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Radioisotopes from Space

Earth Science in Space 2


cosmogenic isotopes

Energetic particles from space generate small amounts of radioactive beryllium and carbon.

U.S. Geological Survey image

One of the most important tools a geologist has is isotopic dating. The classic example is uranium-lead (U-Pb) dating, based on the slow radioactive decay of uranium to lead—if you measure the precise amounts of each in a rock, you can figure out how old the rock is by how much of the uranium has turned into lead. The science is quite accurate these days, now that we can count practically every atom going through a mass spectrograph.

Carbon Isotopes from Space

Some of the isotopes we study originate from space—more precisely, they form when cosmic rays collide with atoms in the uppermost atmosphere. The most familiar of these is carbon-14, or radiocarbon. (We'll use the name that scientists use: 14C.) This atomic species, carbon whose nucleus has 6 protons and 8 neutrons instead of the usual 6, forms in the upper atmosphere when cosmic rays strike nitrogen, so it is being produced all the time. It decays back to nitrogen, 14N, with a half-life of 5,730 years.

All living things constantly circulate 14C through the carbon in their tissues, and when they die the 14C stops being replenished and starts to disappear. So for about ten half-lives, until it can't be detected any more, the 14C content of a sample—ancient buried charcoal, perhaps, or an old textile—is a good gauge of the sample's age.

In recent years we've learned that at different times in prehistory, the 14C clock has been fast or slow by a few percent. We can tell this by comparing 14C ages to absolute ages known from tree rings, for instance.

The Wavering Radiocarbon Clock

The explanation is complex, with many different sources of error to untangle, and some of them involve outer space. Something affects the chain of events that underlie 14C ages. More cosmic rays are reaching the nitrogen in the atmosphere at some times, less at other times.

Cosmic rays are actually highly energetic particles—bare atomic nuclei and their subatomic pieces—that bombard us from all directions, all the time. They're a long-standing mystery, really. Magnetic fields affect them since they are electrically charged (you can make a detector at home and see for yourself).

Around Earth there are at least two different magnetic fields to consider, the Earth's and the Sun's, both of which are known to vary. When either field is stronger, it shields us from cosmic rays.

Another possibility is that the upper atmosphere fluctuates. When the Sun's energy output changes (see Part 1), the outer atmosphere heats or cools in response. As it expands or contracts accordingly, more or less air is exposed to cosmic rays. Finally, the atmosphere itself can change its contents by a few percent. This may sound like a lot of uncertainties to deal with, but as they say in science, one guy's noise is another one's signal.

Other Space Isotopes

Other useful cosmogenic radioisotopes are created in the atmosphere: beryllium-10 (10Be) forms from oxygen-16 and iodine-129 is fantastically rare, since it forms from cosmic bombardment of xenon, but modern labs like Purdue's PRIME Lab can study it too despite the difficulties. These isotopes can tell us the ages of things—like how long a rock has been exposed on the Earth's surface—that could never be quantified before.

Satellites have been observing the magnetosphere and near-Earth environment, where these isotopes form, ever since the 1950s . . . platoons of geophysicists are working to understand what goes on there. The wandering 14C clock is some sort of clue.

PS: Cosmic rays create other isotopes in rocks on the ground, not just in space. Chlorine-36 is one that allows us to extract information from such oddities as packrat urine, which sits crystallized for millennia in desert caves.

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