Elements Covers

Posts Tagged ‘February 2019’

SOTA7: State of the [Continental] Arc

In September 2018, 32 participants from 25 institutions took part in the highest-altitude State of the Arc (SOTA) meeting yet: SOTA7. Participants spent a week in San Pedro in the Chilean Atacama Desert, situated on top of the world’s thickest continental arc crust. A mixture of keynote, regular, and “pop-up” talks and posters explored the realms of geochemistry, geophysics, experimental petrology, and numerical models to address topics in arc magmatism: these included volcano “personality”, the movement of volatiles, compositional evolution, how to match geophysical studies with petrology, and how eruptions are triggered.

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Impact Earth: A New Resource for Outreach, Teaching, and Research

When one mentions the word “geology”, most people will likely think of volcanoes, glaciers, or majestic mountain ranges. Beginning in the late 18th century with the work of pioneering Scottish geologist James Hutton (1726–1797), uniformitarianism emerged as a central tenet of geology and remained so well into the 20th century. Central to the idea of uniformitarianism is the concept of gradualism, whereby processes throughout time occur at the same or similar rates, leading to the famous concept that “The present is the key to the past.”

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v15n1 From the Editors

With the start of 2019, John M. Eiler joins the Elements editorial team. He is taking on the role as our geochemistry principal editor.
There are so many more topics to feature in Elements. In March 2019, the editorial team will meet to evaluate proposals for inclusion in our lineup. We invite you to contact one of the Elements editors and submit a thematic proposal for consideration!

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The Role of Reducing Conditions in Building Mercury

Extremely reducing conditions, such as those that prevailed during the accretion and differentiation of Mercury, change the “normal” pattern of behaviour of many chemical elements. Lithophile elements can become chalcophile, siderophile elements can become lithophile, and volatile elements can become refractory. In this context, unexpected elements, such as Si, are extracted to the core, while others (S, C) concentrate in the silicate portion of the planet, eventually leading to an exotic surface mineralogy. In this article, experimental, theoretical and cosmochemical arguments are applied to the understanding of how reducing conditions influenced Mercury, from the nature of its building blocks to the dynamics of its volcanism.

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The Surface Composition of Mercury

Geochemical data from MESSENGER have revealed details of Mercury’s surface composition, showing that it differs from the other rocky planets in the inner solar system. For example, the planet’s surface is enriched in S and C, and depleted in Fe, indicating that Mercury formed under much more reducing conditions than other planets. The surface is also enriched in Mg and depleted in Al and Ca. Observed elemental heterogeneities and percent levels of graphite suggest that Mercury underwent a magma ocean phase early in its history. These findings have important implications for understanding Mercury’s origin and evolution.

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Volcanism on Mercury

Mercury’s volcanic nature has been revealed by NASA’s MESSENGER mission. We now know that all, or most, of the surface has, at some point, been flooded by lavas, sometimes in extremely voluminous eruptions. The ages of Mercury’s lava surfaces reveal that large-volume effusive volcanism ceased about 3.5 billion years ago due to planetary cooling. Mercury’s crust then went into a state of global contraction, thereby impeding further magma ascent. However, some smaller-scale volcanism continued at zones of crustal weakness, particularly at impact craters. Much of this later volcanism has been violently explosive, with volatile gases potentially helping the magma rise and ripping it apart when released to the vacuum at the surface.

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Mercury: Inside the Iron Planet

NASA’s MESSENGER spacecraft orbited Mercury from 2011 to 2015 and has provided new insights into the interior of the innermost planet. Mercury has a large metallic core ~2,000 km in radius covered by a thin layer of rock only ~420 km thick. Furthermore, a surprisingly large fraction of this outer layer was produced by melting of deeper rocks, forming a light crust ~35 km thick. The core is now known to produce a magnetic field that has intriguing similarities and differences compared to Earth’s field. Some rocks near the surface are magnetized, and the strongest magnetizations are likely to be >3.5 billion years old. This new understanding of Mercury’s interior is helping reveal how rocky planets operate.

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The Exploration of Mercury by Spacecraft

The planet Mercury is sufficiently close to the Sun to pose a major challenge to spacecraft exploration. The Mariner 10 spacecraft flew by Mercury three times in 1974–1975 but viewed less than half of the surface. With the three flybys of Mercury by the MESSENGER spacecraft in 2008–2009 and the insertion of that probe into orbit about Mercury in 2011, our understanding of the innermost planet substantially improved. In its four years of orbital operations, MESSENGER revealed a world more geologically complex and compositionally distinctive, with a more dynamic magnetosphere and more diverse exosphere–surface interactions, than expected. With the launch of the BepiColombo dual-orbiter mission, the scientific understanding of the innermost planet has moved another major step forward.

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