Elements Covers

Thematic Articles

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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The Origin and Differentiation of Planet Mercury

Unique physical and chemical characteristics of Mercury have been revealed by measurements from NASA’s MESSENGER spacecraft. The closest planet to our Sun is made up of a large metallic core that is partially liquid, a thin mantle thought to be formed by solidification of a silicate magma ocean, and a relatively thick secondary crust produced by partial melting of the mantle followed by volcanic eruptions. However, the origin of the large metal/silicate ratio of the bulk planet and the conditions of accretion remain elusive. Metal enrichment may originate from primordial processes in the solar nebula or from a giant impact that stripped most of the silicate portion of a larger planet leaving Mercury as we know it today.

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Changing Trace Element Cycles in the 21st Century Ocean

Human activity is altering the ocean. This is happening through climate change, the release of pollutants, and direct exploitation of the marine environment. Recent advances in understanding the chemical cycling of trace elements within the global ocean comes at a critical time. Society is now increasingly viewing the ocean as a resource while also recognising that ocean systems are vulnerable to change.

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Trace-Metal Contaminants: Human Footprint on the Ocean

Anthropogenic activities have increased the fluxes of many trace metals into the oceans, changing their concentrations and distribution patterns. Despite their low dissolved concentrations, a number of these metals can still pose human and ecological risks. Some of these metals are well known (e.g. Pb, Hg), while others, such as the rare earth elements, represent emerging problems that impose new analytical challenges and environmental concerns. Defining the baselines of trace contaminants, identifying and quantifying the processes that control their transport, fate, and cycling are important issues to protect the ocean environment, safeguard human health, and support national and international marine decision-making.

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Discovering the Ocean’s Past Through Geochemistry

Trace elements and isotopes underlie many of the proxies used to reconstruct past ocean conditions. These proxies, recorded in diverse archives, are used to reconstruct seawater properties such as temperature, pH, and oxygen, or oceanic processes such as circulation, nutrient uptake, and biological productivity. Proxy calibration and validation requires a combination of ocean sediment core-top measurements, sediment trap studies, and laboratory- or field-based observations. New measurements of proxies in the modern ocean are rapidly illuminating the scope and limitations of each proxy while also helping to identify and evaluate new geochemical proxies that are based on trace elements and their isotopes.

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Oceanic Micronutrients: Trace Metals that are Essential for Marine Life

Trace metals are essential for life in the oceans but are present in extremely low concentrations. The availability of trace elements in surface waters frequently regulates the growth of microscopic marine plants called phytoplankton. As phytoplankton are responsible for taking up atmospheric carbon dioxide and exporting this to the deep ocean, trace elements are key components regulating the carbon cycle. New observations of the distribution of trace metals across all ocean basins from the GEOTRACES program have revealed a fascinating story of how the combination of trace metals interact with the ocean to regulate biological activity in new and surprising ways.

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