ac detective
Unraveling the mystery of the solar system Will NASA’s comet analysis rewrite astronomical history?
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ur solar system is thought to have formed from a thick cloud of gas and dust left behind from an ancient exploding star. Then, through a process of mutual gravitational attraction among particles, the cloud began to condense and collapse, eventually giving birth to a new protostar —our Sun. The enormous swirling disk of gas and dust, called the solar nebula, encircled our Sun during this early period and, over millions of years, condensed to form planets, moons, asteroids, and comets. The elements for the inner “rocky” planets, Mercury, Venus, Earth, and Mars, were forged in the hottest regions of the nebula, closest to the newborn Sun. The cold gas giant planets, Jupiter, Saturn, Uranus, and Neptune, were formed in the outer regions, where it was cold enough for ice to be stable. Although this theory of how the solar system formed is generally accepted, scientists have little evidence in hand to back it up. Astronomers view comets as the oldest surviving pristine materials left over from the formation of our solar system. Analysis of these comets could therefore solve the mystery of exactly how the solar system was born. In February 1999, NASA sent a robotic spacecraft to a comet with the hope that it could safely bring back samples to Earth for the first time. The Stardust spacecraft successfully passed within 234 km of the surface of comet Wild 2 in January 2004, and an automated sample collector was exposed to the comet’s coma and tail. The samples were sealed within a specially designed capsule and returned to Earth in January of this year. The sample collector is estimated to have captured millions of submicrometer- and micrometer-sized particles. An initial examination of the samples by the NASA Stardust team is expected to take 6 months. After preliminary analyses, the samples will be distributed to various institutions and universities for intense study. “Since 3498
Artist’s impression of the Stardust encounter with comet Wild 2.
we have samples in hand, we will be able to explore the nature of cometary materials at a level of detail that is unprecedented, and this will lead to major advances in our understanding of both the solar system and comets,” says Scott Sanford, an astrophysicist and Stardust team member from the NASA–Ames Research Center. In February, David Brownlee, the lead scientist for the Stardust mission, isolated and identified two of the tiny particles by transmission electron microscopy. While conducting an analysis with FTIR spectroscopy, he obtained surprising information about the chemical bonding and molecular structure of the particles. He confirmed the presence of high-temperature minerals, such as olivine, spinel, calcium, aluminum, and even titanium. These minerals only form in very hot environments close to stars at temperatures >1600 K. For such high temperatures to have existed, the minerals must have formed inside the orbit of
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Mercury. But how did the high-temperature minerals become entombed deep within the comet? For decades, many astronomers thought that comets were born in the outer reaches of our solar system between the orbits of Saturn and Neptune. Comets were thought to be made up of the same materials that formed the gas giant planets—Jupiter, Saturn, Uranus, and Neptune—and to have remained frozen and unchanged for billions of years. Could the high-temperature minerals found in the Stardust samples simply be asteroid material that had been floating in space that was captured and incorporated into the comet? Sanford considered the idea. “While it is possible that a few stray interplanetary particles could have hit our collector, such particles cannot account for all the minerals we are seeing in the collection,” he says. Brownlee thinks that the high-temperature minerals must have traveled a long distance. “The inside of the comet, where the dust came from, has been sealed for 4.5 billion years. All the components that the comet was made from were at the edge of the solar system at the time of formation.” The Stardust discovery questions the very heart of the theory of how the solar system formed. Because comets have high-temperature minerals buried deep within their ice layers, they could have been formed in the inner region of the solar system within the orbit of Mercury—a completely new idea that threatens to unseat how the gas giant planets were formed. Alternatively, the solar system could have formed in a vast molecular cloud with other possible protostars. Determining which explanation is correct will have to wait for the results of isotope ratio analyses, which the science team is now conducting with secondary ion MS. a —Barry E. DiGregorio © 2006 AMERICAN CHEMICAL SOCIETY
