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Cassini finds interstellar dust during its Solstice flyby of Enceladus

Saturn’s moon Enceladus is a frozen world, but it isn’t frozen solid. Colossal cryovolcanoes near the ice-locked moon’s south pole send plumes from its briny global ocean so far into space that they manage to feed bits of water ice into Saturn’s outermost rings. As part of the extended Solstice mission, NASA and the ESA sent Cassini diving through the plumes of these cryovolcanoes on a recent flyby of Enceladus. The intent was to scoop up particulate directly from the eruptions and find out what’s really going on below the moon’s icy surface. Far from just boring old water, Cassini found complex organic molecules, CO2, and even bits of Enceladus’ seafloor in the vaporous plumes.

But among the millions of pieces of ambient space dust and cryovolcano ejecta the orbiter found floating around Saturn, there were a very special few: thirty-six special particles that didn’t come from Enceladus, or even from Saturn. Cassini found grains of stardust condensed from the interstellar medium inside Saturn’s orbit.

Stardust inside the heliosphere isn’t totally unexpected. Back in the 90s, the Huygens probe found particles that the Galileo spacecraft later confirmed had come from the local interstellar cloud. But Cassini has a lot more instrumentation than Huygens. Cassini was designed to analyze “waves, particles, and fields” on the spot. That’s how the Cassini team decided these particles came from beyond our solar system: they were going too fast, and in the wrong direction, to have come from Saturn or even belong in an orbit around our Sun. When we pointed Cassini’s mass spec at those 36 dust particles, it reported that they were made not of ice, but of elements like calcium, magnesium, silicon and iron. These are rock-forming elements, and they’re produced by the death of huge stars.

Grains of stardust can be found preserved inside micrometeorites, and in that state they stay unchanged for billions of years. But the pieces of stardust that Cassini found are curiously homogenized in composition. “Surprisingly, the grains we’ve detected aren’t old, pristine and compositionally diverse like the stardust grains we find in ancient meteorites,” added Mario Trieloff, a co-author also at the University of Heidelberg. “They have apparently been made rather uniform through some repetitive processing in the interstellar medium.”

The authors speculate in their report on how this processing of dust might happen. Grains of dust and debris floating in a stellar nursery could be vaporized and recondense over and over, as shock waves from dying stars passed through. This could result in homogenized grains like the ones Cassini found zipping through the heliosphere out near Saturn. Whatever their extraction, lead author Nicolas Altobelli remarked that “[the] long duration of the Cassini mission has enabled us to use it like a micrometeorite observatory, providing us privileged access to the contribution of dust from outside our solar system that could not have been obtained in any other way.”

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