Radioactive inheritances

At the bottom of the deep ocean there is a never-ending snowfall of particles. Some of this comes from rivers, bringing eroded material from the land. There is dust from the atmosphere, including material from distant volcanoes and from burned out meteors from space. Then there is the steady fall of the remains of minute ocean creatures.

Over thousands or millions of years, this material has accumulated into thousands of feet of sediment, and it contains something that shouldn’t be there. There is a layer, around 33,000 years thick, containing a radioactive isotope of iron.

All normal matter is made out of atoms. Each atom consists of a nucleus, containing protons and neutrons, surrounded by orbiting electrons.Most of the elements we encounter in our everyday lives are stable, however, some elements aren’t.

Really large atoms, like uranium come apart very easily, breaking into more stable atoms and giving off energy in the form of particles and radiation. This is called radioactivity, and uranium is described as radioactive.

Then there are other elements that have forms, or isotopes, that are unstable. Carbon atoms with six neutrons are stable. Add another couple and we get a radioactive form of carbon, which gradually breaks down, emitting radiation. This process is referred to as radioactive decay.

The rate of decay of a given atom is fixed, and is described in terms of how long half of a given lump of an isotope takes to decay. This is referred to as the half-life of the isotope. This may range from tiny fractions of a second for some atoms, to billions of years for others.

The radioactive form of iron found in those ocean sediments is iron-60, which means its nucleus contains 30 protons and 30 neutrons, and its half-life is 2.6 million years.

Typically, for a radioactive element, after a time period of ten or so half-lives has elapsed, there is basically none of it left.

Moreover, there is nothing that can slow or speed up the rate of decay. For the iron-60 in the ocean sediments this raises a problem. Where could it have come from?

The Earth is around 4.5 billion years old, and formed from material in a cosmic gas and dust cloud that was probably far older. This means that anything with a half-life of around 45 million years or less should be long-gone by now.

Carbon-14, which we use for dating purposes, has a half-life of 5700 years, so any we inherited when the Earth formed would be long gone. We find it in our environment and in living things because it is constantly being manufactured in the upper atmosphere by the impact of cosmic rays.However, you cannot make iron-60 this way.There is only one place to get it, which is in an exploding star, a supernova.

Moreover, it looks as though new iron-60 atoms are still arriving. Spacecraft measurements have detected their presence in nearEarth space.

When a large star dies, it explodes, sending much of its material out into space, where it mixes with the existing clouds of gas and dust.This “nuclear waste” is critically important in that much of the material needed to make planets and life as we know it comes from the waste products of energy production in stars. We are largely made from accumulations of cosmic nuclear waste.

The most likely explanation for the presence of iron-60 in those sediments is that for the last 33,000 years or so, the Solar System, which includes us, has been moving through clouds of material ejected by supernova explosions.

Moreover, these explosions cannot have happened more than a few million years ago. There are plans to dig deeper into those sediments, reaching even further back in time.

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After dark, Saturn and brilliant Jupiter lie low in the south, with Mars rising in the east. Venus, which is even brighter, rises in the early hours. The Moon will reach Last Quarter on the 10th.

Ken Tapping is an astronomer with the National Research Council’s Dominion Radio Astrophysical Observatory in