Wish upon a sea floor

By Oliver Morton

There is a wind that blows from the heart of dying stars, a wind so strong that it reshapes the atoms in its path and drives them out as spindrift into space. This storm-tossed spray spreads out across the galaxy, filling the space between stars. And it settles on everything. It falls on the other stars, and on the nurseries where stars are born. It falls on comets and planets. It falls into puddles. It falls into oceans. And there it settles.

The information that sediments on the ocean floor provide about the history of the Earth has been a source of insight for decades; cores drilled from sea floors around the world were fundamental to the acceptance, in the 1970s, of the modern theory of the ice ages. But more recently, the sediments have started to attract people looking for a record of goings-on farther afield—such as in supernovae. All chemical elements other than hydrogen, helium and a little bit of lithium, which were made in the Big Bang, are produced by nuclear reactions that go on in stars. Some, like carbon and oxygen, are made throughout a star’s life; others come into being only at the end. Many of the atoms thus made last for ever. Some, though, are radioactive, and decay.

Hence the excitement when, in 1999, a team of German scientists published an analysis of a rock sample brought up from the sea floor near Mona Pihoa in the South Pacific. It was not any old rock; it was coated in a ferromanganese crust, a chemical precipitate that had grown very slowly and steadily over a very very long time. Every million years, the crust had got a few millimetres thicker. Analysing this crust, the scientists showed that a bit of it which was about 3m years old contained a measurable amount of iron-60—measurable, that is, if you had instruments capable of distinguishing one atom out of a thousand trillion.

Even this tiniest trace of iron-60 was remarkable. By rights the Earth should have none at all. Iron-60 has a half life of 2.6m years, which means that if you had started off with a kilo of the stuff 2.6m years ago you would now have just 500 grams. If you had started out with a kilo 190m years ago—that is, back among the dinosaurs—then you would now have a single atom. This rate of decay means that none of the iron-60 that the Earth acquired at the time of its formation, 4.5 billion years ago, is still around, and that the iron-60 in that ocean-floor rock must have had an unearthly source. Iron-60 is formed in tiny amounts by cosmic rays hitting the Earth—but it is also formed in copious amounts in supernovae. One exploding star can produce a mass of iron-60 six times that of the Earth. A relatively close supernova a few million years ago would have delivered enough iron-60 to the Pacific—and to the oceans and puddles of every other planet within a hundred light years or so—to explain the signal in the deep-sea crust.

The rate at which supernovae create isotopes other than iron-60 depends on processes which, with no supernovae nearby to observe, scientists can only model—or approximate with particle accelerators. By seeking out the abundances of these yet rarer materials in appropriate rocks, some of the scientists working in the area think it may be possible to say more precisely what really goes on in supernovae. From that they might be able to work out whether there are some ele­ments that supernovae cannot make, and which must instead be formed in yet more esoteric catastrophes, such as the merger of two neutron stars.

A recent study by Anton Wallner and colleagues, many of whom were involved in the iron-60 work, looked for plutonium-244 in similar deep-sea rocks and found, more or less, none. The work suggests—though these are early days—that supernovae may indeed not be able to produce the range of elements previously thought. This may have impli­cations for science’s view of the origin of the solar system. If it were to turn out that some of the heavier elements found in the Earth could only have been made by a collision between neutron stars, it might mean that the Earth is a rarer sort of planet than had been thought. That could have implications for the likelihood of life elsewhere.

This sort of work would still delight, though, even if it turned out to have no bearing on such matters. The links between physics that plays out on the largest scale, that of the cosmos as a whole, and the smallest, that of the fundamental particles studied at CERN and elsewhere, are well rehearsed. No science magazine or mind-stretching television series is complete if it does not include the idea that there are connections between cosmology and particle physics that provide fundamental insights into the universe. The astronomical and the geological are not quite as disparate in scale, and their links not as conceptually deep; accordingly, perhaps, they get less play. But I can’t help thinking their specificity may make these connections, if less profound, no less wonderful—and possibly more so. That humans should find a rock that grows a few millimetres every million years and tease from a scant handful of its atoms the secrets of a dying star does as much to make the world a richer place as anyone could ask.■

Illustration Pete Gamlen