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Riddle of comet dust delivered back to Earth

Paul Sutherland, Feature writer
May 18, 2015, 10:48 UTC

Sen—While the Rosetta space probe studies one comet—67P/Churyumov-Gerasimenko—like never before, other scientists around the world have been busy examining fragments from another of these ancient relics of the Solar System.

They are the teams working in laboratories that were provided with samples from a NASA spacecraft called Stardust which flew through the coma, or dusty atmosphere, of Comet Wild-2 in January 2004.

One team of researchers says it has been presented with something of a riddle. Wild-2 is known to have spent most of its existence in the depths of the Solar System, beyond Neptune, yet it contains material that we know is commonly found in the inner Solar System.

Stardust, launched in February 1999, got within 240 km (150 miles) of Wild-2’s 5.3 km (3.3-mile) wide nucleus, collecting dust with a gel-covered, tennis-racket sized device resembling a high-tech fly swat. Then, amazingly, it returned the samples by firing a capsule as it sped past Earth in 2006. The capsule parachuted into the Utah Desert.

The microscopic grains have been analysed to sort the comet’s own ejected particles from much rarer pieces of interstellar dust collected during the probe’s seven-year, 4.6 billion km (2.88 billion mile) journey.

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The Stardust return capsule, with its precious cargo, after landing in the Utah Desert. Image credit: NASA/JPL

One team that has spent years examining the Stardust samples is from the University of Hawaii Manoa and the University of California-Berkeley, led by Hawaii Institute of Geophysics and Planetology assistant researcher Ryan Ogliore. They have been investigating the different types (or isotopes) of oxygen and the comet dust’s mineral composition.

Stardust scientists had expected that everything brought back from the comet would either be primitive dust or grains of rocks and minerals that formed around other stars—circumstellar material. But this was not so. Ogliore and his colleagues discovered that the larger-sized dust appears to be similar to material found in primitive meteorites called chondrites, according to their study published in the journal Geochimica et Cosmochimica Acta.

The smaller-sized dust, however, shows the full range of known oxygen isotopic compositions that have been measured for objects from the inner Solar System—from the Sun to the asteroid belt between Mars and Jupiter. This is puzzling because Wild-2 is a comet that began life beyond the orbit of distant planet Neptune, and was only sent in closer to Earth’s orbit in 1974, due to the effects of Jupiter’s powerful gravity.

The team will now study their grains more closely to try to solve the riddle of why Wild-2 contains material like that in the inner Solar System, and to learn more about its origins.

Ogliore told Sen: “The results from our recent paper suggest a dichotomy between the really fine dust and the larger dust we captured with Stardust. (Of course this is all relative, the largest particles we captured cannot be seen with the naked eye!) Nearly all the large grains we see look like igneous rocks that we see in meteorites, which came from asteroids that formed in the inner Solar System: e.g. large crystals embedded in a matrix, mineral phases that form at really high temperatures. And also, we measure the oxygen isotopic composition of these grains, and they match the compositions of igneous rocks in meteorites.

“Now the fine-grained dust looks totally different. It spans the range of everything that we’ve ever seen from the Solar System: from the composition of the Sun to these mysterious things found in one meteorite that likely came from interaction with outer Solar System water. And all these compositions were present in tiny dust grains that were fractions of a millimetre from each other!”

Ogliore added: “We could interpret this in one of two ways. Either the cometary fines are a mixture of all this stuff from the inner Solar System that was transported to the outer Solar System, or this is something new—the primitive building blocks of the Solar System that the comet was thought to be mostly made of. We have a couple ways to test which of these answers is true!”

Meanwhile Rosetta continues to give amazing close-up images of Comet 67P/Churyumov-Gerasimenko producing its own gas and dust. Our main image was taken on May 3, 2015, at a distance of 135 km from the centre of the nucleus.

Ogliore told Sen: “The Rosetta mission has been great, I’ve immensely enjoyed following all the fascinating, and daring science they’ve been doing. My favourite stuff from Rosetta has obviously been the isotope and dust measurements. They showed some great pictures of fluffy, porous dust particles they collected at a conference I attended in March. If only we could get that dust back to Earth!”

Stardust’s principal investigator, Professor Don Brownlee, of the University of Washington, told Sen: “Based on laboratory analyses of samples from comet Wild 2, it is evident that most of the rocky materials in this comet, and probably other comets as well, were made at white-hot temperatures in the inner regions of the Solar System and then transported to the cold outer regions where they combined with ice and organics to form comets, 4.6 billion years ago. 

“The fantastic structures seen by the Rosetta mission are probably all composed of very fine mixtures of the products of ‘fire and ice’, rocky material made closer to the Sun and icy materials formed at the fringe of the Solar System. This unexpected mix of inner and outer Solar System materials shows that solid materials were efficiently mixed across the full breath of our Solar System at the time when planets were forming. 

“This discovery provides important insight into the way the life-building organic materials are transported to Earth-like planets around stars.”