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Scientists thought that comets might be made of the same type of interstellar material that first formed our solar system. But NASA's Stardust and Deep Impact missions revealed otherwise.
In 2004, Stardust scooped up a sample of dust from comet Wild 2; in 2005, the Deep Impact mission slammed into comet Temple 1 to learn about its composition and structure. These missions showed that some minerals in the comets formed at temperatures greater than about 700° Celsius (about 1200° Fahrenheit) while others formed at temperatures less than about -70° Celsius (about -100° Fahrenheit). For example, the dust collected by Stardust contains minerals that had to have condensed in the scorching heat close to the sun. Those minerals must then have been blown out to the Oort Cloud, a cold region of the solar system beyond the orbit of Pluto where comets form.
The implication is that instead of containing an untouched record of the solar system's original ingredients, comets are actually made of materials that have been mixed and processed. In forming comets, some mechanism must have blended the material in the inner and the outer regions of the solar nebula—the disk of gas and dust that surrounded the young sun and eventually coalesced into the planets.
Like in comets, the chemical composition and interior structure of Jupiter holds many clues to the origin of the solar system—clues that Juno hopes to unveil.