Where is all the dust?
Stars pump out dust into the Universe, only for it to vanish from view over time
“We know, from looking at the dust content of galaxies, that the amount of dust in the Universe has been dropping for the last eight billion years”
Dust is everywhere in the Universe. It plays a crucial role in star formation, allowing material in stellar nurseries such as the Orion Nebula to cool enough that gravitational collapse is likely, and once that’s done it forms the raw material for planets. On a larger scale, no attempt to observe or understand a distant galaxy – or our own Milky Way – can fail to take into account the effect of dust on what we see. A single dust grain may be tiny, no more than a tenth of the size of a sand grain, but the effects of dust as a whole are mighty.
As a result, much attention has been paid to how this cosmic dust forms, with the atmospheres of giant, cool stars likely to be responsible for most of it. The dramatic fading of Betelgeuse in the constellation of Orion, the Hunter more than a year ago, for example, seems to have been caused by the appearance of new dust in the star’s tenuous outer atmosphere. Yet, relatively little attention has been paid to the other end of cosmic dust’s life cycle. This month’s paper does just that, working out how dust is destroyed.
It’s an important problem because we know, from looking at the dust content of galaxies, that the amount of dust in the Universe has been dropping for at least the last eight billion years or so. This is surprising as the amount of raw material for dust, in the form of carbon, silicon and the other heavy elements astronomers call ‘metals’, is increasing. Production is continuing apace and so something not accounted for in most models must be destroying the dust.
On the dust trail
Andrea Ferrara from Pisa and Céline Péroux from ESO (the European Organisation for Astronomical Research) in Garching think they have the answer
– or rather answers. In their theory, about 40 per cent of the dust is destroyed by supernovae. It makes sense that powerful explosions which send shock waves through the cosmos might destroy delicate dust grains in the supernova’s neighbourhood. But what’s impressive about Ferrera and Péroux’s idea is that they get significant loss of dust overall, while noting that each individual supernova might be less good at this than expected. The trouble is that massive stars which go on to produce supernovae tend to have powerful stellar winds, and these winds can sculpt a ‘bubble’ of material around them. Dust caught up in such a bubble forms a dense barrier, and it’s therefore protected somewhat from the shock waves produced by the supernova. As a result, a supernova on average destroys about half a solar mass worth of dust – about a sixth of what less detailed modelling suggests. Given the supernova rate over the period studied, that’s enough to account for 40 per cent of the dust loss we see. What happens to the rest? The answer is rooted in the fact that star formation requires dust. Dusty clouds can cool, and as they do so they collapse under their own gravity. The dust is not exempt from this process, becoming heated and incorporated into the star – a process rather wonderfully known as ‘astration’. This process is efficient enough to account for the majority of lost dust in the last few billion years and explains why our Universe seems to be undergoing a long, slow spring clean.
The Winchcombe meteorite was found after a lot of observations of a fireball on 28 February. Ashley King, who is a meteorite specialist at the Natural History Museum and helps the UK Fireball Network, went on local television and radio in Gloucestershire and alerted people, saying, “If anybody’s got any unusual rocks that weren’t there yesterday, send me a picture.”
As a result we got loads of pictures, but there was only one that might have been a meteorite. That morning, a family had found this pile of grey dust spread out on their driveway with some bigger chunks sitting on top of it. They picked up the bigger bits using aluminum foil and put them in a sandwich bag. King asked my colleague Richard Greenwood if he could go have a look. And, yes, it was a meteorite, which is brilliant! I then started putting into action our recovery team along with colleagues from the Natural History Museum, as well as Imperial College London and the University of Manchester, but you can’t just start wandering around people’s fields and driveways in the time of a pandemic, so there was a lot of admin. Fortunately, everybody seemed to drop everything as they realised that this was something special.
Moving quickly
The meteorite was seen on Sunday, identified on Wednesday lunchtime, and by Thursday we had it in a mass spectrometer at the Open University. By Thursday night we had classified it. We reckon this is the fastest journey from an asteroid to a laboratory – certainly in UK history and possibly in global history – because it was collected and analysed so quickly.
There is some analysis that has to be done instantly: though the meteorite isn’t radioactive it does contain some isotopes which decay on a very short timescale – a matter of days. Those can help us
calculate how big it was before it actually came through the atmosphere. There are a couple of other tests that need to be done quickly; because even though it’s sitting in clean conditions, the longer it’s on Earth, the more chance it will be contaminated.
There are also minerals which might lose their water and a lot of very volatile organic compounds – I haven’t sniffed it yet, but my colleague said it smells like compost and that’s those organic volatiles escaping. We usually lose those and don’t get a chance to analyse them. After that, we’ll take a chip of rock, embed it in resin and polish it to see what the surface looks like and what minerals are in there.
Meteorites come in two groups – primitive and processed. Primitive ones are made from dust left over from when the Solar System formed. Processed meteorites have been part of a planet at some point; these tell you about planetary cores and surfaces. The Winchcombe meteorite is a primitive meteorite, so it’s unchanged since the dawn of the Solar System. It’s a particularly exciting type because it’s a carbonaceous chondrite. It’s got lots of organic compounds in it, which are the stuff which eventually formed life. We’re looking at a meteorite which contains the building blocks of life.
Words by Ezzy Pearson