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DAVID POGUE (Technology Guru): Scientists now know that most elements come in more than one version. The different versions are called isotopes.
Consider carbon, the backbone of life. It has three natural isotopes, or versions. Each has six protons and six electrons. That's what makes them all carbon. The difference between them is the number of neutrons in the nucleus.
Neutrons are electrically neutral particles that act as glue to hold atoms together. What we think of as normal carbon is called carbon-12: six protons plus six neutrons. But about one percent of carbon atoms have an extra neutron, giving them seven. They're called carbon-13.
And about one in a million have eight neutrons. That's carbon-14.
And that rare version of carbon has proven to be a crucial tool for unlocking the past.
Several times a year, scientist Scott Stine travels to the shores of Mono Lake, near Yosemite National Park.
So this, then, is Mono Lake.
SCOTT STINE (Paleoclimatologist): Mono Lake, yeah. Right here, at the foot of the Sierra Nevada.
DAVID POGUE: He's studying the long history of droughts in California, trying to determine how frequently they occur and how long they last. Over the millennia, the water level has risen and fallen, as the area has cycled between wet periods and dry times.
So that sandy area should be the level?
SCOTT STINE: During times when the climate was dry, Mono Lake dropped down, exposed the shore lands, and allowed trees and shrubs to grow. When the dry periods ended and the water level rose, the trees drowned, marking the end of the droughts.
Since then, the remains of those trees have been well preserved by the arid climate.
These droughts were long and persistent.
DAVID POGUE: To determine how long ago these droughts occurred, Scott is using carbon-14 to date the trees.
Unlike the other natural isotopes of carbon, carbon-14 is unstable. Over time, its atoms begin to deteriorate. One of its neutrons turns into a proton and spits out an electron. Now, with seven protons instead of six, it's turned into nitrogen.
That process is called radioactive decay. And scientists know exactly how long it will take for half of any amount of carbon-14 to decay away. Scientists call that time its "half-life."
Living things constantly replenish the carbon in their bodies, animals from food, plants from the atmosphere, but after death, that process stops.
The amount of carbon-12 stays the same, but the carbon-14 decays away, at a constant rate, making carbon-14 a ticking atomic clock.
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