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Making simulated cosmic dust - in the microwave!

Discovery of how to make Space Dust in Microwave could explain evolution of planets and life itself

$The 3-circle powder diffractometer on beamline I11, used for the present work$: The 3-circle powder diffractometer on beamline I11, used for the present work

To coincide with the annual campus wide Stargazing event at STFC Rutherford Appleton Laboratory on Friday 31^st Jan from 5.30 – 8.30 pm, scientists at Diamond Light Source, the UK’s national synchrotron, will be revealing their recent discovery of how to make cosmic dust samples in a domestic microwave.

Senior Beamline Scientist, Dr Stephen Thompson led a team of Diamond researchers whose work has recently been published in Astronomy & Astrophysics¹. He explains;

"The composition of cosmic dust is not well understood and it’s not currently possible to collect samples for analysis. So being able to make analogue dust samples in the microwave could help to shed light on our early solar system history."

Dust is the first solid matter formed, and investigating cosmic dust is a very active field within astrophysics. There are a few examples that have arrived on Earth as interplanetary dust particles, comet dust and in meteorites their complicated history means they may not be representative. The primary methods of investigating the properties of cosmic dust, therefore, are astronomical observations and laboratory experiments on analogue materials.

Dr Thompson adds;

We cannot replicate the formation conditions of cosmic dust here on Earth exactly, and no single method of producing analogue dust samples in the laboratory can simulate all of the dust we observe around stars and in the interstellar medium. However, by creating and characterising these samples, and comparing them to astronomical data to see where they are similar, and where they differ, we increase our understanding of the formation, composition and evolution of their cosmic counterparts.

: An example of the amorphous magnesium and iron (Mg-Fe) silicates - analogues to the dust grains that form in the atmospheres around Red Giant stars - studied in this experiment

In their recent paper Dr Thompson and his team demonstrated that microwave drying can be used to cheaply and easily produce amorphous magnesium and iron (Mg-Fe) silicates as analogues to the dust grains that form in the atmospheres around Red Giant stars. They then investigated their crystallisation by in situ thermal annealing and considered the results in the context of modelling dust grains in protoplanetary disks which represent a point in time just prior to planet formation.

Much of the experimental work was carried out by Anna Herlihy during her Year in Industry placement at Diamond. Anna was in the middle of a degree at St. Andrews University and came to Diamond to investigate the production of amorphous nanoparticles. The application to cosmic dust arose from her work, and - inspired by her experience - Anna has completed her degree and is now studying for a PhD at Warwick University.

The results demonstrate that this is an excellent, quick, easy and cheap method for producing analogue dust samples. The team hope that it will be adopted by other laboratory astrophysicists, but it could also have industrial applications, e.g. as a means of producing nanostructured materials.

For the Diamond team, this research is just the beginning. They will continue exploring their new microwave method, using it to produce dust samples with different compositions. Anna Herlihy says;

Each sample gets us one step closer to understanding more about cosmic dust and how planetary systems form. Who would have guessed a kitchen microwave could help with that!

There were many challenges to finding the best solutions to creating space dust. For example, the sol-gel process is a chemical method used to produce solid materials from small molecules. Sol-gels have a consistency similar to hand cream and must be dried to form the dust samples. Air-drying takes about 24 hours and is time-consuming for researchers who wish to produce multiple samples.

Another challenging aspect of producing analogue dust samples is the inclusion of iron, which - on Earth - tends to form rust (iron oxides) that isn't seen in space. Dr Thompson explains;

Although we see evidence of iron in stars and planets, we don't see it in the interstellar medium. This is the 'missing iron' problem, and one possible explanation is that the iron exits as nanoparticles. Another is that iron is 'locked away' in silicate minerals, in quantities too low (less than 10%) to greatly affect the spectral properties of what otherwise appears as pure magnesium silicate dust.

Using sol-gel to incorporate iron into the silicate structure requires special drying conditions and Dr Thompson and his team had previously developed a vacuum drying process². However, this took several days to complete from start to finish.

The researchers, therefore, investigated whether they could speed up the production of analogue samples, and produce iron-bearing silicate dust, using an off-the-shelf microwave oven.

The research team microwave-dried gels with and without iron, and compared them to samples produced from the same gel but dried using conventional in-air oven and vacuum furnace. To confirm the new samples did indeed contain iron in their structure they used X-ray powder diffraction and total X-ray scattering on beamline I11, along with mid-IR FTIR spectroscopy and small-angle X-ray scattering on I22. They then used I11 to observe how their structure evolved at high temperatures, to mimic the conditions experienced by dust grains in protoplanetary disks

Related publications:

¹Thompson SP et al. Amorphous Mg–Fe silicates from microwave-dried sol-gels: Multi-scale structure, mid-IR spectroscopy and thermal crystallisation. Astronomy & Astrophysics 624, A136 (2019). DOI:10.1051/0004-6361/201834691.

² Thompson et al J Non-crystalline Solids 447, 255 (2016) DOI: 10.1016/j.jnoncrysol.2016.06.017

Diamond Light Source is the UK's national synchrotron science facility, located at the Harwell Science and Innovation Campus in Oxfordshire.

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