April 2018 - Volume 14, Number 2

Cover April 2018 v14 n2


Michael E. Zolensky and Monica M. Grady Guest Editors

Table of Contents


The articles in this 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, about their complex organic geochemistry, the surprising find of free O2, and the likelihood of a comet impact on Earth. Articles present concise reviews of what has been learned regarding the mineralogy, geochemistry, and geology of comets from the Giotto, Vega, Stardust, Stardust NEXT, Deep Impact, and Rosetta missions to comets Halley, Wild 2, Borrelly, Tempel 1, and Churyumov–Gerasimenko. Processes that force comets to interact with other Solar System bodies are also discussed.

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2018 Topics

Thematic Articles

Comets: Where We Are, How We Got Here, and Where We Want To Go Next

By and

We introduce the principal mysteries surrounding comets; discuss the proposed importance of comets to the origin of water and organic compounds in the inner solar system; and summarize the history of cometary observation, study, and exploration over the past 22 centuries.


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


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.


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.


Comets in the Path of Earth

By and

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.


The Formation of the Solar Systems: A Recipe for Worlds


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.