles pulls them together According to this theory called the nebu lar hypothesis the cloud consisted mainly of hydrogen and helium most of which ended up in the Sun The outer planets Jupiter Saturn Uranus and Neptune are quite different from Earth They were far enough from the Sun and therefore cold enough that these gases along with other compounds such as methane ammonia and water were able to gravitationally collect around small rocky or metallic cores I2
EARTH's ORIGINS Disks of gas and dust around young stars in the Orion nebula about 1 500 light years away may be new solar systems in the making Four images taken by the Hubble Space Telescope show disks surrounding four different stars that are only about a million years old The planets of our solar system are thought to have arisen from such a protoplan etary disk about 4 6 billion years ago The central glow in each image is the star The disks range in size from about two to eight times the diameter of our solar system NASA 3
