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12 February 2015

A handful of cosmic dust: revealing the roots of our existence

It's time to appreciate space dust.

By Michael Brooks

All the talk of a Nobel Prize for cosmology has turned to dust. Last year, scientists thought they saw ripples in spacetime caused by the Big Bang. These “gravitational waves” would have been the first confirmation of a theory called inflation, which says the universe went through a very short but very fast growth spurt in the first split second of the cosmos’s existence.

Inflation is central to the Big Bang story, and some kind of observational confirmation is necessary if we want to put our full trust in the idea. What we hailed as a reason to trust the theory, however, turned out to be a mirage caused by intergalactic dust. The final analysis suggests that dust distorts the patterns of radiation in the same way as gravitational waves.

Rather than presenting space dust as a problem, however, let’s use its moment in the spotlight to appreciate it a little. This is surprisingly interesting and informative stuff, perhaps even the key to understanding our existence.

To understand why, imagine taking a close look at the dust in your house – its structure and distribution through your home. It is, essentially, the debris of your body and the materials of your shelter. From careful inspection you could infer the material composition of the house, the genetic make-up of the people living there – along with the animals (and parasites) they share with – and all kinds of information about their lifestyle.

Similarly, space dust tells us about what elements formed when and where, and how they became spread through the universe. And you don’t even have to go into space to find it.

At the end of January, a team of researchers from the Australian National University published their analysis of space dust they had gathered from the ocean floor. From 25 million years’ worth of settlement, which corresponds to a 10cm layer of dust, the researchers inferred that we are the result of a very happy accident.

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We know that the heavy elements necessary for life – iron, potassium and iodine, for example – are manufactured in stars. If those stars explode as supernovae, the elements get fired out into space. They exist as cosmic dust particles that sometimes get caught up and trapped as gravity pulls the dust together to form planets.

To work out whether that really is the whole story for earth, researchers looked at the radioactive plutonium-244 in the seabed sediment. Knowing the speed at which this element emits radioactive particles and the products of its radioactive decay allows scientists to infer how long it has been around and how much of it there was to begin with. It turns out that there is 100 times less of it than there would be if it came from supernovae.

According to their paper, it is therefore much more likely our planet’s heavier elements were formed in a freak explosion: a collision between two neutron stars that happened at roughly the same time and place as earth’s formation.

It’s a fascinating result because the dust provides more than just the physical building blocks for life. It also provides the necessary source of energy. Gravity pushed the heavy radioactive elements into earth’s core. The heat they give out provides the energy source for continental movement and the generation of earth’s magnetic field, both of which were essential ingredients for our emergence. We may well be the result of an extremely rare cosmic phenomenon that made a unique kind of dust.

So, while cosmologists might be cursing space dust and trying to find ways to reduce its impact on their observations, they should also be celebrating it. Without that dust, especially the very special dust of our neighbourhood’s recent past, they wouldn’t have evolved to carry out their frustrating inquiries.