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n its earliest youth the solar system was inhabited by a huge swarm of small rocky and icy bodies called planetesimals, orbiting the Sun in a giant disk. And, although the full details are still being debated (see the section, Formation of the Giant Planets), it is clear that collisions between planetesimals led to the for- mation of increasingly larger objects, with the end result being the planets we know today. Many of the diverse Above: Cometary material bombarding early Earth, as illustrated in this artist's concept, may have carried organic mate- rials from which life arose. Right: A wide-angle view of the twin tails of Comet Hale-Bopp on the night of March 10, 1997. small bodies now seen in the solar sys- tem are directly related to the primor- dial population of planetesimals. Beyond the orbit of Pluto, for example, many small icy objects remain in their archaic forms. For the most part these so-called Kuiper Belt objects and their relatives remain in their orbits far from the Sun. But every so often, one gets flung into a new path taking it into the inner solar system we call these objects comets. As comets travel through the solar system, their surfaces undergo a num- ber of changes. The most obvious example is the development of the comet's characteristic tail as the Sun warms the surface ices and causes them to turn from a solid into a vapor. Other physical and chemical changes occur owing to the effects of fuzz {~: ~~ ~~f~ ~;f5~:ffff~ ~f/~'f:~ radiation, micrometeorite impacts, and other processes. By studying these changes in a comet's surface material, scientists glean a significant amount of information about the his- tory of the Sun and the solar system, much as reading the rings of a tree can teach us about the history of cli- mate changes on Earth. One change of particular interest to researchers results from the discovery that complex carbon compounds are created when a comet's surface ices (water, ammonia, carbon dioxide, methane, and so on) are irradiated by ultraviolet light, high-energy particles from the Sun, or cosmic rays. Labora- tory simulations indicate that, when exposed to liquid water, such radiation- processed material produces amino acids and other organic molecules found in living systems. Radiation on the surfaces of planets and their satel- lites can also create and destroy com- plex organic molecules, but the details of the required conditions and the bal- ance between destruction and creation are not known. Studying the differ- ences between cometary and planetary organic molecules can help scientists understand this balance by showing which molecules are likely to have come from comets and which have been formed by other processes. This understanding will provide important insights into the origin and evolution of life on Earth. The Comet Surface Sample Return mission emphasizes the return to Earth of a sample of cometary surface material that will provide the first direct set of data on cometary

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Come! Surface Sample Return Profile Comet Surface Sample Return Mission Type: Sample Return Cost Class: Medium Priority Measurements: Characterize the target comet. Select, document, and return mate- rial collected at one or more sites, preferably in or near an active vent. processes and answer whether the water in comets is very close to the surface. Such a sample will also show if the organic materials we believe to exist there actually do. If they do, then the sample will also give us our first look at ancient organic mole- cules similar to those first brought to our planet by cometary impacts 4 bil- lion years ago. Knowing the nature of these molecules should provide exciting new insights into the origin of life on Earth. The Comet Surface Sample Return (CSSR) mission would address other scientific interests as well. It would provide the first direct data on the dif- ference between a comet's nucleus and the relatively well studied material in the cometary tail. In addition, data from this mission would enable scien- tists to resolve questions about the wealth of large-mass molecules and fragments seen by spacecraft in the cloud of gas and dust around Halley's comet during its last visit to the inner solar system in 1986. Finally, the CSSR mission concept would give scientists their first look at how a comet is actually put together. Is it really a dirty snowball, as conceived in popular and scientif- ic imagination? Is a comet a homo- genous mixture of dust and ices, or are there pockets of differing materi- als scattered throughout its body? If the latter is true, what holds these different pieces together? Studying the structure of the material returned by CSSR should answer many of these questions. Artist's impression of the lander portion of the Comet Surface Sample Return space- craft just after it has deployed its twin solar arrays. Guiding Themes Addressed Important Planetary Science Questions Addressed ~ ~ ~ ~ . . ~