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Topic Name: Physicists unveil the history of the solar system in grains of comet dust

Category: STAR (Space, Telecommunications & Radioscience)

Research persons: Robert Pepin

Location: University of Minnesota, United States

Details

Four years ago, NASA's Stardust spacecraft chased down a comet and collected grains of dust blowing off its nucleus. When the spacecraft Comet Wild-2 returned, comet dust was shipped to scientists all over the world, including University of Minnesota physics professor Bob Pepin. After testing helium and neon trapped in the dust specks, Pepin and his colleagues report that while the comet formed in the icy fringes of the solar system, the dust appears to have been born close to the infant sun and bombarded by intense radiation from these and other gases before being flung out beyond Neptune and trapped in the comet. The research appears in the Jan. 4 issue of the journal Science.

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 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." One early event was the birth of Earth's moon, about 50 million years after the solar system formed.

Also, the gases he studies have relevance even closer to home. "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," he said.

Comet Wild-2 (pronounced Vilt-two) is thought to have originated in the Kuiper Belt, a comet-rich region stretching from just inside the orbit of Neptune to well beyond Pluto. As it grew in this roughly -360 F region, it incorporated grains of dust and ambient gas.

The comet received a visit from the Stardust spacecraft in early January 2004, two years after its launch. Veering as close as 149 miles to the comet nucleus, Stardust used a spongy, ultralight glass-fiber material called aerogel to trap the dust. At the moment of encounter, the spacecraft exposed a sheet of aerogel -- supported by a framework -- to the stream of particles blowing off the nucleus.

"It looked like a tennis racket," said Pepin. "It was exposed for approximately 20 minutes."

The aerogel trapped aggregates of fine particles that hit at 13,000 miles per hour and split on impact. The collisions left drumstick-shaped trails pointing inward from the surface of the aerogel.

After the collection, the spacecraft headed home and parachuted its payload safely back to Earth in January 2006. A few months later, Pepin received three sub-samples of particles and colleagues at Nancy University, France, received two others, all from the same particle "hit."

Their task was to analyze gases locked in tiny dust grains about a quarter of a billionth of a gram in weight. As a first step, the researchers heated the grains to about 1,400 degrees C., liberating gases imprisoned for eons.

"The particles probably came from the first million years or even less, of the solar system's existence," Pepin said. 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.

Note for Stardust spacecraft

Stardust is an American interplanetary spacecraft, whose primary purpose was to investigate the makeup of the comet Wild 2 and its coma. It was launched on February 7, 1999 by NASA, travelled nearly 3 billion miles (5·109 km), and returned to Earth on January 15, 2006 to release a sample material capsule. It is the first sample return mission to collect cosmic dust and return the sample to Earth. On July 3, 2007 a second mission was approved to revisit the comet Tempel 1. In 1996 NASA started to make the Stardust space craft. After launch in 1999, the Stardust spacecraft travelled in an initial orbit beyond- but intersecting- Earth's orbit. The Delta II booster did not have enough energy to reach Wild 2 directly. The Stardust spacecraft then re-approached Earth in January 2001 for a gravity assist maneuver. The encounter with Earth enlarged the spacecraft's orbit to intersect that of Wild 2. On the second orbit, Stardust flew by the comet to earth Wild 2 on January 2, 2004. During the flyby it collected dust samples from the comet's coma and took detailed pictures of its icy nucleus. Additionally, the spacecraft accomplished several other goals. It passed within 3300 km of the asteroid 5535 Annefrank on November 2, 2002 and took several photographs. The aerogel collector also acquired interstellar dust. In March-May 2000 and July-December 2002, the spacecraft angled itself into a dust stream believed to originate outside the solar system. The reverse side of the aerogel collector then caught a sample of such particles.

Note for Comet dust

Comet dust refers to cosmic dust that originates from a comet. Comet dust can provide clues to comets' origin.
The models for the origin of comets are: 1) the interstellar model, 2) the solar system model, 3) primordial rubble piles 4) aggregation of planetisimals in the dust disk around the Uranus-Neptune region 5) cold shells of material swept out by the protostellar wind. Bulk properties of the comet dust such as density as well as the chemical composition can distinguish between the models. For example the isotope ratios of comet and of interstellar dust are very similar, indicating a common origin.
A comet and its dust allow investigation of the solar system beyond the main planetary orbits. Comets are distinguished by their orbits; long period comets have long elliptical orbits, randomly inclined to the plane of our solar system, and with periods greater than 200 years. Short period comets are usually inclined less than 30 degrees to the plane of our solar system, revolve around the Sun in the same counterclockwise direction as the planets orbit, and have periods less than 200 years.