Thematic Articles

One’s Trash is Another’s Treasure: Cosmic Rubble Piles

Until 15 years ago, meteorites and cosmic dust were the only extraterrestrial materials available for investigating the nature and chemical evolution of the early Solar System. Since then, three major sample return missions have significantly advanced our understanding of the material composing the small bodies that populate our Solar System. The asteroid sample return mission Hayabusa first proved the direct link between an asteroid type and the most common type of meteorites falling to Earth. The Hayabusa2 and OSIRIS-REx missions recently collected and returned material from two carbonaceous asteroids, Ryugu and Bennu, respectively. Together, the results from those samples are revealing information not gleaned from studies of meteorites and are revolutionizing our understanding of the formation and evolution of planetary bodies at the dawn of our Solar System.

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Seeing Red: Retrieving Rocks from Mars and Phobos

Mars Sample Return (MSR) missions have been a priority for the planetary community for decades. The NASA Perseverance rover mission is collecting diverse samples from Mars for potential return to Earth, whereas the JAXA Martian Moons eXploration (MMX) mission will bring back samples from Phobos, the largest of Mars’ two moons. High-resolution analyses of these samples in Earth-based laboratories will enable us to answer key questions that current martian data (meteorites, rovers, and orbiters) are unable to fully address. MSR results will better inform our understanding of the geological and planetary evolution of the red planet, the possibility of habitability and life on Mars, the potential for human exploration, and the formation of its moons and the martian system.

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It’s Not Just a Phase: Over 50 Years of Lunar Sample Science

Sample Return Through the Ages, Thematic Articles / Katherine Joy

Landed robots, rovers, and orbital spacecraft provide regional-scale information about the nature of the Moon’s surface, but such data require ground truth information made accessible through lunar samples. Such samples include a range of material including hand-specimen-sized rocks, pieces of rocks chipped from boulders by astronauts wielding geologic hammers, to soil—scooped, trenched, and drilled from the upper few meters of the Moon’s surface by robots as well as humans. This chapter provides an overview of recent discoveries made using the lunar sample collection, highlights outstanding questions about the Moon’s origin and evolution, and discusses how these knowledge gaps will be addressed by future sample return missions.

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To See a World in a Grain of Sand

Sample Return Through the Ages, Thematic Articles / Jessica J. Barnes

In the 1960s and 1970s, NASA’s Apollo and the Soviet Union’s Luna missions captured imaginations across the world and revolutionized our under-standing of Earth’s moon and the Solar System. Over 50 years on, the realm of space exploration has expanded significantly, both in terms of the celestial bodies that have been explored and the nations working on these endeavors. In the coming decades, we will return samples from Mars and one of its moons, and humans will return to the Moon. This article sets the scene for this Elements issue, which will explore what we have learned about the formation and evolution of planetary bodies, including Earth, from analysis of returned samples, the links with orbital datasets, and priorities for the future.

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The Osmium Isotope Perspective on the Dynamics of the Post-Archean Mantle

Re-Os – Clock with Clout, Thematic Articles / Ambre Luguet

The ¹⁸⁷Re–¹⁸⁷Os system offers a unique perspective among the isotopic approaches used to understand planetary evolution because of the chalcophile and siderophile affinities of the parent and daughter elements and their contrasting behaviors during partial melting. Considered the geochronometer of choice to study the Earth’s mantle, from the scale of individual minerals to large-scale outcrops, this system has revealed the survival of Archean and Proterozoic mantle in younger tectonic settings, and has demonstrated local to regional coupling, and sometimes decoupling, between the crust and mantle. Osmium isotopes are also key tracers of melt–peridotite and mantle–crust interactions and recycling processes in subduction zones, and have furthered our understanding of the origin of multi-scale geochemical and isotopic heterogeneities.

