Posted on by Nancy Atkinson

New Results from Stardust Mission Paint Chaotic Picture of Early Solar System

Secondary electron image of the Coki section analyzed in this study showing mineral shards surrounded by compressed aerogel. Credit: Lawrence Livermore National Lab.

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One of the most surprising results from the Stardust mission – which returned comet dust samples to Earth in 2006 – is that comets don’t just consist of particles from the icy parts of the outer solar system, which was the common assumption, but also includes sooty dust from the hot, inner region close to the Sun. A new study confirms this finding, and also provides the first chronological information from the Wild 2 comet (pronounced like Vilt 2). The find paints a chaotic picture of the early solar system.

Even some of the first looks at the cometary particles returned by Stardust showed that contrary to the popular scientific notion, there was enough mixing in the early solar system to transport material from the sun’s sizzling neighborhood and deposit it in icy deep-space comets. Whether the mixing occurred as a gentle eddy in a stream or more like an artillery blast is still unknown.

“Many people imagined that comets formed in total isolation from the rest of the solar system. We have shown that’s not true,” said Donald Brownlee back in 2006, principal investigator for Stardust.

The new study, conducted by scientists from Lawrence Livermore (Calif.) National Laboratory, shows the dust from comet 81P/Wild 2 has been altered by heating and other processes, which could have only occurred if a transport of space dust took place after the solar system formed some 4.57 billion years ago.

“The mission was expected to provide a unique window into the early solar system,” the team, led by Jennifer Matzel wrote in their paper, “by returning a mix of solar system condensates, amorphous grains from the interstellar medium, and true stardust – crystalline grains originating in distant stars. Initial results, however, indicate that comet Wild 2 instead contains an abundance of high-temperature silicate and oxide minerals analogous to minerals in carbonaceous chondrites.”

Corresponding false color mineral map overlaid on a montage of brightfield Transmission Electron Microscope (TEM) images.

They analyzed a particle from the comet, about five micrometers across, known as Coki. The particle does not appear to contain any of the radiogenic isotope aluminum-26, which implies that this particle crystallized 1.7 million years after the formation of the oldest solar system solids. This means that material from the inner solar system must have traveled to the outer solar system, across a period of at least two million years.

“The inner solar system material in Wild 2 underscores the importance of radial transport of material over large distances in the early solar nebula,” said Matzel. “These findings also raise key questions regarding the timescale of the formation of comets and the relationship between Wild 2 and other primitive solar nebula objects.”

The presence of CAIs in comet Wild 2 indicates that the formation of the solar system included mixing over radial distances much greater than anyone expected.

Sources: LLNL, Astrobiology

Posted on by Nancy Atkinson

Comet, Cometary Dust Formed in Different Parts of Solar System

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Scientists studying the particles of comet dust brought to Earth by the Stardust spacecraft have uncovered a bit of a mystery. Research on the particles seem to indicate that while the comet formed in the icy fringes of the solar system, the dust appears to have been formed close to the sun and was bombarded by intense radiation before being flung out beyond Neptune and trapped in the comet. The finding opens the question of what was going on in the early life of the solar system to subject the dust to such intense radiation and hurl them hundreds of millions of miles from their birthplace.

The Stardust spacecraft flew to Comet Wild-2 in 2004, coming approximately 150 miles from the comet’s nucleus, and captured particles of dust and gases from the comet’s coma and then returned those particles to Earth in 2006.

Researchers from the University of Minnesota and Nancy University in France analyzed gases locked in the tiny dust grains, which are about a quarter of a billionth of a gram in weight. They were looking for helium and neon, two noble gases that don’t combine chemically with other elements, and therefore would be in the same condition as when the comet dust formed.

The analysis of the helium and neon isotopes suggests that some of the Stardust grains match a special type of carbonaceous material found in meteorites. The gases most likely came from a hot environment exposed to magnetic flares that must have been close to the young 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. 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.

“Somehow these little high-temperature particles were transported out very early in the life of the solar system,” said Bob Pepin from the University of Minnesota. “The particles probably came from the first million years or even less, of the solar system’s existence.” That would be close to 4.6 billion years ago. If our middle-aged sun were 50 years old, then the particles were born in the first four days of its life.

The studies of cometary dust are part of a larger effort to trace the history of our celestial neighborhood.
“We want to establish what the solar system looked like in the very early stages,” said Pepin. “If we establish the starting conditions, we can tell what happened in between then and now.”

Stardust launched in February 1999, began collecting interstellar dust in 2000 and met up with Wild-2 in January 2004. It’s tennis raquet-sized collector made of an ultra-light material called aerogel, trapped aggregates of fine particles that hit at 13,000 miles per hour and split on impact. It is the first spacecraft to bring cometary dust particles back to Earth.

This study also has relevance in learning about the history of our own planet. “Because some scientists have proposed that comets have contributed these gases to the atmospheres of Earth, Venus and Mars, learning about them in comets would be fascinating,” Pepin said.

The research appears in the Jan. 4 issue of the journal Science

Original News Sources: University of Minnesota Press Release, Lawrence Livermore National Laboratory Press Release