Cassini samples dust from outside the solar system

During its last 12 years in orbit around Saturn, the Cassini probe has spent some of that time testing dust samples – with most of the dust it sees coming off Saturn’s moon Enceladus.

However, scientists recently announced that out of the millions of dust grains the faraway probe has analyzed, 36 grains appear to be from outside the Solar System, according to a new study published in the journal Science.

Nicolas Altobelli, a study co-author and Cassini project scientist at the European Space Agency, noted that dust from interstellar space was actually first detected in the 1990s by the ESA/NASA Ulysses mission.

“From that discovery, we always hoped we would be able to detect these interstellar interlopers at Saturn with Cassini. We knew that if we looked in the right direction, we should find them,” Altobelli said in a press release. “Indeed, on average, we have captured a few of these dust grains per year, travelling at high speed and on a specific path quite different from that of the usual icy grains we collect around Saturn.”

The fastest dust this side of the galaxy

The study reported that the speed at which the interstellar dust was travelling – 45,000 mph – kept them from getting trapped by the gravity of Saturn and its moons.

“We’re thrilled Cassini could make this detection, given that our instrument was designed primarily to measure dust from within the Saturn system, as well as all the other demands on the spacecraft,” said study author Marcia Burton, a Cassini fields and particles scientist at NASA’s Jet Propulsion Laboratory in California.

Cassini advanced on the initial discovery of interstellar dust by analyzing its composition. The probe found it to be made of a very particular mixture of minerals, major rock-forming elements like magnesium, silicon, iron and calcium in the typical cosmic levels. On the other hand, more reactive elements like sulfur and carbon were found to be less abundant than what might be expected.

“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 co-author Frank Postberg, a co-investigator of Cassini’s dust analyzer from the University of Heidelberg in Germany.

Stardust grains are discovered in some kinds of meteorites, which have conserved them since the birth of our solar system. They are usually old, clean and diverse in their composition. But remarkably, the grains found by Cassini aren’t like that. They have evidently been made rather homogeneous through some recurring processing in the interstellar medium, the scientists said.

The study speculated on how this refinement of dust might take place: Dust in a star-forming area might be destroyed and formed numerous times as shock waves from dying stars passed through, causing grains like the ones Cassini detected streaming into our solar system.

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Image credit: NASA/JPL-Caltech

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