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Osmium and Tungsten Isotopes Reveal Earth’s Youthful Exuberance

Re-Os – Clock with Clout, Thematic Articles / Igor S. Puchtel

The siderophile elements, which include Re, Pt, Os, and W, directly constrain the accretionary history of Earth. The largely chondritic ¹⁸⁶,¹⁸⁷Os/¹⁸⁸Os ratios of Earth’s mantle, coupled with excesses in siderophile element abundances, provide nearly incontrovertible evidence that some meteoritic addition continued after core formation was complete. Osmium and W isotope systematics of plume-derived mafic-ultramafic rocks reveal the complex chemical evolution of their deep mantle sources. In the upper mantle, Re-Os dating of whole-rock xenoliths and sulfide inclusions in diamonds hosted by kimberlites indicate both ancient melt depletion and subsequent modification of the mantle lithosphere beneath the earliest continents, with Re-Os ages of eclogitic diamonds possibly recording the transition to a sustained plate tectonic regime on Earth.

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Oil and Water – Intimate Conversations

Re-Os – Clock with Clout, Thematic Articles / Svetoslav V. Georgiev

Re-Os geochronology is a powerful tool for unravelling the complexity of petroleum systems. Because of their organophile nature, Re and Os are enriched in sedimentary organic matter and the hydrocarbons it produces upon heating. Rhenium and Os isotopes are used to date hydrocarbon generation, to fingerprint oils, to distinguish different pulses of oil generation, and to determine interactions between oil, host rock, and formation water. Here we summarize knowledge on the content, distribution, and isotopic composition of Re and Os in petroleum and associated waters, highlight the main technical advances for Re-Os analyses, review experimental studies on water–oil interaction, and discuss the diverse Re-Os applications to petroleum systems.

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Reel-to-Reel Re-Os Records: Earth System Transactions Preserved in Sediments

Re-Os – Clock with Clout, Thematic Articles / Brian Kendall

A unique feature of the Re-Os isotope system is its ability to provide precise and accurate depositional ages from organic-rich sedimentary rocks. Applications include geologic timescale calibration, stratigraphic correlation, and dating key events such as biological innovations, mass extinctions, carbon cycle perturbations, Snowball Earth glaciations, and atmospheric oxygenation. Multiple sediment types reveal temporal variations in the osmium isotope composition of seawater, driven by changes in osmium inputs from continental weathering, seafloor hydrothermal systems, and extraterrestrial material. These variations provide valuable information on climate–tectonic interactions, glacial–interglacial cycles, large igneous province magmatism, bolide impacts, and crustal evolution. Continental processes can be inferred from lake sediment records. These diverse applications highlight the central role of the Re-Os isotope pair in understanding Earth’s evolution.

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Sulfides and Their Little Darling, Molybdenite

Re-Os – Clock with Clout, Thematic Articles / Holly Stein

Rhenium-osmium geochronology of sulfides and its Os tracer accompaniment have taken their place among geochronometers, although the journey was not without doubters and disparagers. This review highlights several historical hurdles overcome in dating sulfides. The opening act was the debut of molybdenite, which provided an accessible radiometric clock and early insight into the accuracy of the ¹⁸⁷Re decay constant. Once controversies surrounding newly minted and game-changing Re-Os molybdenite ages died down, the door flung wide open to begin dating other sulfides, most notably, arsenopyrite and pyrite. Applications sprinted from the ore geology community to constraining the timing of important events, from Earth’s oxygenation to the amalgamation of tectonic terranes. The power of Re-Os sulfide dating in crustal environments was unleashed.

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The Re-Os Revolution: Mighty Messages From Two of Earth’s Rarest Elements

Re-Os – Clock with Clout, Thematic Articles / Laurie Reisberg

The exceptional power and versatility of the Re-Os radioactive decay system for Earth science stems from the distinctive geochemical behavior of its constituent elements. Here, we first explain how the positions of Re and Os in the periodic table are responsible for their highly siderophile, chalcophile, and organophile properties. We then discuss how these properties dictate the distribution of Re and Os within and at the surface of the Earth and other planetary bodies. Lastly, we describe how the analytical challenges posed by the unusual geochemistry of these elements were overcome with major technological advances, leading to a dramatic decrease in the amount of sample material required for Re-Os isotopic analysis, thereby sparking an explosion of new applications.

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