Why the universe might be fluffier than we thought

Space dust provides more than just awe-inspiring pictures like the Pillars of Creation. It can provide the necessary materials to build everything from planets to asteroids. But what it actually looks like, especially in terms of its “porosity” (i.e., how many holes it has) has been an area of debate for astrochemists for decades. A new paper from Alexey Potapov of Friedrich Schiller University Jena and his co-authors published in The Astronomy and Astrophysics Review suggests that the dust that makes up so much of the universe might be “spongier” than originally thought.

So why does the porosity of dust matter? One of the main factors affecting the chemistry that goes on in the clouds of the stuff that float around between planets and stars. Specifically, the surface area of dust available to act as a catalyst in important chemical reactions, such as the formation of H2, is drastically higher if the dust is “porous” rather than “compact,” as it is in more traditional models. More porous dust also has ways of “trapping” volatiles in their structure, keeping them safe from the harsh conditions outside, and potentially allowing the dust particles themselves to ferry these fragile substances (such as water) to protoplanetary sites, like early Earth.

It’s important to note there are two different types of porosity when talking about the holes between particles of space dust. One is “intrinsic” porosity, where there are intentional holes in the material itself—something equivalent to a buckyball with a large hole in the middle of the structure. The other is extrinsic porosity, where there are gaps between particles that were smashed together as part of the gravitational pull between themselves.

The authors base their argument on four different pieces of observational evidence. First is dust samples that were collected as part of various missions, including Stardust and Rosetta. Second is remote observations of the spectra of dust in the interstellar medium. Third is experimental growth of synthetic dust in a laboratory. And finally, the fourth is simulations, both on a particle collision scale, but also on an “atomistic” scale of the structure of the dust itself.

Stardust was launched in 1999 with the express purpose of passing through the coma of Comet Wild 2, and returning to Earth with that sample so it could be analyzed with advanced laboratory equipment. Rosetta was launched in 2004 with the intention of visiting Comet 67P/Churyumov-Gerasimenko and studying its overall surroundings, including the dust forming its coma. Both missions found significant amounts of both “compact” and “porous” dust, with some porous samples reaching as high as 99% porosity.

Polarization studies of dust in the interstellar medium put slightly lower limits on the “fluffiness” of the dust particles. Data from one particular ALMA study of HL Tau put the porosity of the dust at around 90%, which the authors think might have been lowered by repeated inter-dust collisions compacting it down. A different study of the IM Lup system collected data on the scattering of the system that fit models of dust as “Fractal aggregates” with relatively small radii.

Growing cosmic dust on a planet seems counterintuitive, but researchers have tried to do just that by using a laser to ablate rocks and then attempting to deposit the resulting gas and dust. In these laboratory simulations, the resulting deposition is always extremely porous, matching the data gathered by Rosetta and Stardust.

Modeling confirmed a similar amount of porosity, especially for “hit-and-stick” models of early dust interactions, which were particularly good at causing extrinsic porosity. Atomistic modeling also showed how having internal “micropores” on samples that were intrinsically porous could harbor water molecules and make them less likely to sublimate away in interplanetary space.

Ultimately, while there is abundant anecdotal evidence for highly porous space dust, there isn’t enough to prove conclusively that most of the dust in space looks like a sponge rather than a solid pillar. As always, the authors believe more data is needed before a definitive answer as to whether that’s common throughout the universe or not. If it is, maybe someone will ask an AI to update the famous Pillars of Creation picture to add some noticeable holes to it.