Samples of the material picked up during the NASA Stardust mission indicate that parts of the comet Wild 2 actually formed in an area close to the sun.
New research by an international collaboration including Livermore researcher Saša Bajt analyzed noble gases within Stardust samples.The helium and neon isotope analysis suggests that some of the Stardust grains match a special type of carbonaceous material found in meterorites; hence both must have spent time in the same gas reservoir, which was close to the sun.
About 10 percent of the mass of Wild 2 is estimated to be from particles transported out from hot inner zones to the cold zone where Wild 2 formed. The paper concludes that this is how these grains with unusual isotope ratios go incorporated into a comet.
Earlier research showed that the comet formed in the Kuiper Belt, outside the orbit of Neptune, and only recently entered the inner regions of the solar system.
Wild 2 spent most of its life orbiting in the Kuiper Belt, far beyond Neptune, and in 1974 had a close encounter with Jupiter that placed it into its current orbit. The Stardust spacecraft’s seven-year mission returned to earth in January 2006 with particles that are the same material that accreted along with ice to shape the comet about 4.57 billion years ago, when the sun and planets formed.
But during its lifetime, Wild 2 gathered material that formed much closer to the sun.
And the new research, which appears in the Jan. 4 issue of the journal Science, shows that some of the particles in Stardust are consistent with the early solar nebula.
“The unusual isotope ratio of helium and neon demonstrate that materials in comet Wild 2 had been much closer to the young sun than previously expected,” Bajt said.
Bajt, who studied tracks in aerogel caused by cometary particles rich in noble gases, used infrared spectroscopy, which is very sensitive in detecting organic molecules. She found none, at least not in the pieces of aerogel she examined. The group concluded that the carriers of the noble gases must be the refractory metal-metal sulfide-metal carbide grains, unlike what many expected would be a meteoritic Q-phase, which is known to be organic.
“That’s the first-order finding of the paper, and it’s a rather startling one,” said lead author Robert Pepin from the University of Minnesota.
The second conclusion is that the ion irradiation is the only known mechanism that could load the grains (by ion implantation) to the very high concentrations based on mass density estimates from X-ray absorption spectroscopy by Andrew Westphal and his team at the (Space Science Laboratory, UC Berkeley.
Noble gases are excellent tracers of contributions from various solar system volatile reservoirs and of physical processing of gases acquired from these reservoirs. Their elemental and isotopic compositions in primitive meteorites differ from those in the Sun. Planetary atmospheres display noble gas signatures distinct from both solar and meteoritic patterns.
X-ray absorption spectroscopy in the current study showed that the grains are composed primarily of high-temperature metal.
The X-ray and isotopic analyses point to gas acquisition in a hot, high-ion flux nebular environment close to the young sun.
Stardust is a part of NASA’s series of Discovery missions and is managed by the Jet Propulsion Laboratory. Stardust launched in February 1999 and set off on three giant loops around the sun. It began collecting interstellar dust in 2000 and met Wild 2 in January 2004, when the spacecraft was slammed by millions of comet particles, nearly halting the mission. It is the first spacecraft to safely make it back to Earth with cometary dust particles in tow.
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