Dissipation of the Solar System’s debris disk recorded in primitive meteorites.

This post was written by Prof Jamie Gilmour

We understand that systems of planets form alongside their parent stars.  Part of a rotating cloud of gas and dust collapses to form a rotating disk.  Most of the material is eventually fed into the centre where the star is growing, but dust grains condense in the disk, sticking together and accumulating.  Once they become planetesimals – bodies around 50 km in diameter – they can avoid being consumed by the central star.  Most planetesimals are eventually incorporated into planets, but some survive. There comes a time when these left-over planetesimals begin to collide, generating progressively smaller fragments until dust is produced that is small enough to be lost from the system.  This process gives rise to a new cloud of dust around a recently formed star – a debris disk – that dissipates as the population of parent planetesimals is depleted.  Planet formation obliterated much of the record of this stage of our solar system’s history, but asteroids are descended from leftover planetesimals that survived the debris disk.  Meteorites from these asteroids allow us to investigate what was going on over the first 100 Myr of our solar system’s history as the planets were forming.

An atrist’s impression of a debris disk. Image credit: T. Pyle (SSC), JPL-Caltech, NASA

By studying these meteorites, we have learnt that the early solar system was a radioactive place.  Short lived isotopes recently made in nearby massive stars were still alive, giving us a way of determining the relative timing of events; the earlier a mineral formed, the more of a radioactive isotope it incorporated and the more of its decay product is present today.  The radioactive material also shaped our environment – one isotope of aluminium was so abundant that, as it decayed, it released heat that warmed up the planetesimals, leading to changes to the structures of their rocks and, in some cases, melting them so they separated out into metallic cores and rocky mantles. 

Left undisturbed, a planetesimal heated by radioactive decay will warm up and cool down in a predictable way.  Higher peak temperatures will be reached towards the centre.  More deeply buried regions will also cool more slowly than regions closer to the surface, just as the centre of a pie can still be scorching hot when the pastry case has cooled.  When we date different bits of a small rock that went through this process, they should all record the same age because they all heated and cooled together. 

The iodine-xenon dating scheme, which I’ve been working on for nearly 30 years, relies on the decay of a short-lived iodine isotope to produce a xenon isotope over the first ~100 Myr of the solar system.  It was the first such scheme to be developed, but was long considered “puzzling” because the ages it generated did not correspond to expectation that more processed material had been processed for longer.  One classic paper in particular showed a wide range of ages from various bits of a relatively unprocessed sample, and work in our group seemed to show the same story; adjacent fragments record events that occurred as much as 50 million years apart. 

Ten to fifteen years ago I began to wonder if this record was telling us something about the pattern of impacts in the early solar system.  The samples that record the spread of iodine-xenon ages are only lightly processed – they can’t have been buried very deeply or the heat from aluminium decay would have processed them more.  So they must have been close to the surface where they would have been exposed to impacts – maybe this caused the processing they record.  My coauthor, who was a Phd student at the time, and I began to learn about astronomical observations of debris disks and the theory of how they dissipate.  This suggested the impact rate should decline over time, so we began to think how this might be constrained from the literature data.  We ended up with a model that could account for the observations well; it involves small minerals that can record an I-Xe age being created over time as starting material was processed by impacts into the rock we study to day.  It was encouraging when, a few years ago, other scientists showed that shocks from small impacts can affect part of one grain while leaving an adjacent grain relatively undisturbed. 

We combined the results of this model with those from some of my group’s previous work on the I-Xe record preserved in nanometre-scale diamonds found in meteorites.  These nanodiamonds seem to have been present across the whole of the solar system.  Because they presumably all started off the same, we can use them to understand the history of the particular sample we isolate them from.  They can also tell us what fraction of the processing recorded by a rock took place while radioactive iodine was alive.  Putting together the story from nanodiamonds and the output from our model, we can conclude that the debris disk dissipated over a timescale of 40-50 Myr in the first 100 Myr or so of our solar system’s history.  This is as predicted by current models of solar system formation.

The record preserve in meteorite samples tells us about small scale impacts – planetesimals would be disrupted by larger impacts so we wouldn’t have samples to study.  So our work tells us how the amount of dust and few-metre-sized bodies declined over the early years of the solar system.  This material would be contributing to the terrestrial planetary bodies more-or-less in proportion to their surface area, delivering volatile elements and precious metals in a predictable way.  But the Earth’s crust seems to have more precious metals than would be expected based on the amount present on the Moon and on Mars.  It has been suggested that this is because the Earth mostly gained such material in one large impact rather than through the continuous arrival of dust.  So it’s possible that such an impact is a rare event for planets that we observe in our history because it helped shape an environment in which we could develop.  (This is an example of Poisson statistics – it’s much more likely to see place-to-place variation if a process uses a few big events than if it used a lot of small events;  one person winning the lottery is the same average income as everybody who plays getting a few pounds each.)

Of course, our inferences about the debris disk are based on studies of one sample.  Time will tell whether our model survives being tested against other samples, or whether something different is going on.  We can also test our idea by looking in detail at the rocks that record these iodine-xenon ages and trying to understand the processes that “set” the clock.  But whatever the fate of our idea, it’s clear that the I-Xe system gives us a unique opportunity to study the unfolding of our solar system’s history across its first hundred million years when the starting conditions for the planets were set.

Of course, our inferences about the debris disk are based on studies of one sample.  Time will tell whether our model survives being tested against other samples, or whether something different is going on.  We can also test our idea by looking in detail at the rocks that record these iodine-xenon ages and trying to understand the processes that “set” the clock.  But whatever the fate of our idea, it’s clear that the I-Xe system gives us a unique opportunity to study the unfolding of our solar system’s history across its first hundred million years when the starting conditions for the planets were set.

Full citation: Dissipation of the Solar System’s debris disk recorded in primitive meteorites.  Jamie D. Gilmour and Michal J. Filtness, (2019) Nature Astronomy, doi: 10.1038/s41550-019-0696-0

Jamie also talks about this work in the lastest edition of our podcast, The Cosmic CastEpisode # 8: Relaxing in the early Solar System.

Material from the Chainpur meteorite was used in this study. Chainpur was observed to fall in India in 1907. Image credit: Don Edwards, Direct link to photo: http://www.encyclopedia-of-meteorites.com/test/Chainpur_don_edwards.jpg

About Sarah Crowther

I'm a Post Doc in the Isotope Geochemistry and Cosmochemistry group. I study xenon isotope ratios using the RELAX mass spectrometer, to try to learn more about the origins and evolution of our solar system. I look at a wide range of samples from solar wind returned by NASA's Genesis mission to zircons (some of the oldest known terrestrial rocks), from meteorites to presolar grains.
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