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When a rock forms, some of its atoms will be radioactive ‣Radioactive “parents” decay to “daughter atoms” over time ‣Rate of decay is measured in half-lives o‣Every half-life, 50% of the remaining parent atoms decay o‣After 1 half-life, 50% left o‣After 2 half-lives, 25% left...
To measure the passage of long periods of time, scientists take advantage of a regularity in certain radioactive atoms. In a radioactive decay, the original radioactive atom is called the “parent” and the resulting atom after the decay is called the “daughter”. For example, Uranium-238 is the parent that breaks apart to form the daughter Lead-204. Radioactive isotopes will decay in a regular way such that one-half of a given amount of parent material will decay to form daughter material in a time period called a half-life. Radiometric dating is an absolute dating system because you can determine accurate ages from the number of remaining radioactive atoms in a rock sample. Radioactive dating gives the solidification age of the rock. Figure: Note that at time 0, the time of the mineral's formation, the crystal contains only parent atoms. At time 1, 50% of the parent atoms remain; at time 2, only 25% remain, and so on. The oldest rocks in the solar system ‣Oldest Earth rocks: 4.3 billion years old ‣Oldest Moon rocks: 4.48 billion years old ‣Oldest Mars rocks: 4.5 billion years old ‣Oldest meteorites (asteroid fragments): 4.56 billion years old ‣Meteorites tell us the age of the solar system! The only celestial bodies from which scientists have samples are Earth, the moon, Mars, and meteorites. • • • The oldest Earth rocks so far discovered and dated are tiny zircon crystals from Australia, 4.3 billion years old. The surface of Earth is active, and the crust is continually destroyed and reformed.Unlike Earth’s surface, the moon’s surface is not being recycled by constant geologic activity. The oldest rocks brought back by the Apollo astronauts are 4.48 billion years old. That means the moon must be at least 4.48 billion years old. Over a dozen meteorites found on Earth have been identified by their chemical composition as having come from Mars. The oldest has an age of approximately 4.5 billion years.The most important source for determining the age of the solar system is meteoritesfrom asteroids that are unchanged since they condensed and accreted from the solarnebula. They have a range of ages with an upper limit of 4.56 billion years. •This is accepted as the age of the solar system; often rounded of to 4.6 billion years. Mercury is a small, airless, extensively cratered world. Venus is nearly the same size as Earth, but it is shrouded by a perpetual cloudcover that hides its surface from view. Only with radar can we observe its surface. (click to reveal a radar image of Venus’ surface)
The Earth is unique in many ways. It has an abundance of liquid water, a thin but nottoo thin atmosphere, an active surface, and life. The Moon’s airless, dry surface is covered with plains and craters. Mars has captivated the popular imagination like no other planet, showing evidence of having had liquid water on its surface for extended periods.