Cassini Finds Grains Of Dust From Beyond The Solar System

Of the millions of dust grains Cassini has sampled at Saturn, a few dozen appear to have come from beyond our solar system. Scientists believe these special grains have interstellar origins because they moved much faster and in different directions compared to dusty material native to Saturn. Image credit: NASA/JPL-Caltech
Of the millions of dust grains Cassini has sampled at Saturn, a few dozen appear to have come from beyond our solar system. Scientists believe these special grains have interstellar origins because they moved much faster and in different directions compared to dusty material native to Saturn. Credit: NASA/JPL-Caltech

We may have come from the stars but try finding any leftover stardust in the solar system these days. It would seem all the frosting’s been licked from the spoon. Well, maybe not all. NASA’s Cassini spacecraft has detected the faint but distinct signature of dust coming from between the stars. The probe’s been studying Saturn’s rings, atmosphere and moons close up since 2004. Using its onboard cosmic dust analyzer, it’s sampled sampled millions of ice-rich dust grains, the vast majority of which spew from active jets on the surface of Saturn’s geologically active moon Enceladus.

Right now the Sun is inside of a cloud (Local cloud) that is so tenuous that the interstellar gas detected by IBEX is as sparse as a handful of air stretched over a column that is hundreds of light years long. These clouds are identified by their motions, indicated in this graphic with blue arrows.
Right now the Sun is inside of a cloud, nicknamed the Local Fluff, that’s so tenuous that the interstellar gas detected is as sparse as a handful of air stretched over a column that is hundreds of light years long. These clouds are identified by their motions, indicated in this graphic with blue arrows. Credit: NASA/Goddard/Adler/U. Chicago/Wesleyan

But among all the minute particles passing into the analyzer a special few — just 36 in all — appear to originate from beyond the solar system in the wide open spaces between the stars. We half expected them. In the 1990s, the ESA/NASA Ulysses mission made the first in-situ observations of this material, which were later confirmed by NASA’s Galileo spacecraft. The dust was traced back to the local interstellar cloud: a nearly empty bubble of gas and dust that our solar system is traveling through with a distinct direction and speed.

The three dozen dust grains stand out because all were speeding through the Saturn system at over 45,000 mph (72,000 kps) and on a specific path quite different from that of the usual icy grains Cassini collects around Saturn. That’s fast enough to avoid being trapped inside the solar system by the gravity of Saturn, the sun or any of the planets for that matter.

The Cosmic Dust Analyser on the international Cassini spacecraft has detected the faint but distinct signature of dust coming from outside our Solar System, from the local interstellar cloud: an almost empty bubble of gas and dust we are travelling through with a distinct direction and speed. Credit: ESA; dust grain inset: NASA/JPL; Saturn image: NASA/JPL/Space Science Institute
The Cosmic Dust Analyser on the international Cassini spacecraft detected the faint but distinct signature of dust coming from outside our solar system, from the local interstellar cloud: an almost empty bubble of gas and dust we are travelling through with a distinct direction and speed. Credit: ESA; dust grain inset: NASA/JPL; Saturn image: NASA/JPL/Space Science Institute

Unlike Ulysses and Galileo, Cassini was able to analyze the composition of the dust for the first time, showing it to be made of a very specific mixture of minerals, not ice. The grains were surprisingly similar, containing major rock-forming elements like magnesium, silicon, iron and calcium in average cosmic proportions. Carbon and sulfur were found to be less abundant than average. All these elements are common in both meteorites and in Earth’s crust.

But the results were unexpected.

 Rich star fields and glowing hydrogen gas silhouette dense, opaque clouds of interstellar gas and dust in this Hubble Space Telescope close-up of IC 2944, a bright star forming region in Centaurus, 5,900 light-years away. Credit: Hubble Heritage Team (STScI/AURA), NASA
Rich star fields and glowing hydrogen gas silhouette dense, opaque clouds of interstellar gas and dust in this Hubble Space Telescope close-up of IC 2944, a bright star forming region in Centaurus, 5,900 light-years away. The universe abounds in dust lofted into space as stars evolve. Credit: Hubble Heritage Team (STScI/AURA), NASA

“Cosmic dust is produced when stars die, but with the vast range of types of stars in the universe, we naturally expected to encounter a huge range of dust types over the long period of our study,” said Frank Postberg of the University of Heidelberg, a co-author of the paper and co-investigator of Cassini’s dust analyzer. Stardust grains are found in some types of meteorites, which have preserved them since the birth of our solar system. They are generally old, pristine and have diverse compositions; grains include interstellar diamonds, graphite and silicon carbide. But surprisingly, the grains detected by Cassini aren’t like that. They’ve apparently been homogenized through some kind of repetitive processing in the interstellar medium, the vast spaces between the stars.

As for how they were processed, the researchers speculate that dust in a star-forming region could be destroyed and re-condense multiple times as shock waves from dying stars passed through, resulting in grains like the ones Cassini observed streaming into our solar system. In so many words — cooked!