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Planetary News: Stardust (2006)Stardust Samples Rewrite History of Solar SystemBy Amir Alexander16 December, 2006
It has been eleven months since Stardust’s sample return capsule made a perfect landing in the Utah desert, bringing with it priceless samples of cometary particles and interstellar dust. During this time hundreds of scientists from around the world have been intensely studying the samples retrieved from comet Wild 2, when Stardust flew within 150 miles its nucleus on January 2, 2004. Now, in a special issue of Science magazine dedicated to the mission, the Stardust investigators present their early results to the scientific world. Slowly but surely, they show, the comet is yielding up its secrets, providing surprising clues to origins and history of our solar system. “We’ve taken a pinch of comet dust and are learning incredible things” said Donald Brownlee of the University of Washington, Stardust’s Principal Investigator. The surprises began early on, with the second grain Brownlee worked on in his laboratory -- a calcium-aluminum inclusion, a rare material found in some meteorites. The stunning thing about this particle is that it could only form in the very hottest part of the early solar system, near its center. As a result, this tiny grain calls for a major rethinking of theories of the formation of the solar system. According to common scientific wisdom, comets formed in the outer edges of the “solar nebula,” the swirling disk of gas and dust which gave birth to the solar system 4.6 billion years ago. In those cold and distant regions, the dust particles were not affected by the heat and radiation from the Sun, and remained largely unchanged to this day. That is why scientists consider comets to be time capsules, mementos from the early solar system that can provide invaluable clues as to our origins. The calcium-aluminum inclusion brought to Earth by Stardust boldly challenges this elegant picture. For if comets formed from material unaffected by the Sun’s heat and radiation, what is a particle that was formed at extremely high temperatures doing in the mix? Somehow, this particle had traveled from the very center of the solar system, inside the orbit of Mercury, all the way to the cold outer regions where comets roamed. “Truthfully,” said Brownlee, “we didn’t expect to find anything from the inner solar system” among the Wild 2 samples. “Instead, it showed up in the second particle we looked at.” "Truthfully, we didn’t expect to find anything from the inner solar system among the Wild 2 samples. Instead, it showed up in the second particle we looked at." Don Brownlee, Principal Investigator, Stardust Other Stardust samples confirm that particles formed in the inner solar system had a part in the formation of comets. Magnesium olivine and pyroxene are silicate minerals formed at high temperatures, and they were found in substantial numbers among the Wild 2 samples. Studies of the isotopic composition of the Stardust samples, conducted by scientists from the Carnegie Institution in Washington DC., also point in the same direction. Isotopes are different varieties of the same element, which have identical chemical properties but differ in atomic weight. The ratios of the different isotopes differ depending on where and how an element was formed, and are therefore an important clue to the conditions of the element’s formation. Among the Stardust particles, the investigators found that the ratio of oxygen isotopes was similar to that found in meteorites formed in the inner solar system. Overall, Brownlee and his collaborators estimate that as much as 10% of the material in comets came from the inner solar system, and 90% came from matter in the outer reaches of the solar nebula. This requires a major rethinking of the early history of the solar system, explained Brownlee. “Many people imagined that comets formed in total isolation from the rest of the solar system. We have shown that’s not true.” In fact, Brownlee and his collaborators write in the main Science article, the presence of the “high-temperature minerals that appear to have been formed in the inner regions of the solar nebula … proves that the formation of the solar system included mixing on the grandest scales.” In other words, the tiny grains collected by Stardust prove that matter from the inner solar nebula blew all the way to the outer edges of the disk and mixed with matter that was formed in that region. Why or how this happened, and what it means for the later development of the solar system, are questions that scientists will be addressing for years to come. Other Stardust results are also creating excitement in different parts of the scientific community. The same group that analyzed the ratios of oxygen isotopes in the samples, also analyzed hydrogen and nitrogen with startlingly different results. The excess of heavy isotopes of these elements compared to what is found elsewhere in the solar system, explained the Carnegie Institute’s Larry Nittler, “is a strong indication that some of the comet dust was around before the Sun formed.” In other words, whereas some of the particles that went into forming comet Wild 2 came from the inner solar system, other particles came from far beyond the solar system, from interstellar space.
Apart from what they tell us of the formation of the solar system as a whole, comets are also believed to have played a crucial role in the history of our planet, Earth. In it early days Earth underwent an intense bombardment by comets, that many scientists believe provided our planet with water and the necessary ingredients for life. Studies of the Stardust samples by scientists at several NASA centers lend support to this hypothesis, by detecting the presence of two nitrogen-rich organic molecules – methylamine and ethylamine. “The two molecules we discovered in comet Wild 2” said Jason Dworkin of NASA’s Goddard Flight Center in Greenbelt, Maryland, “provide a source of fixed nitrogen, a commodity that would have been rare on the ancient Earth.” “These results show that comets could have delivered nitrogen-rich organic compounds to the early Earth where they would have been available for the origin of life,” added Scott Sanford of NASA’s Ames Research Center in Moffett Field, California. Less than a year after arriving on Earth the Stardust cometary samples have both challenged and confirmed scientists’ most basic assumptions on the nature of comets, the history of the solar system, and the origins of life on Earth. And this, no doubt, is only the beginning, for the spoonful of cometary grains brought to Earth by Stardust will continue to be studied for decades to come. There is no telling what wonders they have yet to reveal. |
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