Q. How old is the Earth?

Concord

A. Probably about 4.5 or 4.6 billion years old. Nobody has found any Earth minerals that old, but a meteorite that fell in Mexico in 1969 has been dated to 4.56 billion years, give or take 15 million years.

What, you ask, does something from space have to do with the age of the Earth? Geologists think the Earth and the moon formed at about the same time as other objects in our neighborhood of the solar system, including meteorites, many of which have been dated to the same age as the one found in Mexico.

The terrestrial oldest rocks found so far have come from the Canadian Shield in the Northwest Territories of Canada. Sam Bowring, an MIT geologist, says in a paper to be published soon that samples of Acasta Gneiss, a type of metamorphic rock, are 4.03 billion years old.

The oldest Earth minerals are tiny grains of zircon found in sandstone in Australia. The zircon is 4.3 billion years old, but the sandstone in which it was found is just a teenager by comparison, at 3 billion years. The zircon eroded out of rocks that no one has located and got trapped in the younger sediments that turned into the sandstone.

How can we measure the age of minerals that old? Radioactive decay causes certain elements to change their atomic structure and break down into other elements, a process that Bowring says is called the ``parent-daughter relationship.'' An unstable ``parent'' isotope breaks down into other isotopes until it turns into a stable ``daughter'' form.

In rocks, uranium decays finally to lead, rubidium ends up as strontium, samarium finishes as neodymium. Scientists know how fast the decay takes, so by measuring how much of the parent and the daughter they find in a rock sample, they can calculate how long it took to produce the lead, strontium, or neodymium.