The composition of the dust between stars in our galaxy provides a window into some of the material that went into forming our Solar System. The local dust left behind from this process has been through many shake-ups in its history that have changed its composition; interstellar dust should be relatively pristine. For a long time, however, our efforts to understand interstellar dust have relied largely on inferences, as it’s difficult to directly observe the dim, diffuse material using telescopes.

Luckily, there is a way to get a direct measurement. There’s a cloud of interstellar dust near our Solar System, known as the Local Interstellar Cloud (LIC, sometimes called the “Local Fluff”). Some of it is streaming into our Solar System. This stream of LIC material was first observed by the Ulysses spacecraft in 1993, and grains of dust were captured by the Stardust spacecraft in the early 2000s and analyzed by a citizen science project.

The inward dust flow also passes by Saturn, where NASA happens to have a spacecraft with a dust collection system. This is Cassini, which was outfitted with that capability with the intention of capturing dust native to Saturn's rings. It just happened to be in the right place with the right tools to catch some interstellar dust.

New Results



Cassini has been collecting this interstellar dust for just under a full decade (from 2004 to 2013), and the results have recently been published. In all, the probe picked up 36 grains coming from the direction of the LIC dust flow, all moving at high speed. Analysis of these grains showed that they were made from magnesium, silicon, iron, and calcium—the stuff rocks are made from.

Based on this material, the interstellar medium appears to be mostly homogenous. There are some small variations from grain to grain, but the similarities are more significant. Carbon and sulfur are mostly absent, which the researchers conclude is probably because those elements weren’t able to fully condense under the conditions prevalent in the LIC.

The fact that the collection took place at Saturn, far from the contamination of the inner Solar System, increases the researchers’ confidence that they're getting an even sampling from interstellar space. “We are therefore convinced that our compositional results are not significantly affected by a selection bias related to the grain dynamics in the Solar System,” the researchers write in their paper.

Previous observations of the LIC had given a general idea of the dust's composition that it was largely homogenous; the new study shows that this homogeneity extends down to the grain scale.

Just how homogenous?



The researchers can’t rule out the possibility that grain populations with different compositions exist within the LIC, but this study puts a new constraint on them. If such populations exist, they make up 10 percent of the LIC at most.

The aforementioned Stardust mission also returned with grain samples from the interstellar medium, and its data largely agrees with the new results. The one notable difference is the presence of sulfide in three of Stardust’s four samples. However, these grains are larger than the ones detected by Cassini, so they might have formed by different mechanisms and could have different compositions. Cassini’s instruments aren’t sensitive enough to detect larger grains, so the researchers note there could be additional grains up to twice as large.

The researchers speculate that the cloud may have started off more diverse and then undergone processes that homogenized it. The cloud’s dust grains may have been destroyed and then reconstituted over time, leading to a more even distribution of material.

What could drive such a wholesale rearrangement? The interstellar dust would be expected to be hit by the debris from a supernova explosion every 500 million years. Compare that timing to the normal residence of material in the interstellar medium—an average two and a half billion years. That means the grains would have likely stayed there long enough to experience several supernovae.

Obviously, it would be good to look at some dust that hasn't repeatedly evaporated and recondensed. The authors think that the best place for this may be inside a meteorite. Meteorites, were they to condense around some grains, could offer a place that has been shielded from these violent upheavals, and thus any dust they contain could be relatively pristine. While these grains might be “isotopically inconspicuous,” as the researchers put it, if we could find it, the material could teach us a lot about the gas that gave rise to the Sun, the Earth, and us.

Science, 2015. DOI: doi:10.1126/science.aac6397 (About DOIs)