Cassini Has Detected Dust Grains From Outside The Solar System

Artistic impression of the ESA/NASA Cassini mission around Saturn. ESA/NASA

Interstellar space is mostly empty, but not as empty as you might think. There are clouds of interstellar dust – made of small particles from supernova explosions – and this dust comes into the Solar System as we move around the galaxy. The composition of this dust is mysterious, so the Cassini spacecraft around Saturn has been collecting it since 2004 to shed more light.

ESA planetary scientist Nicolas Altobelli and his collaborators have now analyzed the 36 interstellar dust grains detected by Cassini; the number might seem small, but it's more than five times that of previous missions. The dust was composed mainly of magnesium, calcium, iron, silicon, and oxygen. The grains seem to have gone through repeated processes, periodically thawing and reforming, before reaching the Solar System.

The results, published in Science, will help astronomers understand the origin and evolution of interstellar dust, its role in the cosmic cycle of matter, and the formation of new stars and planets. The detection is important, because it is normally extremely difficult to find and detect interstellar dust in the Solar System.

“The Sun is moving around the galaxy center, and it encounters different interstellar clouds,” Dr. Altobelli told IFLScience. “We have been flying through the Local Interstellar Cloud for a few tens of thousands of years. We are moving through the cloud with a velocity of 26 kilometers [16 miles] per second, so there’s a flow of neutral helium and interstellar dust entering the Solar System.”

The grains interact with the Sun’s gravity, its light, and the magnetic interplanetary field. The dust grains flow in a specific direction but they mix with local material. In the case of Saturn, this would be tiny ice crystals from the E-ring and the cryoactivity of its moon Enceladus.

“These observations were like looking for a needle in a haystack. We have millions of detections of Saturn-bound particles, and we detected very few interstellar grains,” added Dr. Altobelli. But more than a decade of observations paid of. “It is very nice to have seen their signature in the dusty Saturnian background.”

And the grains were found to be different from the dust found in the Solar System. “We showed that these grains are representative of a well-mixed and homogeneous population, and they are different from what you’d find in all meteorites, which are more closely related to their parent star," said Dr. Altobelli.

Further studies of grains like this could even help us better understand how planets have formed.

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