The Formation of the Solar Systems: A Recipe for Worlds

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This paper summarises the recipe – the raw and processed ingredients plus some of the processes – behind making our solar system 4,600 million years ago. Like a gourmand recipe, the solar system formed from many disparate ingredients, many of these ingredients themselves being the products of complex processes. Thus, to create the habitable solar system we see today required extensive work and processing. However, unlike a food recipe, much of how this happened is poorly understood, although a combination of new observations and analysis is ensuring that progress continues to be made.

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Comets in the Path of Earth

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Earth’s atmosphere offers little protection against comet impacts, because many comets are bigger than 1 km. Fewer comets hit Earth than asteroids of the same size, except perhaps for sizes larger than 10 km. Comets release copious amounts of solid debris called meteoroids, and these meteoroids disperse to form meteoroid streams, some of which cause meteor showers on Earth. Recent meteor shower observations reveal the presence of potentially dangerous parent comets and trace their dynamical evolution. In addition, some showers leave a signature of “cosmic dust” in our atmosphere.

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Organic Molecules and Volatiles in Comets

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Organic molecules and volatiles (e.g. H2O, CO, CO2) are the major components of comets. The majority of the organic compounds found within comets were produced by ice irradiation in dense molecular clouds and in the protoplanetary disk prior to comet formation. Comets are essentially repositories of protocometary material. As a result, comets do not show the clear trends in chemical and isotopic compositions that would be expected from our understanding of their formation locations. Rather, comets record chemical evolution in the protoplanetary disk and allow us to unveil the formation history of the organics and volatiles.

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The Rosetta Mission and the Chemistry of Organic Species in Comet 67P/Churyumov-Gerasimenko

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Comets are regarded as probably the most primitive of solar system objects, preserving a record of the materials from which the solar system aggregated. Key amongst their components are organic compounds – molecules that may trace their heritage to the interstellar medium from which the protosolar nebula eventually emerged. The most recent cometary space mission, Rosetta, carried instruments designed to characterize, in unprecedented detail, the organic species in comet 67P/Churyumov–Gerasimenko (67P). Rosetta was the first mission to match orbits with a comet and follow its evolution over time, and also the first mission to land scientific instruments on a comet surface. Results from the mission revealed a greater variety of molecules than previously identified and indicated that 67P contained both primitive and processed organic entities.

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Flyby Missions to Comets and Return Sample Analysis

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Images from flyby missions show comets to be geomorphically diverse bodies that spew jets of gas, dust, and rocks into space. Comet surfaces differ from other small bodies because of their ejection of mass into space. Comet solids >2 µm are similar to primitive meteorite ingredients and include the highest temperature materials made in the early solar system. The presence of these materials in ice-rich comets is strong evidence for large-scale migration of solid grains in the early solar system. Cometary silicates appear to have formed in numerous hot solar system regions. Preserved interstellar grains are rare, unless they have eluded identification by having solar isotopic compositions

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Credibility of Scientific Writing: An Appeal for Responsibility

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The world of scientific publishing is evolving rapidly. Alongside the usual “big sharks”—the monopolistic publishing houses that have been dominating the market and pressurizing institutional subscribers for decades—there are a wealth of new, online players emerging under the banner of “Open Access”. Open access journals are web-based scientific journals that are free to read by anyone but require authors to pay a fee for publishing their paper. These open access journals are a true challenge, not only for journals published by non-profit, learned, societies, but also for scientific publishing in general.

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v14n2 From the Editors

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We are excited to announce that John M. Eiler, Robert P. Sharp Professor of Geology and Geochemistry at the California Institute of Technology (USA) has agreed to join the Elements editorial team as our next geochemistry editor. His official term begins January 2019. He will replace Friedhelm von Blanckenburg whose term of office ends December 2018. We will introduce John more formally at a later date.

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The Excitement of Science Discoveries in the Blue Sky

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The articles in this “Comets” issue of Elements provide a fascinating account of comets and the making of our planetary system. We learn why comets are visible to the naked eye and about their complex organic geochemistry, the surprising find of free O2, and the likelihood of a comet impact on Earth. Perhaps most impressively, we learn about the tremendous effort that goes into the exploration of comets. These missions require decades of design, planning, and instrument miniaturization and their culmination captures our imagination in a way little else can. Who could not be enthralled by the evocatively named Stardust mission returning a few thousand grains of dust from comet Wild 2 to Earth? We collectively shared the despair when communication was lost from Rosetta’s Philae lander on comet 67P/Churyumov–Gerasimenko, and the excitement about the unparalleled wealth of information sent back during its 70 hours of life on the comet’s surface.

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