The effects of impacts and how they might affect us here on Earth, global climate change (Venus vs.Earth) and what could happen to Earth in an extreme case, etc.Any item that derives from a previously living organism—for example, a basket made of plant fibers or a scrap of animal hide—can be subjected to carbon-14 dating.

what is radioactive dating and what isotopes are used-66what is radioactive dating and what isotopes are used-50what is radioactive dating and what isotopes are used-81

But for humans whose life span rarely reaches more than 100 years, how can we be so sure of that ancient date? Even the Greeks and Romans realized that layers of sediment in rock signified old age.

But it wasn't until the late 1700s -- when Scottish geologist James Hutton, who observed sediments building up on the landscape, set out to show that rocks were time clocks -- that serious scientific interest in geological age began.

Using this technique, called radiometric dating, scientists are able to "see" back in time.

We have rocks from the Moon (brought back), meteorites, and rocks that we know came from Mars.

Using fossils as guides, they began to piece together a crude history of Earth, but it was an imperfect history.

After all, the ever-changing Earth rarely left a complete geological record.

From Wikipedia, radioactive decay is the process in which an unstable atomic nucleus spontaneously loses energy by emitting ionizing particles and radiation.

This decay, or loss of energy, results in an atom (element) of one type, called the parent nuclide transforming to an atom of a different type (another element or another isotope of the same element), named the daughter nuclide.

Measuring the uranium-to-lead ratios in the oldest rocks on Earth gave scientists an estimated age of the planet of 4.6 billion years.

Segment from A Science Odyssey: "Origins."Geologists have calculated the age of Earth at 4.6 billion years.

For example, about 1.5 percent of a quantity of Uranium 238 will decay to lead every 100 million years.